@article{bhowmick_additional_2026,
	title = {An additional water is introduced into the manganese cluster during the formation of the {S3} state of photosystem {II}},
	volume = {17},
	copyright = {2026 The Author(s)},
	issn = {2041-1723},
	url = {https://www.nature.com/articles/s41467-026-76805-9},
	doi = {10.1038/s41467-026-76805-9},
	abstract = {During photosynthetic water oxidation, the Mn4Ca cluster in Photosystem II progresses through five intermediate Si (i = 0–4) states. X-ray crystallography studies have reported the insertion of one new O ligand during the formation of the S3 state, but recent studies question the presence of this additional ligand based on cryo-EM and earlier room-temperature crystallography data. There is also controversy about whether the O-O bond interaction already occurs in the S3 state or in the subsequent S3 to S0 transition. Here we report conventional high-resolution data for the S1, S2, and S3 states to a resolution of {\textasciitilde}1.9 Å, and anomalous diffraction data at two energies (9.5 keV and 7 keV), that was used to model the Mn positions, followed by determination of oxygen positions using the high-resolution maps. We show that the new oxygen atom, OX (or O6), in the S3 state is observable as a distinct peak without any restraints, confirming its ligation to Mn1 and Ca. The OX-O5 distance is {\textasciitilde}2.1 Å, supporting no strong interaction between them in the S3 state, suggesting that if this is the O-O bond formation site, it is formed during the S3 to S0 transition initiated by the final oxidation of the cluster.},
	language = {en},
	number = {1},
	urldate = {2026-09-02},
	journal = {Nature Communications},
	publisher = {Nature Publishing Group},
	author = {Bhowmick, Asmit and Zhang, Miao and Simon, Philipp S. and Makita, Hiroki and Nangca, Isabela I. and Szilagyi, Erzsi and Kretzschmar, Moritz and Doyle, Margaret D. and Minnetian, Natalie M. and Hussein, Rana and Hart, Olli and Chatterjee, Kuntal and Aydin, A. Orkun and Shevela, Dmitry and Cheah, Mun Hon and Croy, Nicholas and Chernev, Petko and Fransson, Thomas and Tiwari, Vandana and Sanchez, Humberto and Schleissner, Pamela and Lemons, Randy and Gate, Greg and Henstridge, Meredith and Glownia, James M. and Poitevin, Frédéric and Rosenberg, Daniel J. and Dehe, Sebastian and Gee, Leland B. and Tono, Kensuke and Owada, Shigeki and Oggenfuss, Roland and Ozerov, Dmitry and Sander, Mathias and Mankowsky, Roman and Lemke, Henrik T. and Young, Iris D. and Holton, James M. and Mittan-Moreau, David W. and Paley, Daniel W. and Afonine, Pavel V. and Moriarty, Nigel W. and Adams, Paul D. and Mamedov, Fikret and Dobbek, Holger and Zouni, Athina and Alonso-Mori, Roberto and Bergmann, Uwe and Brewster, Aaron S. and Sauter, Nicholas K. and Messinger, Johannes and Kern, Jan F. and Yachandra, Vittal K. and Yano, Junko},
	month = aug,
	year = {2026},
	keywords = {Bioenergetics, Physical chemistry, Structural biology},
	pages = {8818},
}

During photosynthetic water oxidation, the Mn4Ca cluster in Photosystem II progresses through five intermediate Si (i = 0–4) states. X-ray crystallography studies have reported the insertion of one new O ligand during the formation of the S3 state, but recent studies question the presence of this additional ligand based on cryo-EM and earlier room-temperature crystallography data. There is also controversy about whether the O-O bond interaction already occurs in the S3 state or in the subsequent S3 to S0 transition. Here we report conventional high-resolution data for the S1, S2, and S3 states to a resolution of ~1.9 Å, and anomalous diffraction data at two energies (9.5 keV and 7 keV), that was used to model the Mn positions, followed by determination of oxygen positions using the high-resolution maps. We show that the new oxygen atom, OX (or O6), in the S3 state is observable as a distinct peak without any restraints, confirming its ligation to Mn1 and Ca. The OX-O5 distance is ~2.1 Å, supporting no strong interaction between them in the S3 state, suggesting that if this is the O-O bond formation site, it is formed during the S3 to S0 transition initiated by the final oxidation of the cluster.

@article{forsman_structure_2026,
	title = {The structure of intact and active {Photosystem} {II} from {Arabidopsis} thaliana at 2.44 Å resolution},
	volume = {250},
	copyright = {© 2026 The Author(s). New Phytologist © 2026 New Phytologist Foundation.},
	issn = {1469-8137},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/nph.71085},
	doi = {10.1111/nph.71085},
	abstract = {Photosystem II (PS II) is a large membrane-bound protein complex that catalyses light-driven water oxidation in plants and cyanobacteria. The structure of PS II is well studied in cyanobacteria; however, there are very few PS II structures from plants. The currently available plant PS II structures are comparatively low resolution and are frequently incomplete, that is, missing subunits or cofactors. We optimized the procedure for isolating PS II from Arabidopsis thaliana and employed cryo-electron microscopy to generate a high-resolution structure of an intact and oxygen-evolving PS II from Arabidopsis thaliana at 2.44 Å resolution, which to date represents the highest resolution structure of PS II from higher plants. At this resolution, many water molecules within the PS II structure can be detected, including waters around the water-splitting manganese cluster, the nonheme iron, and within the water/proton channels connecting these active sites to the protein exterior, allowing for the first detailed description of the water networks in Arabidopsis thaliana and comparison with the highly resolved cyanobacterial PS II. Our findings further the understanding of design principles of protein–water–cofactor interactions in photosynthetic water splitting, quinone reduction/exchange, and about the role of lipids at the interface between PS II and the light-harvesting proteins.},
	language = {en},
	number = {5},
	urldate = {2026-05-15},
	journal = {New Phytologist},
	author = {Forsman, Jack and Graça, André T. and Aydin, Abuzer Orkun and Hall, Michael and Hussein, Rana and Schröder, Wolfgang P. and Messinger, Johannes},
	year = {2026},
	note = {\_eprint: https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.71085},
	keywords = {Arabidopsis thaliana, Cryo-EM, Photosystem II structure, manganese cluster, photosynthesis, protein–water–cofactor interactions, water channels},
	pages = {3014--3025},
}

Photosystem II (PS II) is a large membrane-bound protein complex that catalyses light-driven water oxidation in plants and cyanobacteria. The structure of PS II is well studied in cyanobacteria; however, there are very few PS II structures from plants. The currently available plant PS II structures are comparatively low resolution and are frequently incomplete, that is, missing subunits or cofactors. We optimized the procedure for isolating PS II from Arabidopsis thaliana and employed cryo-electron microscopy to generate a high-resolution structure of an intact and oxygen-evolving PS II from Arabidopsis thaliana at 2.44 Å resolution, which to date represents the highest resolution structure of PS II from higher plants. At this resolution, many water molecules within the PS II structure can be detected, including waters around the water-splitting manganese cluster, the nonheme iron, and within the water/proton channels connecting these active sites to the protein exterior, allowing for the first detailed description of the water networks in Arabidopsis thaliana and comparison with the highly resolved cyanobacterial PS II. Our findings further the understanding of design principles of protein–water–cofactor interactions in photosynthetic water splitting, quinone reduction/exchange, and about the role of lipids at the interface between PS II and the light-harvesting proteins.

@article{aydin_probing_2025,
	title = {Probing substrate water access through the {O1} channel of {Photosystem} {II} by single site mutations and membrane inlet mass spectrometry},
	volume = {163},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-025-01147-4},
	doi = {10.1007/s11120-025-01147-4},
	abstract = {Light-driven water oxidation by photosystem II sustains life on Earth by providing the electrons and protons for the reduction of CO2 to carbohydrates and the molecular oxygen we breathe. The inorganic core of the oxygen evolving complex is made of the earth-abundant elements manganese, calcium and oxygen (Mn4CaO5 cluster), and is situated in a binding pocket that is connected to the aqueous surrounding via water-filled channels that allow water intake and proton egress. Recent serial crystallography and infrared spectroscopy studies performed with PSII isolated from Thermosynechococcus vestitus (T. vestitus) support that one of these channels, the O1 channel, facilitates water access to the Mn4CaO5 cluster during its S2→S3 and S3→S4→S0 state transitions, while a subsequent CryoEM study concluded that this channel is blocked in the cyanobacterium Synechocystis sp. PCC 6803, questioning the role of the O1 channel in water delivery. Employing site-directed mutagenesis we modified the two O1 channel bottleneck residues D1-E329 and CP43-V410 (T. vestitus numbering) and probed water access and substrate exchange via time resolved membrane inlet mass spectrometry. Our data demonstrates that water reaches the Mn4CaO5 cluster via the O1 channel in both wildtype and mutant PSII. In addition, the detailed analysis provides functional insight into the intricate protein-water-cofactor network near the Mn4CaO5 cluster that includes the pentameric, near planar ‘water wheel’ of the O1 channel.},
	language = {en},
	number = {3},
	urldate = {2025-04-25},
	journal = {Photosynthesis Research},
	author = {Aydin, A. Orkun and de Lichtenberg, Casper and Liang, Feiyan and Forsman, Jack and Graça, André T. and Chernev, Petko and Zhu, Shaochun and Mateus, André and Magnuson, Ann and Cheah, Mun Hon and Schröder, Wolfgang P. and Ho, Felix and Lindblad, Peter and Debus, Richard J. and Mamedov, Fikret and Messinger, Johannes},
	month = apr,
	year = {2025},
	keywords = {CP43-V410, D1-E329, O1 channel, Oxygen evolving complex, Photosystem II, Substrate water exchange, Synechocystis sp. PCC 6803, Water delivery, Water oxidation, Water wheel},
	pages = {28},
}

Light-driven water oxidation by photosystem II sustains life on Earth by providing the electrons and protons for the reduction of CO2 to carbohydrates and the molecular oxygen we breathe. The inorganic core of the oxygen evolving complex is made of the earth-abundant elements manganese, calcium and oxygen (Mn4CaO5 cluster), and is situated in a binding pocket that is connected to the aqueous surrounding via water-filled channels that allow water intake and proton egress. Recent serial crystallography and infrared spectroscopy studies performed with PSII isolated from Thermosynechococcus vestitus (T. vestitus) support that one of these channels, the O1 channel, facilitates water access to the Mn4CaO5 cluster during its S2→S3 and S3→S4→S0 state transitions, while a subsequent CryoEM study concluded that this channel is blocked in the cyanobacterium Synechocystis sp. PCC 6803, questioning the role of the O1 channel in water delivery. Employing site-directed mutagenesis we modified the two O1 channel bottleneck residues D1-E329 and CP43-V410 (T. vestitus numbering) and probed water access and substrate exchange via time resolved membrane inlet mass spectrometry. Our data demonstrates that water reaches the Mn4CaO5 cluster via the O1 channel in both wildtype and mutant PSII. In addition, the detailed analysis provides functional insight into the intricate protein-water-cofactor network near the Mn4CaO5 cluster that includes the pentameric, near planar ‘water wheel’ of the O1 channel.

@article{bogacz_x-ray_2025,
	title = {X-ray {Absorption} {Spectroscopy} of {Dilute} {Metalloenzymes} at {X}-ray {Free}-{Electron} {Lasers} in a {Shot}-by-{Shot} {Mode}},
	volume = {16},
	url = {https://doi.org/10.1021/acs.jpclett.5c00399},
	doi = {10.1021/acs.jpclett.5c00399},
	abstract = {X-ray absorption spectroscopy (XAS) of 3d transition metals provides important electronic structure information for many fields. However, X-ray-induced radiation damage under physiological temperature has prevented using this method to study dilute aqueous systems, such as metalloenzymes, as the catalytic reaction proceeds. Here we present a new approach to enable operando XAS of dilute biological samples and demonstrate its feasibility with K-edge XAS spectra from the Mn cluster in photosystem II and the Fe–S centers in photosystem I. This approach combines highly efficient sample delivery strategies and a robust signal normalization method with high-transmission Bragg diffraction-based spectrometers at X-ray free-electron lasers (XFELs) in a damage-free, shot-by-shot mode. These photon-out spectrometers have been optimized for discriminating the metal Mn/Fe Kα fluorescence signals from the overwhelming scattering background present on currently available detectors for XFELs that lack suitable energy discrimination. We quantify the enhanced performance metrics of the spectrometer and discuss its potential applications for acquiring time-resolved XAS spectra of biological samples during their reactions at XFELs.},
	number = {15},
	urldate = {2025-04-22},
	journal = {The Journal of Physical Chemistry Letters},
	publisher = {American Chemical Society},
	author = {Bogacz, Isabel and Szilagyi, Erzsi and Makita, Hiroki and Simon, Philipp S. and Zhang, Miao and Doyle, Margaret D. and Chatterjee, Kuntal and Kretzschmar, Moritz and Chernev, Petko and Croy, Nicholas and Cheah, Mun-Hon and Dasgupta, Medhanjali and Nangca, Isabela and Fransson, Thomas and Bhowmick, Asmit and Brewster, Aaron S. and Sauter, Nicholas K. and Owada, Shigeki and Tono, Kensuke and Zerdane, Serhane and Oggenfuss, Alexander and Babich, Danylo and Sander, Mathias and Mankowsky, Roman and Lemke, Henrik T. and Gee, Leland B. and Sato, Takahiro and Kroll, Thomas and Messinger, Johannes and Alonso-Mori, Roberto and Bergmann, Uwe and Sokaras, Dimosthenis and Yachandra, Vittal K. and Kern, Jan and Yano, Junko},
	month = apr,
	year = {2025},
	pages = {3778--3787},
}

X-ray absorption spectroscopy (XAS) of 3d transition metals provides important electronic structure information for many fields. However, X-ray-induced radiation damage under physiological temperature has prevented using this method to study dilute aqueous systems, such as metalloenzymes, as the catalytic reaction proceeds. Here we present a new approach to enable operando XAS of dilute biological samples and demonstrate its feasibility with K-edge XAS spectra from the Mn cluster in photosystem II and the Fe–S centers in photosystem I. This approach combines highly efficient sample delivery strategies and a robust signal normalization method with high-transmission Bragg diffraction-based spectrometers at X-ray free-electron lasers (XFELs) in a damage-free, shot-by-shot mode. These photon-out spectrometers have been optimized for discriminating the metal Mn/Fe Kα fluorescence signals from the overwhelming scattering background present on currently available detectors for XFELs that lack suitable energy discrimination. We quantify the enhanced performance metrics of the spectrometer and discuss its potential applications for acquiring time-resolved XAS spectra of biological samples during their reactions at XFELs.

@article{de_lichtenberg_assignment_2024,
	title = {Assignment of the slowly exchanging substrate water of nature’s water-splitting cofactor},
	volume = {121},
	url = {https://www.pnas.org/doi/full/10.1073/pnas.2319374121},
	doi = {10.1073/pnas.2319374121},
	abstract = {Identifying the two substrate water sites of nature’s water-splitting cofactor (Mn4CaO5 cluster) provides important information toward resolving the mechanism of O-O bond formation in Photosystem II (PSII). To this end, we have performed parallel substrate water exchange experiments in the S1 state of native Ca-PSII and biosynthetically substituted Sr-PSII employing Time-Resolved Membrane Inlet Mass Spectrometry (TR-MIMS) and a Time-Resolved 17O-Electron-electron Double resonance detected NMR (TR-17O-EDNMR) approach. TR-MIMS resolves the kinetics for incorporation of the oxygen-isotope label into the substrate sites after addition of H218O to the medium, while the magnetic resonance technique allows, in principle, the characterization of all exchangeable oxygen ligands of the Mn4CaO5 cofactor after mixing with H217O. This unique combination shows i) that the central oxygen bridge (O5) of Ca-PSII core complexes isolated from Thermosynechococcus vestitus has, within experimental conditions, the same rate of exchange as the slowly exchanging substrate water (WS) in the TR-MIMS experiments and ii) that the exchange rates of O5 and WS are both enhanced by Ca2+→Sr2+ substitution in a similar manner. In the context of previous TR-MIMS results, this shows that only O5 fulfills all criteria for being WS. This strongly restricts options for the mechanism of water oxidation.},
	number = {11},
	urldate = {2024-10-16},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {de Lichtenberg, Casper and Rapatskiy, Leonid and Reus, Michael and Heyno, Eiri and Schnegg, Alexander and Nowaczyk, Marc M. and Lubitz, Wolfgang and Messinger, Johannes and Cox, Nicholas},
	month = mar,
	year = {2024},
	pages = {e2319374121},
}

Identifying the two substrate water sites of nature’s water-splitting cofactor (Mn4CaO5 cluster) provides important information toward resolving the mechanism of O-O bond formation in Photosystem II (PSII). To this end, we have performed parallel substrate water exchange experiments in the S1 state of native Ca-PSII and biosynthetically substituted Sr-PSII employing Time-Resolved Membrane Inlet Mass Spectrometry (TR-MIMS) and a Time-Resolved 17O-Electron-electron Double resonance detected NMR (TR-17O-EDNMR) approach. TR-MIMS resolves the kinetics for incorporation of the oxygen-isotope label into the substrate sites after addition of H218O to the medium, while the magnetic resonance technique allows, in principle, the characterization of all exchangeable oxygen ligands of the Mn4CaO5 cofactor after mixing with H217O. This unique combination shows i) that the central oxygen bridge (O5) of Ca-PSII core complexes isolated from Thermosynechococcus vestitus has, within experimental conditions, the same rate of exchange as the slowly exchanging substrate water (WS) in the TR-MIMS experiments and ii) that the exchange rates of O5 and WS are both enhanced by Ca2+→Sr2+ substitution in a similar manner. In the context of previous TR-MIMS results, this shows that only O5 fulfills all criteria for being WS. This strongly restricts options for the mechanism of water oxidation.

@article{guo_closing_2024,
	title = {Closing {Kok}’s cycle of nature’s water oxidation catalysis},
	volume = {15},
	copyright = {2024 The Author(s)},
	issn = {2041-1723},
	url = {https://www.nature.com/articles/s41467-024-50210-6},
	doi = {10.1038/s41467-024-50210-6},
	abstract = {The Mn4CaO5(6) cluster in photosystem II catalyzes water splitting through the Si state cycle (i = 0–4). Molecular O2 is formed and the natural catalyst is reset during the final S3 → (S4) → S0 transition. Only recently experimental breakthroughs have emerged for this transition but without explicit information on the S0-state reconstitution, thus the progression after O2 release remains elusive. In this report, our molecular dynamics simulations combined with density functional calculations suggest a likely missing link for closing the cycle, i.e., restoring the first catalytic state. Specifically, the formation of closed-cubane intermediates with all hexa-coordinate Mn is observed, which would undergo proton release, water dissociation, and ligand transfer to produce the open-cubane structure of the S0 state. Thereby, we theoretically identify the previously unknown structural isomerism in the S0 state that acts as the origin of the proposed structural flexibility prevailing in the cycle, which may be functionally important for nature’s water oxidation catalysis.},
	language = {en},
	number = {1},
	urldate = {2024-07-19},
	journal = {Nature Communications},
	publisher = {Nature Publishing Group},
	author = {Guo, Yu and He, Lanlan and Ding, Yunxuan and Kloo, Lars and Pantazis, Dimitrios A. and Messinger, Johannes and Sun, Licheng},
	month = jul,
	year = {2024},
	keywords = {Bioinorganic chemistry, Catalytic mechanisms, Reaction mechanisms},
	pages = {5982},
}

The Mn4CaO5(6) cluster in photosystem II catalyzes water splitting through the Si state cycle (i = 0–4). Molecular O2 is formed and the natural catalyst is reset during the final S3 → (S4) → S0 transition. Only recently experimental breakthroughs have emerged for this transition but without explicit information on the S0-state reconstitution, thus the progression after O2 release remains elusive. In this report, our molecular dynamics simulations combined with density functional calculations suggest a likely missing link for closing the cycle, i.e., restoring the first catalytic state. Specifically, the formation of closed-cubane intermediates with all hexa-coordinate Mn is observed, which would undergo proton release, water dissociation, and ligand transfer to produce the open-cubane structure of the S0 state. Thereby, we theoretically identify the previously unknown structural isomerism in the S0 state that acts as the origin of the proposed structural flexibility prevailing in the cycle, which may be functionally important for nature’s water oxidation catalysis.

@article{hussein_cryoelectron_2024,
	title = {Cryo–electron microscopy reveals hydrogen positions and water networks in photosystem {II}},
	volume = {384},
	url = {https://www.science.org/doi/10.1126/science.adn6541},
	doi = {10.1126/science.adn6541},
	abstract = {Photosystem II starts the photosynthetic electron transport chain that converts solar energy into chemical energy and thus sustains life on Earth. It catalyzes two chemical reactions: water oxidation to molecular oxygen and plastoquinone reduction. Coupling of electron and proton transfer is crucial for efficiency; however, the molecular basis of these processes remains speculative owing to uncertain water binding sites and the lack of experimentally determined hydrogen positions. We thus collected high-resolution cryo–electron microscopy data of fully hydrated photosystem II from the thermophilic cyanobacterium Thermosynechococcus vestitus to a final resolution of 1.71 angstroms. The structure reveals several previously undetected partially occupied water binding sites and more than half of the hydrogen and proton positions. This clarifies the pathways of substrate water binding and plastoquinone B protonation.},
	number = {6702},
	urldate = {2024-06-26},
	journal = {Science},
	publisher = {American Association for the Advancement of Science},
	author = {Hussein, Rana and Graça, André and Forsman, Jack and Aydin, A. Orkun and Hall, Michael and Gaetcke, Julia and Chernev, Petko and Wendler, Petra and Dobbek, Holger and Messinger, Johannes and Zouni, Athina and Schröder, Wolfgang P.},
	month = jun,
	year = {2024},
	pages = {1349--1355},
}

Photosystem II starts the photosynthetic electron transport chain that converts solar energy into chemical energy and thus sustains life on Earth. It catalyzes two chemical reactions: water oxidation to molecular oxygen and plastoquinone reduction. Coupling of electron and proton transfer is crucial for efficiency; however, the molecular basis of these processes remains speculative owing to uncertain water binding sites and the lack of experimentally determined hydrogen positions. We thus collected high-resolution cryo–electron microscopy data of fully hydrated photosystem II from the thermophilic cyanobacterium Thermosynechococcus vestitus to a final resolution of 1.71 angstroms. The structure reveals several previously undetected partially occupied water binding sites and more than half of the hydrogen and proton positions. This clarifies the pathways of substrate water binding and plastoquinone B protonation.

@incollection{shevela_measurements_2024,
	address = {New York, NY},
	title = {Measurements of {Oxygen} {Evolution} in {Photosynthesis}},
	isbn = {978-1-0716-3790-6},
	url = {https://doi.org/10.1007/978-1-0716-3790-6_8},
	doi = {10.1007/978-1-0716-3790-6_8},
	abstract = {This chapter compares two different techniques for monitoring photosynthetic O2 production; the wide-spread Clark-type O2 electrode and the more sophisticated membrane inlet mass spectrometry (MIMS) technique. We describe how a simple membrane inlet for MIMS can be made out of a commercial Clark-type cell and outline the advantages and drawbacks of the two techniques to guide researchers in deciding which method to use. Protocols and examples are given for measuring O2 evolution rates and for determining the number of chlorophyll molecules per active photosystem II reaction center.},
	language = {en},
	urldate = {2024-10-16},
	booktitle = {Photosynthesis : {Methods} and {Protocols}},
	publisher = {Springer US},
	author = {Shevela, Dmitry and Schröder, Wolfgang P. and Messinger, Johannes},
	editor = {Covshoff, Sarah},
	year = {2024},
	keywords = {Clark-type electrode, Membrane-inlet mass spectrometry, O2 evolution, Oxygenic photosynthesis, Photosynthetic water oxidation, Photosynthetic water splitting, Photosystem II},
	pages = {133--148},
}

This chapter compares two different techniques for monitoring photosynthetic O2 production; the wide-spread Clark-type O2 electrode and the more sophisticated membrane inlet mass spectrometry (MIMS) technique. We describe how a simple membrane inlet for MIMS can be made out of a commercial Clark-type cell and outline the advantages and drawbacks of the two techniques to guide researchers in deciding which method to use. Protocols and examples are given for measuring O2 evolution rates and for determining the number of chlorophyll molecules per active photosystem II reaction center.

@article{chernev_simulation_2024,
	title = {On the simulation and interpretation of substrate-water exchange experiments in photosynthetic water oxidation},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-024-01084-8},
	doi = {10.1007/s11120-024-01084-8},
	abstract = {Water oxidation by photosystem II (PSII) sustains most life on Earth, but the molecular mechanism of this unique process remains controversial. The ongoing identification of the binding sites and modes of the two water-derived substrate oxygens (‘substrate waters’) in the various intermediates (Si states, i = 0, 1, 2, 3, 4) that the water-splitting tetra-manganese calcium penta-oxygen (Mn4CaO5) cluster attains during the reaction cycle provides central information towards resolving the unique chemistry of biological water oxidation. Mass spectrometric measurements of single- and double-labeled dioxygen species after various incubation times of PSII with H218O provide insight into the substrate binding modes and sites via determination of exchange rates. Such experiments have revealed that the two substrate waters exchange with different rates that vary independently with the Si state and are hence referred to as the fast (Wf) and the slow (WS) substrate waters. New insight for the molecular interpretation of these rates arises from our recent finding that in the S2 state, under special experimental conditions, two different rates of WS exchange are observed that appear to correlate with the high spin and low spin conformations of the Mn4CaO5 cluster. Here, we reexamine and unite various proposed methods for extracting and assigning rate constants from this recent data set. The analysis results in a molecular model for substrate-water binding and exchange that reconciles the expected non-exchangeability of the central oxo bridge O5 when located between two Mn(IV) ions with the experimental and theoretical assignment of O5 as WS in all S states. The analysis also excludes other published proposals for explaining the water exchange kinetics.},
	language = {en},
	urldate = {2024-10-16},
	journal = {Photosynthesis Research},
	author = {Chernev, Petko and Aydin, A. Orkun and Messinger, Johannes},
	month = mar,
	year = {2024},
	keywords = {Mechanism of water oxidation, Membrane inlet mass spectrometry (MIMS), Oxygen-evolving complex, Photosystem II, Substrate-water exchange},
}

Water oxidation by photosystem II (PSII) sustains most life on Earth, but the molecular mechanism of this unique process remains controversial. The ongoing identification of the binding sites and modes of the two water-derived substrate oxygens (‘substrate waters’) in the various intermediates (Si states, i = 0, 1, 2, 3, 4) that the water-splitting tetra-manganese calcium penta-oxygen (Mn4CaO5) cluster attains during the reaction cycle provides central information towards resolving the unique chemistry of biological water oxidation. Mass spectrometric measurements of single- and double-labeled dioxygen species after various incubation times of PSII with H218O provide insight into the substrate binding modes and sites via determination of exchange rates. Such experiments have revealed that the two substrate waters exchange with different rates that vary independently with the Si state and are hence referred to as the fast (Wf) and the slow (WS) substrate waters. New insight for the molecular interpretation of these rates arises from our recent finding that in the S2 state, under special experimental conditions, two different rates of WS exchange are observed that appear to correlate with the high spin and low spin conformations of the Mn4CaO5 cluster. Here, we reexamine and unite various proposed methods for extracting and assigning rate constants from this recent data set. The analysis results in a molecular model for substrate-water binding and exchange that reconciles the expected non-exchangeability of the central oxo bridge O5 when located between two Mn(IV) ions with the experimental and theoretical assignment of O5 as WS in all S states. The analysis also excludes other published proposals for explaining the water exchange kinetics.

@article{guo_alternative_2023,
	title = {Alternative {Mechanism} for {O2} {Formation} in {Natural} {Photosynthesis} via {Nucleophilic} {Oxo}–{Oxo} {Coupling}},
	volume = {145},
	issn = {0002-7863},
	url = {https://doi.org/10.1021/jacs.2c12174},
	doi = {10.1021/jacs.2c12174},
	abstract = {O2 formation in photosystem II (PSII) is a vital event on Earth, but the exact mechanism remains unclear. The presently prevailing theoretical model is “radical coupling” (RC) involving a Mn(IV)-oxyl unit in an “open-cubane” Mn4CaO6 cluster, which is supported experimentally by the S3 state of cyanobacterial PSII featuring an additional Mn-bound oxygenic ligand. However, it was recently proposed that the major structural form of the S3 state of higher plants lacks this extra ligand, and that the resulting S4 state would feature instead a penta-coordinate dangler Mn(V)=oxo, covalently linked to a “closed-cubane” Mn3CaO4 cluster. For this proposal, we explore here a large number of possible pathways of O–O bond formation and demonstrate that the “nucleophilic oxo–oxo coupling” (NOOC) between Mn(V)=oxo and μ3-oxo is the only eligible mechanism in such a system. The reaction is facilitated by a specific conformation of the cluster and concomitant water binding, which is delayed compared to the RC mechanism. An energetically feasible process is described starting from the valid S4 state through the sequential formation of peroxide and superoxide, followed by O2 release and a second water insertion. The newly found mechanism is consistent with available experimental thermodynamic and kinetic data and thus a viable alternative pathway for O2 formation in natural photosynthesis, in particular for higher plants.},
	number = {7},
	urldate = {2024-10-16},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Guo, Yu and Messinger, Johannes and Kloo, Lars and Sun, Licheng},
	month = feb,
	year = {2023},
	pages = {4129--4141},
}

O2 formation in photosystem II (PSII) is a vital event on Earth, but the exact mechanism remains unclear. The presently prevailing theoretical model is “radical coupling” (RC) involving a Mn(IV)-oxyl unit in an “open-cubane” Mn4CaO6 cluster, which is supported experimentally by the S3 state of cyanobacterial PSII featuring an additional Mn-bound oxygenic ligand. However, it was recently proposed that the major structural form of the S3 state of higher plants lacks this extra ligand, and that the resulting S4 state would feature instead a penta-coordinate dangler Mn(V)=oxo, covalently linked to a “closed-cubane” Mn3CaO4 cluster. For this proposal, we explore here a large number of possible pathways of O–O bond formation and demonstrate that the “nucleophilic oxo–oxo coupling” (NOOC) between Mn(V)=oxo and μ3-oxo is the only eligible mechanism in such a system. The reaction is facilitated by a specific conformation of the cluster and concomitant water binding, which is delayed compared to the RC mechanism. An energetically feasible process is described starting from the valid S4 state through the sequential formation of peroxide and superoxide, followed by O2 release and a second water insertion. The newly found mechanism is consistent with available experimental thermodynamic and kinetic data and thus a viable alternative pathway for O2 formation in natural photosynthesis, in particular for higher plants.

@article{simon_capturing_2023,
	title = {Capturing the sequence of events during the water oxidation reaction in photosynthesis using {XFELs}},
	volume = {597},
	copyright = {© 2022 Federation of European Biochemical Societies.},
	issn = {1873-3468},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/1873-3468.14527},
	doi = {10.1002/1873-3468.14527},
	abstract = {Ever since the discovery that Mn was required for oxygen evolution in plants by Pirson in 1937 and the period-four oscillation in flash-induced oxygen evolution by Joliot and Kok in the 1970s, understanding of this process has advanced enormously using state-of-the-art methods. The most recent in this series of innovative techniques was the introduction of X-ray free-electron lasers (XFELs) a decade ago, which led to another quantum leap in the understanding in this field, by enabling operando X-ray structural and X-ray spectroscopy studies at room temperature. This review summarizes the current understanding of the structure of Photosystem II (PS II) and its catalytic centre, the Mn4CaO5 complex, in the intermediate Si (i = 0–4)-states of the Kok cycle, obtained using XFELs.},
	language = {en},
	number = {1},
	urldate = {2024-10-16},
	journal = {FEBS Letters},
	author = {Simon, Philipp S. and Makita, Hiroki and Bogacz, Isabel and Fuller, Franklin and Bhowmick, Asmit and Hussein, Rana and Ibrahim, Mohamed and Zhang, Miao and Chatterjee, Ruchira and Cheah, Mun Hon and Chernev, Petko and Doyle, Margaret D. and Brewster, Aaron S. and Alonso-Mori, Roberto and Sauter, Nicholas K. and Bergmann, Uwe and Dobbek, Holger and Zouni, Athina and Messinger, Johannes and Kern, Jan and Yachandra, Vittal K. and Yano, Junko},
	year = {2023},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/1873-3468.14527},
	keywords = {X-ray free-electron laser, X-ray spectroscopy, manganese metalloenzymes, oxygen evolving complex, photosystem II, water-oxidation/splitting},
	pages = {30--37},
}

Ever since the discovery that Mn was required for oxygen evolution in plants by Pirson in 1937 and the period-four oscillation in flash-induced oxygen evolution by Joliot and Kok in the 1970s, understanding of this process has advanced enormously using state-of-the-art methods. The most recent in this series of innovative techniques was the introduction of X-ray free-electron lasers (XFELs) a decade ago, which led to another quantum leap in the understanding in this field, by enabling operando X-ray structural and X-ray spectroscopy studies at room temperature. This review summarizes the current understanding of the structure of Photosystem II (PS II) and its catalytic centre, the Mn4CaO5 complex, in the intermediate Si (i = 0–4)-states of the Kok cycle, obtained using XFELs.

@article{hussein_evolutionary_2023,
	title = {Evolutionary diversity of proton and water channels on the oxidizing side of photosystem {II} and their relevance to function},
	volume = {158},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-023-01018-w},
	doi = {10.1007/s11120-023-01018-w},
	abstract = {One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the ‘water wheel’ to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624–12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proton release. The comparison does not eliminate the potential role of O4 channel as a water intake channel. However, the highly ordered hydrogen-bonded water wire connected to the Mn4CaO5 cluster via the O4 may strongly suggest that it functions in proton release, especially during the S0 → S1 transition (Saito et al., Nat Commun 6:8488, 2015; Kern et al., Nature 563:421–425, 2018; Ibrahim et al., Proc Natl Acad Sci USA 117:12624–12635, 2020; Sakashita et al., Phys Chem Chem Phys 22:15831–15841, 2020; Hussein et al., Nat Commun 12:6531, 2021).},
	language = {en},
	number = {2},
	urldate = {2024-10-16},
	journal = {Photosynthesis Research},
	author = {Hussein, Rana and Ibrahim, Mohamed and Bhowmick, Asmit and Simon, Philipp S. and Bogacz, Isabel and Doyle, Margaret D. and Dobbek, Holger and Zouni, Athina and Messinger, Johannes and Yachandra, Vittal K. and Kern, Jan F. and Yano, Junko},
	month = nov,
	year = {2023},
	keywords = {Evolution, Oxygen evolving complex, Photosystem II, Water oxidation, Water transport},
	pages = {91--107},
}

One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the ‘water wheel’ to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624–12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proton release. The comparison does not eliminate the potential role of O4 channel as a water intake channel. However, the highly ordered hydrogen-bonded water wire connected to the Mn4CaO5 cluster via the O4 may strongly suggest that it functions in proton release, especially during the S0 → S1 transition (Saito et al., Nat Commun 6:8488, 2015; Kern et al., Nature 563:421–425, 2018; Ibrahim et al., Proc Natl Acad Sci USA 117:12624–12635, 2020; Sakashita et al., Phys Chem Chem Phys 22:15831–15841, 2020; Hussein et al., Nat Commun 12:6531, 2021).

@article{bag_flavodiiron-mediated_2023,
	title = {Flavodiiron-mediated {O2} photoreduction at photosystem {I} acceptor-side provides photoprotection to conifer thylakoids in early spring},
	volume = {14},
	copyright = {2023 The Author(s)},
	issn = {2041-1723},
	url = {https://www.nature.com/articles/s41467-023-38938-z},
	doi = {10.1038/s41467-023-38938-z},
	abstract = {Green organisms evolve oxygen (O2) via photosynthesis and consume it by respiration. Generally, net O2 consumption only becomes dominant when photosynthesis is suppressed at night. Here, we show that green thylakoid membranes of Scots pine (Pinus sylvestris L) and Norway spruce (Picea abies) needles display strong O2 consumption even in the presence of light when extremely low temperatures coincide with high solar irradiation during early spring (ES). By employing different electron transport chain inhibitors, we show that this unusual light-induced O2 consumption occurs around photosystem (PS) I and correlates with higher abundance of flavodiiron (Flv) A protein in ES thylakoids. With P700 absorption changes, we demonstrate that electron scavenging from the acceptor-side of PSI via O2 photoreduction is a major alternative pathway in ES. This photoprotection mechanism in vascular plants indicates that conifers have developed an adaptative evolution trajectory for growing in harsh environments.},
	language = {en},
	number = {1},
	urldate = {2023-06-09},
	journal = {Nature Communications},
	publisher = {Nature Publishing Group},
	author = {Bag, Pushan and Shutova, Tatyana and Shevela, Dmitry and Lihavainen, Jenna and Nanda, Sanchali and Ivanov, Alexander G. and Messinger, Johannes and Jansson, Stefan},
	month = jun,
	year = {2023},
	note = {Number: 1},
	keywords = {Abiotic, Light responses, Photosystem I},
	pages = {3210},
}

Green organisms evolve oxygen (O2) via photosynthesis and consume it by respiration. Generally, net O2 consumption only becomes dominant when photosynthesis is suppressed at night. Here, we show that green thylakoid membranes of Scots pine (Pinus sylvestris L) and Norway spruce (Picea abies) needles display strong O2 consumption even in the presence of light when extremely low temperatures coincide with high solar irradiation during early spring (ES). By employing different electron transport chain inhibitors, we show that this unusual light-induced O2 consumption occurs around photosystem (PS) I and correlates with higher abundance of flavodiiron (Flv) A protein in ES thylakoids. With P700 absorption changes, we demonstrate that electron scavenging from the acceptor-side of PSI via O2 photoreduction is a major alternative pathway in ES. This photoprotection mechanism in vascular plants indicates that conifers have developed an adaptative evolution trajectory for growing in harsh environments.

@article{bhowmick_going_2023,
	title = {Going around the {Kok} cycle of the water oxidation reaction with femtosecond {X}-ray crystallography},
	volume = {10},
	copyright = {https://creativecommons.org/licenses/by/4.0/},
	issn = {2052-2525},
	url = {https://journals.iucr.org/m/issues/2023/06/00/it5029/},
	doi = {10.1107/S2052252523008928},
	abstract = {The water oxidation reaction in photosystem II (PS II) produces most of the molecular oxygen in the atmosphere, which sustains life on Earth, and in this process releases four electrons and four protons that drive the downstream process of CO2 fixation in the photosynthetic apparatus. The catalytic center of PS II is an oxygen-bridged Mn4Ca complex (Mn4CaO5) which is progressively oxidized upon the absorption of light by the chloro­phyll of the PS II reaction center, and the accumulation of four oxidative equivalents in the catalytic center results in the oxidation of two waters to di­oxy­gen in the last step. The recent emergence of X-ray free-electron lasers (XFELs) with intense femtosecond X-ray pulses has opened up opportunities to visualize this reaction in PS II as it proceeds through the catalytic cycle. In this review, we summarize our recent studies of the catalytic reaction in PS II by following the structural changes along the reaction pathway via room-temperature X-ray crystallography using XFELs. The evolution of the electron density changes at the Mn complex reveals notable structural changes, including the insertion of OX from a new water molecule, which disappears on completion of the reaction, implicating it in the O—O bond formation reaction. We were also able to follow the structural dynamics of the protein coordinating with the catalytic complex and of channels within the protein that are important for substrate and product transport, revealing well orchestrated conformational changes in response to the electronic changes at the Mn4Ca cluster.},
	language = {en},
	number = {6},
	urldate = {2024-10-16},
	journal = {IUCrJ},
	publisher = {International Union of Crystallography},
	author = {Bhowmick, A. and Simon, P. S. and Bogacz, I. and Hussein, R. and Zhang, M. and Makita, H. and Ibrahim, M. and Chatterjee, R. and Doyle, M. D. and Cheah, M. H. and Chernev, P. and Fuller, F. D. and Fransson, T. and Alonso-Mori, R. and Brewster, A. S. and Sauter, N. K. and Bergmann, U. and Dobbek, H. and Zouni, A. and Messinger, J. and Kern, J. and Yachandra, V. K. and Yano, J.},
	month = nov,
	year = {2023},
	note = {Number: 6},
	pages = {642--655},
}

The water oxidation reaction in photosystem II (PS II) produces most of the molecular oxygen in the atmosphere, which sustains life on Earth, and in this process releases four electrons and four protons that drive the downstream process of CO2 fixation in the photosynthetic apparatus. The catalytic center of PS II is an oxygen-bridged Mn4Ca complex (Mn4CaO5) which is progressively oxidized upon the absorption of light by the chloro­phyll of the PS II reaction center, and the accumulation of four oxidative equivalents in the catalytic center results in the oxidation of two waters to di­oxy­gen in the last step. The recent emergence of X-ray free-electron lasers (XFELs) with intense femtosecond X-ray pulses has opened up opportunities to visualize this reaction in PS II as it proceeds through the catalytic cycle. In this review, we summarize our recent studies of the catalytic reaction in PS II by following the structural changes along the reaction pathway via room-temperature X-ray crystallography using XFELs. The evolution of the electron density changes at the Mn complex reveals notable structural changes, including the insertion of OX from a new water molecule, which disappears on completion of the reaction, implicating it in the O—O bond formation reaction. We were also able to follow the structural dynamics of the protein coordinating with the catalytic complex and of channels within the protein that are important for substrate and product transport, revealing well orchestrated conformational changes in response to the electronic changes at the Mn4Ca cluster.

@article{shevela_solar_2023,
	title = {Solar energy conversion by photosystem {II}: principles and structures},
	volume = {156},
	issn = {1573-5079},
	shorttitle = {Solar energy conversion by photosystem {II}},
	url = {https://doi.org/10.1007/s11120-022-00991-y},
	doi = {10.1007/s11120-022-00991-y},
	abstract = {Photosynthetic water oxidation by Photosystem II (PSII) is a fascinating process because it sustains life on Earth and serves as a blue print for scalable synthetic catalysts required for renewable energy applications. The biophysical, computational, and structural description of this process, which started more than 50 years ago, has made tremendous progress over the past two decades, with its high-resolution crystal structures being available not only of the dark-stable state of PSII, but of all the semi-stable reaction intermediates and even some transient states. Here, we summarize the current knowledge on PSII with emphasis on the basic principles that govern the conversion of light energy to chemical energy in PSII, as well as on the illustration of the molecular structures that enable these reactions. The important remaining questions regarding the mechanism of biological water oxidation are highlighted, and one possible pathway for this fundamental reaction is described at a molecular level.},
	language = {en},
	number = {3},
	urldate = {2024-10-16},
	journal = {Photosynthesis Research},
	author = {Shevela, Dmitry and Kern, Jan F. and Govindjee, Govindjee and Messinger, Johannes},
	month = jun,
	year = {2023},
	keywords = {Educational review, Function of Photosystem II, Mechanism of water oxidation, Oxygen evolution, Photosynthesis, Primary photochemistry},
	pages = {279--307},
}

Photosynthetic water oxidation by Photosystem II (PSII) is a fascinating process because it sustains life on Earth and serves as a blue print for scalable synthetic catalysts required for renewable energy applications. The biophysical, computational, and structural description of this process, which started more than 50 years ago, has made tremendous progress over the past two decades, with its high-resolution crystal structures being available not only of the dark-stable state of PSII, but of all the semi-stable reaction intermediates and even some transient states. Here, we summarize the current knowledge on PSII with emphasis on the basic principles that govern the conversion of light energy to chemical energy in PSII, as well as on the illustration of the molecular structures that enable these reactions. The important remaining questions regarding the mechanism of biological water oxidation are highlighted, and one possible pathway for this fundamental reaction is described at a molecular level.

@article{bhowmick_structural_2023,
	title = {Structural evidence for intermediates during {O2} formation in photosystem {II}},
	volume = {617},
	copyright = {2023 The Author(s)},
	issn = {1476-4687},
	url = {https://www.nature.com/articles/s41586-023-06038-z},
	doi = {10.1038/s41586-023-06038-z},
	abstract = {In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn4CaO5 cluster first stores four oxidizing equivalents, the S0 to S4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry1–3. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S3→[S4]→S0 transition where O2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn4CaO5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom Ox, which was introduced as a bridging ligand between Ca and Mn1 during the S2→S3 transition4–6, disappears or relocates in parallel with Yz reduction starting at approximately 700 μs after the third flash. The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.},
	language = {en},
	number = {7961},
	urldate = {2024-10-16},
	journal = {Nature},
	publisher = {Nature Publishing Group},
	author = {Bhowmick, Asmit and Hussein, Rana and Bogacz, Isabel and Simon, Philipp S. and Ibrahim, Mohamed and Chatterjee, Ruchira and Doyle, Margaret D. and Cheah, Mun Hon and Fransson, Thomas and Chernev, Petko and Kim, In-Sik and Makita, Hiroki and Dasgupta, Medhanjali and Kaminsky, Corey J. and Zhang, Miao and Gätcke, Julia and Haupt, Stephanie and Nangca, Isabela I. and Keable, Stephen M. and Aydin, A. Orkun and Tono, Kensuke and Owada, Shigeki and Gee, Leland B. and Fuller, Franklin D. and Batyuk, Alexander and Alonso-Mori, Roberto and Holton, James M. and Paley, Daniel W. and Moriarty, Nigel W. and Mamedov, Fikret and Adams, Paul D. and Brewster, Aaron S. and Dobbek, Holger and Sauter, Nicholas K. and Bergmann, Uwe and Zouni, Athina and Messinger, Johannes and Kern, Jan and Yano, Junko and Yachandra, Vittal K.},
	month = may,
	year = {2023},
	keywords = {Bioenergetics, Nanocrystallography},
	pages = {629--636},
}

In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn4CaO5 cluster first stores four oxidizing equivalents, the S0 to S4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry1–3. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S3→[S4]→S0 transition where O2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn4CaO5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom Ox, which was introduced as a bridging ligand between Ca and Mn1 during the S2→S3 transition4–6, disappears or relocates in parallel with Yz reduction starting at approximately 700 μs after the third flash. The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.

@article{han_molecular_2022,
	title = {Molecular basis for turnover inefficiencies (misses) during water oxidation in photosystem {II}},
	volume = {13},
	issn = {2041-6539},
	url = {https://pubs.rsc.org/en/content/articlelanding/2022/sc/d2sc00854h},
	doi = {10.1039/D2SC00854H},
	abstract = {Photosynthesis stores solar light as chemical energy and efficiency of this process is highly important. The electrons required for CO2 reduction are extracted from water in a reaction driven by light-induced charge separations in the Photosystem II reaction center and catalyzed by the CaMn4O5-cluster. This cyclic process involves five redox intermediates known as the S0–S4 states. In this study, we quantify the flash-induced turnover efficiency of each S state by electron paramagnetic resonance spectroscopy. Measurements were performed in photosystem II membrane preparations from spinach in the presence of an exogenous electron acceptor at selected temperatures between −10 °C and +20 °C and at flash frequencies of 1.25, 5 and 10 Hz. The results show that at optimal conditions the turnover efficiencies are limited by reactions occurring in the water oxidizing complex, allowing the extraction of their S state dependence and correlating low efficiencies to structural changes and chemical events during the reaction cycle. At temperatures 10 °C and below, the highest efficiency (i.e. lowest miss parameter) was found for the S1 → S2 transition, while the S2 → S3 transition was least efficient (highest miss parameter) over the whole temperature range. These electron paramagnetic resonance results were confirmed by measurements of flash-induced oxygen release patterns in thylakoid membranes and are explained on the basis of S state dependent structural changes at the CaMn4O5-cluster that were determined recently by femtosecond X-ray crystallography. Thereby, possible “molecular errors” connected to the e− transfer, H+ transfer, H2O binding and O2 release are identified.},
	language = {en},
	number = {29},
	urldate = {2024-10-16},
	journal = {Chemical Science},
	publisher = {The Royal Society of Chemistry},
	author = {Han, Guangye and Chernev, Petko and Styring, Stenbjörn and Messinger, Johannes and Mamedov, Fikret},
	month = jul,
	year = {2022},
	pages = {8667--8678},
}

Photosynthesis stores solar light as chemical energy and efficiency of this process is highly important. The electrons required for CO2 reduction are extracted from water in a reaction driven by light-induced charge separations in the Photosystem II reaction center and catalyzed by the CaMn4O5-cluster. This cyclic process involves five redox intermediates known as the S0–S4 states. In this study, we quantify the flash-induced turnover efficiency of each S state by electron paramagnetic resonance spectroscopy. Measurements were performed in photosystem II membrane preparations from spinach in the presence of an exogenous electron acceptor at selected temperatures between −10 °C and +20 °C and at flash frequencies of 1.25, 5 and 10 Hz. The results show that at optimal conditions the turnover efficiencies are limited by reactions occurring in the water oxidizing complex, allowing the extraction of their S state dependence and correlating low efficiencies to structural changes and chemical events during the reaction cycle. At temperatures 10 °C and below, the highest efficiency (i.e. lowest miss parameter) was found for the S1 → S2 transition, while the S2 → S3 transition was least efficient (highest miss parameter) over the whole temperature range. These electron paramagnetic resonance results were confirmed by measurements of flash-induced oxygen release patterns in thylakoid membranes and are explained on the basis of S state dependent structural changes at the CaMn4O5-cluster that were determined recently by femtosecond X-ray crystallography. Thereby, possible “molecular errors” connected to the e− transfer, H+ transfer, H2O binding and O2 release are identified.

@article{guo_reversible_2022,
	title = {Reversible {Structural} {Isomerization} of {Nature}’s {Water} {Oxidation} {Catalyst} {Prior} to {O}–{O} {Bond} {Formation}},
	volume = {144},
	issn = {0002-7863},
	url = {https://doi.org/10.1021/jacs.2c03528},
	doi = {10.1021/jacs.2c03528},
	abstract = {Photosynthetic water oxidation is catalyzed by a manganese–calcium oxide cluster, which experiences five “S-states” during a light-driven reaction cycle. The unique “distorted chair”-like geometry of the Mn4CaO5(6) cluster shows structural flexibility that has been frequently proposed to involve “open” and “closed”-cubane forms from the S1 to S3 states. The isomers are interconvertible in the S1 and S2 states, while in the S3 state, the open-cubane structure is observed to dominate inThermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S3+Yz state to the S3nYz• → S4+Yz step, and report for the first time that the reversible isomerism, which is suppressed in the S3+Yz state, is fully recovered in the ensuing S3nYz• state due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese–ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O2 formation, and exert a significant influence on the water oxidation mechanism in photosystem II.},
	number = {26},
	urldate = {2024-10-16},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Guo, Yu and Messinger, Johannes and Kloo, Lars and Sun, Licheng},
	month = jul,
	year = {2022},
	pages = {11736--11747},
}

Photosynthetic water oxidation is catalyzed by a manganese–calcium oxide cluster, which experiences five “S-states” during a light-driven reaction cycle. The unique “distorted chair”-like geometry of the Mn4CaO5(6) cluster shows structural flexibility that has been frequently proposed to involve “open” and “closed”-cubane forms from the S1 to S3 states. The isomers are interconvertible in the S1 and S2 states, while in the S3 state, the open-cubane structure is observed to dominate inThermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S3+Yz state to the S3nYz• → S4+Yz step, and report for the first time that the reversible isomerism, which is suppressed in the S3+Yz state, is fully recovered in the ensuing S3nYz• state due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese–ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O2 formation, and exert a significant influence on the water oxidation mechanism in photosystem II.

@article{damario_towards_2022,
	title = {Towards time resolved characterization of electrochemical reactions: electrochemically-induced {Raman} spectroscopy},
	volume = {13},
	issn = {2041-6539},
	shorttitle = {Towards time resolved characterization of electrochemical reactions},
	url = {https://pubs.rsc.org/en/content/articlelanding/2022/sc/d2sc01967a},
	doi = {10.1039/D2SC01967A},
	abstract = {Structural characterization of transient electrochemical species in the sub-millisecond time scale is the all-time wish of any electrochemist. Presently, common time resolution of structural spectro-electrochemical methods is about 0.1 seconds. Herein, a transient spectro-electrochemical Raman setup of easy implementation is described which allows sub-ms time resolution. The technique studies electrochemical processes by initiating the reaction with an electric potential (or current) pulse and analyses the product with a synchronized laser pulse of the modified Raman spectrometer. The approach was validated by studying a known redox driven isomerization of a Ru-based molecular switch grafted, as monolayer, on a SERS active Au microelectrode. Density-functional-theory calculations confirmed the spectral assignments to sub-ms transient species. This study paves the way to a new generation of time-resolved spectro-electrochemical techniques which will be of fundamental help in the development of next generation electrolizers, fuel cells and batteries.},
	language = {en},
	number = {36},
	urldate = {2024-10-16},
	journal = {Chemical Science},
	publisher = {The Royal Society of Chemistry},
	author = {D'Amario, Luca and Stella, Maria Bruna and Edvinsson, Tomas and Persico, Maurizio and Messinger, Johannes and Dau, Holger},
	month = sep,
	year = {2022},
	pages = {10734--10742},
}

Structural characterization of transient electrochemical species in the sub-millisecond time scale is the all-time wish of any electrochemist. Presently, common time resolution of structural spectro-electrochemical methods is about 0.1 seconds. Herein, a transient spectro-electrochemical Raman setup of easy implementation is described which allows sub-ms time resolution. The technique studies electrochemical processes by initiating the reaction with an electric potential (or current) pulse and analyses the product with a synchronized laser pulse of the modified Raman spectrometer. The approach was validated by studying a known redox driven isomerization of a Ru-based molecular switch grafted, as monolayer, on a SERS active Au microelectrode. Density-functional-theory calculations confirmed the spectral assignments to sub-ms transient species. This study paves the way to a new generation of time-resolved spectro-electrochemical techniques which will be of fundamental help in the development of next generation electrolizers, fuel cells and batteries.

@article{boniolo_water_2022,
	title = {Water {Oxidation} by {Pentapyridyl} {Base} {Metal} {Complexes}? {A} {Case} {Study}},
	volume = {61},
	issn = {0020-1669},
	shorttitle = {Water {Oxidation} by {Pentapyridyl} {Base} {Metal} {Complexes}?},
	url = {https://doi.org/10.1021/acs.inorgchem.2c00631},
	doi = {10.1021/acs.inorgchem.2c00631},
	abstract = {The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.},
	number = {24},
	urldate = {2024-10-16},
	journal = {Inorganic Chemistry},
	publisher = {American Chemical Society},
	author = {Boniolo, Manuel and Hossain, Md Kamal and Chernev, Petko and Suremann, Nina F. and Heizmann, Philipp A. and Lyvik, Amanda S.L. and Beyer, Paul and Haumann, Michael and Huang, Ping and Salhi, Nessima and Cheah, Mun Hon and Shylin, Sergii I. and Lundberg, Marcus and Thapper, Anders and Messinger, Johannes},
	month = jun,
	year = {2022},
	pages = {9104--9118},
}

The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.

@article{fransson_effects_2021,
	title = {Effects of x-ray free-electron laser pulse intensity on the {Mn} {Kβ1},3 x-ray emission spectrum in photosystem {II}—{A} case study for metalloprotein crystals and solutions},
	volume = {8},
	issn = {2329-7778},
	url = {https://doi.org/10.1063/4.0000130},
	doi = {10.1063/4.0000130},
	abstract = {In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and Kβ x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the Kβ XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn Kβ1,3 XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from ∼5 × 1015 to 5 × 1017 W/cm2 at a pulse length of ∼35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.},
	number = {6},
	urldate = {2024-11-25},
	journal = {Structural Dynamics},
	author = {Fransson, Thomas and Alonso-Mori, Roberto and Chatterjee, Ruchira and Cheah, Mun Hon and Ibrahim, Mohamed and Hussein, Rana and Zhang, Miao and Fuller, Franklin and Gul, Sheraz and Kim, In-Sik and Simon, Philipp S. and Bogacz, Isabel and Makita, Hiroki and de Lichtenberg, Casper and Song, Sanghoon and Batyuk, Alexander and Sokaras, Dimosthenis and Massad, Ramzi and Doyle, Margaret and Britz, Alexander and Weninger, Clemens and Zouni, Athina and Messinger, Johannes and Yachandra, Vittal K. and Yano, Junko and Kern, Jan and Bergmann, Uwe},
	month = nov,
	year = {2021},
	pages = {064302},
}

In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and Kβ x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the Kβ XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn Kβ1,3 XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from ∼5 × 1015 to 5 × 1017 W/cm2 at a pulse length of ∼35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.

@article{kwong_electrochemical_2021,
	series = {Sustainable {Energy} and {Environmental} {Protection}},
	title = {Electrochemical {N2} reduction at ambient condition – {Overcoming} the selectivity issue via control of reactants’ availabilities},
	volume = {46},
	issn = {0360-3199},
	url = {https://www.sciencedirect.com/science/article/pii/S0360319921024678},
	doi = {10.1016/j.ijhydene.2021.06.184},
	abstract = {Ammonia production via the electrochemical N2 reduction reaction (NRR) at ambient conditions is highly desired as an alternative to the Haber-Bosch process, but remains a great challenge due to the low efficiency and selectivity caused by the competing hydrogen evolution reaction (HER). Herein we investigate the effect of availabilities of reactants (protons, electrons and N2) on NRR using a FeOx-coated carbon fiber paper cathode in various electrochemical configurations. NRR is found viable only under the conditions of low proton- and high N2 availabilities, which are achieved using 0.12 vol\% water in LiClO4-ethyl acetate electrolyte and gaseous N2 supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 ± 19 pmolNH3 s−1 cm−2 at a Faradaic efficiency of 70 ± 24\% at the applied potential of −0.1 V vs. NHE. Other conditions (high proton-, or low N2-availability, or both) yield a lower or negligible amount of ammonia due to the competing HER. Our work shows that promoting NRR by suppressing the HER requires optimization of the operational variables, which serves as a complementary strategy to the development of NRR catalysts.},
	number = {59},
	urldate = {2024-10-16},
	journal = {International Journal of Hydrogen Energy},
	author = {Kwong, Wai Ling and Wågberg, Thomas and Messinger, Johannes},
	month = aug,
	year = {2021},
	keywords = {Ammonia, Electrocatalysis, H evolution reaction, Haber-Bosch method, N reduction reaction},
	pages = {30366--30372},
}

Ammonia production via the electrochemical N2 reduction reaction (NRR) at ambient conditions is highly desired as an alternative to the Haber-Bosch process, but remains a great challenge due to the low efficiency and selectivity caused by the competing hydrogen evolution reaction (HER). Herein we investigate the effect of availabilities of reactants (protons, electrons and N2) on NRR using a FeOx-coated carbon fiber paper cathode in various electrochemical configurations. NRR is found viable only under the conditions of low proton- and high N2 availabilities, which are achieved using 0.12 vol% water in LiClO4-ethyl acetate electrolyte and gaseous N2 supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 ± 19 pmolNH3 s−1 cm−2 at a Faradaic efficiency of 70 ± 24% at the applied potential of −0.1 V vs. NHE. Other conditions (high proton-, or low N2-availability, or both) yield a lower or negligible amount of ammonia due to the competing HER. Our work shows that promoting NRR by suppressing the HER requires optimization of the operational variables, which serves as a complementary strategy to the development of NRR catalysts.

@article{boniolo_electronic_2021,
	title = {Electronic and geometric structure effects on one-electron oxidation of first-row transition metals in the same ligand framework},
	volume = {50},
	issn = {1477-9234},
	url = {https://pubs.rsc.org/en/content/articlelanding/2021/dt/d0dt03695a},
	doi = {10.1039/D0DT03695A},
	abstract = {Developing new transition metal catalysts requires understanding of how both metal and ligand properties determine reactivity. Since metal complexes bearing ligands of the Py5 family (2,6-bis-[(2-pyridyl)methyl]pyridine) have been employed in many fields in the past 20 years, we set out here to understand their redox properties by studying a series of base metal ions (M = Mn, Fe, Co, and Ni) within the Py5OH (pyridine-2,6-diylbis[di-(pyridin-2-yl)methanol]) variant. Both reduced (MII) and the one-electron oxidized (MIII) species were carefully characterized using a combination of X-ray crystallography, X-ray absorption spectroscopy, cyclic voltammetry, and density-functional theory calculations. The observed metal–ligand interactions and electrochemical properties do not always follow consistent trends along the periodic table. We demonstrate that this observation cannot be explained by only considering orbital and geometric relaxation, and that spin multiplicity changes needed to be included into the DFT calculations to reproduce and understand these trends. In addition, exchange reactions of the sixth ligand coordinated to the metal, were analysed. Finally, by including published data of the extensively characterised Py5OMe (pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane])complexes, the special characteristics of the less common Py5OH ligand were extracted. This comparison highlights the non-innocent effect of the distal OH functionalization on the geometry, and consequently on the electronic structure of the metal complexes. Together, this gives a complete analysis of metal and ligand degrees of freedom for these base metal complexes, while also providing general insights into how to control electrochemical processes of transition metal complexes.},
	language = {en},
	number = {2},
	urldate = {2024-10-16},
	journal = {Dalton Transactions},
	publisher = {The Royal Society of Chemistry},
	author = {Boniolo, Manuel and Chernev, Petko and Cheah, Mun Hon and Heizmann, Philipp A. and Huang, Ping and Shylin, Sergii I. and Salhi, Nessima and Hossain, Md Kamal and Gupta, Arvind K. and Messinger, Johannes and Thapper, Anders and Lundberg, Marcus},
	month = jan,
	year = {2021},
	pages = {660--674},
}

Developing new transition metal catalysts requires understanding of how both metal and ligand properties determine reactivity. Since metal complexes bearing ligands of the Py5 family (2,6-bis-[(2-pyridyl)methyl]pyridine) have been employed in many fields in the past 20 years, we set out here to understand their redox properties by studying a series of base metal ions (M = Mn, Fe, Co, and Ni) within the Py5OH (pyridine-2,6-diylbis[di-(pyridin-2-yl)methanol]) variant. Both reduced (MII) and the one-electron oxidized (MIII) species were carefully characterized using a combination of X-ray crystallography, X-ray absorption spectroscopy, cyclic voltammetry, and density-functional theory calculations. The observed metal–ligand interactions and electrochemical properties do not always follow consistent trends along the periodic table. We demonstrate that this observation cannot be explained by only considering orbital and geometric relaxation, and that spin multiplicity changes needed to be included into the DFT calculations to reproduce and understand these trends. In addition, exchange reactions of the sixth ligand coordinated to the metal, were analysed. Finally, by including published data of the extensively characterised Py5OMe (pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane])complexes, the special characteristics of the less common Py5OH ligand were extracted. This comparison highlights the non-innocent effect of the distal OH functionalization on the geometry, and consequently on the electronic structure of the metal complexes. Together, this gives a complete analysis of metal and ligand degrees of freedom for these base metal complexes, while also providing general insights into how to control electrochemical processes of transition metal complexes.

@incollection{shevela_photosystem_2021,
	title = {Photosystem {II}},
	copyright = {Copyright © 2021 John Wiley \& Sons, Ltd.},
	isbn = {978-0-470-01590-2},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470015902.a0029372},
	doi = {10.1002/9780470015902.a0029372},
	abstract = {Photosystem II (PSII) of plants, algae and cyanobacteria is a specialised protein complex that uses light energy to transfer electrons from water to plastoquinone, producing molecular oxygen and reduced plastoquinone. The PSII complex includes a peripheral antenna containing chlorophyll and other pigments to absorb light, a reaction centre that utilises the excitation energy transferred to it for charge separation, cofactors that stabilise the charge pair via electron transfer reactions, a Mn4CaO5 cluster that oxidises water, and a binding pocket where plastoquinone is reduced. The electrons and protons that PSII extracts from water are employed in the overall photosynthetic process for the reduction of CO2, which provides the chemical energy for most life on Earth. PSII is the only known biological source of O2 produced from water and is responsible for the molecular oxygen in the atmosphere. Key Concepts Photosystem II (PSII) is a membrane-embedded pigment–protein complex, containing more than 20 subunits and approximately 100 cofactors. The PSII antenna and the PSII reaction centre are distinct protein complexes. Light is absorbed by chlorophyll, carotenoid and phycobilin pigments in the antenna and the excitation energy is rapidly transferred to the reaction centre. At the reaction centre, light-induced charge separation takes place resulting in the formation of a chlorophyll cation and a pheophytin anion that are approximately 10 Å apart; this charge separation is rapidly stabilised by the transfer of the charges to more distant cofactors with smaller differences in redox potentials. The oxidation of water occurs at the Mn4CaO5 cluster, which is embedded in the two protein subunits D1 and CP43 on the luminal side of PSII. To oxidise two molecules of water, four oxidising equivalents must be accumulated in the Mn4CaO5 cluster by four consecutive light-induced charge separations. Water oxidation by PSII occurs at the Mn4CaO5 cluster, likely via oxo-oxyl radical coupling in the so-called S4 state. Hydrogen-bonding networks surrounding the Mn4CaO5 cluster are crucial for its catalytic activity, as well as its structural flexibility. Bicarbonate ions play regulatory roles for electron transfer through PSII. The electrons and protons extracted from water by PSII drive the reduction of NADP+ via Photosystem I, and the production of ATP, respectively.},
	language = {en},
	urldate = {2024-10-16},
	booktitle = {{eLS}},
	publisher = {John Wiley \& Sons, Ltd},
	author = {Shevela, Dmitry and Kern, Jan F and Govindjee, Govindjee and Whitmarsh, John and Messinger, Johannes},
	year = {2021},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/9780470015902.a0029372},
	keywords = {chlorophyll, electron transport, oxygen evolution, photosynthesis, primary photochemistry, reaction centre, solar energy conversion, water oxidation},
	pages = {1--16},
}

Photosystem II (PSII) of plants, algae and cyanobacteria is a specialised protein complex that uses light energy to transfer electrons from water to plastoquinone, producing molecular oxygen and reduced plastoquinone. The PSII complex includes a peripheral antenna containing chlorophyll and other pigments to absorb light, a reaction centre that utilises the excitation energy transferred to it for charge separation, cofactors that stabilise the charge pair via electron transfer reactions, a Mn4CaO5 cluster that oxidises water, and a binding pocket where plastoquinone is reduced. The electrons and protons that PSII extracts from water are employed in the overall photosynthetic process for the reduction of CO2, which provides the chemical energy for most life on Earth. PSII is the only known biological source of O2 produced from water and is responsible for the molecular oxygen in the atmosphere. Key Concepts Photosystem II (PSII) is a membrane-embedded pigment–protein complex, containing more than 20 subunits and approximately 100 cofactors. The PSII antenna and the PSII reaction centre are distinct protein complexes. Light is absorbed by chlorophyll, carotenoid and phycobilin pigments in the antenna and the excitation energy is rapidly transferred to the reaction centre. At the reaction centre, light-induced charge separation takes place resulting in the formation of a chlorophyll cation and a pheophytin anion that are approximately 10 Å apart; this charge separation is rapidly stabilised by the transfer of the charges to more distant cofactors with smaller differences in redox potentials. The oxidation of water occurs at the Mn4CaO5 cluster, which is embedded in the two protein subunits D1 and CP43 on the luminal side of PSII. To oxidise two molecules of water, four oxidising equivalents must be accumulated in the Mn4CaO5 cluster by four consecutive light-induced charge separations. Water oxidation by PSII occurs at the Mn4CaO5 cluster, likely via oxo-oxyl radical coupling in the so-called S4 state. Hydrogen-bonding networks surrounding the Mn4CaO5 cluster are crucial for its catalytic activity, as well as its structural flexibility. Bicarbonate ions play regulatory roles for electron transfer through PSII. The electrons and protons extracted from water by PSII drive the reduction of NADP+ via Photosystem I, and the production of ATP, respectively.

@article{ibrahim_reply_2021,
	title = {Reply to {Wang} et al.: {Clear} evidence of binding of {Ox} to the oxygen-evolving complex of photosystem {II} is best observed in the omit map},
	volume = {118},
	shorttitle = {Reply to {Wang} et al.},
	url = {https://www.pnas.org/doi/10.1073/pnas.2102342118},
	doi = {10.1073/pnas.2102342118},
	number = {24},
	urldate = {2024-10-16},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Ibrahim, Mohamed and Moriarty, Nigel W. and Kern, Jan and Holton, James M. and Brewster, Aaron S. and Bhowmick, Asmit and Bergmann, Uwe and Zouni, Athina and Messinger, Johannes and Yachandra, Vittal K. and Yano, Junko and Dobbek, Holger and Sauter, Nicholas K. and Adams, Paul D.},
	month = jun,
	year = {2021},
	pages = {e2102342118},
}

@article{keable_room_2021,
	title = {Room temperature {XFEL} crystallography reveals asymmetry in the vicinity of the two phylloquinones in photosystem {I}},
	volume = {11},
	copyright = {2021 The Author(s)},
	issn = {2045-2322},
	url = {https://www.nature.com/articles/s41598-021-00236-3},
	doi = {10.1038/s41598-021-00236-3},
	abstract = {Photosystem I (PS I) has a symmetric structure with two highly similar branches of pigments at the center that are involved in electron transfer, but shows very different efficiency along the two branches. We have determined the structure of cyanobacterial PS I at room temperature (RT) using femtosecond X-ray pulses from an X-ray free electron laser (XFEL) that shows a clear expansion of the entire protein complex in the direction of the membrane plane, when compared to previous cryogenic structures. This trend was observed by complementary datasets taken at multiple XFEL beamlines. In the RT structure of PS I, we also observe conformational differences between the two branches in the reaction center around the secondary electron acceptors A1A and A1B. The π-stacked Phe residues are rotated with a more parallel orientation in the A-branch and an almost perpendicular confirmation in the B-branch, and the symmetry breaking PsaB-Trp673 is tilted and further away from A1A. These changes increase the asymmetry between the branches and may provide insights into the preferential directionality of electron transfer.},
	language = {en},
	number = {1},
	urldate = {2024-10-16},
	journal = {Scientific Reports},
	publisher = {Nature Publishing Group},
	author = {Keable, Stephen M. and Kölsch, Adrian and Simon, Philipp S. and Dasgupta, Medhanjali and Chatterjee, Ruchira and Subramanian, Senthil Kumar and Hussein, Rana and Ibrahim, Mohamed and Kim, In-Sik and Bogacz, Isabel and Makita, Hiroki and Pham, Cindy C. and Fuller, Franklin D. and Gul, Sheraz and Paley, Daniel and Lassalle, Louise and Sutherlin, Kyle D. and Bhowmick, Asmit and Moriarty, Nigel W. and Young, Iris D. and Blaschke, Johannes P. and de Lichtenberg, Casper and Chernev, Petko and Cheah, Mun Hon and Park, Sehan and Park, Gisu and Kim, Jangwoo and Lee, Sang Jae and Park, Jaehyun and Tono, Kensuke and Owada, Shigeki and Hunter, Mark S. and Batyuk, Alexander and Oggenfuss, Roland and Sander, Mathias and Zerdane, Serhane and Ozerov, Dmitry and Nass, Karol and Lemke, Henrik and Mankowsky, Roman and Brewster, Aaron S. and Messinger, Johannes and Sauter, Nicholas K. and Yachandra, Vittal K. and Yano, Junko and Zouni, Athina and Kern, Jan},
	month = nov,
	year = {2021},
	keywords = {Biochemistry, Bioenergetics, Photosynthesis, Structural biology},
	pages = {21787},
}

Photosystem I (PS I) has a symmetric structure with two highly similar branches of pigments at the center that are involved in electron transfer, but shows very different efficiency along the two branches. We have determined the structure of cyanobacterial PS I at room temperature (RT) using femtosecond X-ray pulses from an X-ray free electron laser (XFEL) that shows a clear expansion of the entire protein complex in the direction of the membrane plane, when compared to previous cryogenic structures. This trend was observed by complementary datasets taken at multiple XFEL beamlines. In the RT structure of PS I, we also observe conformational differences between the two branches in the reaction center around the secondary electron acceptors A1A and A1B. The π-stacked Phe residues are rotated with a more parallel orientation in the A-branch and an almost perpendicular confirmation in the B-branch, and the symmetry breaking PsaB-Trp673 is tilted and further away from A1A. These changes increase the asymmetry between the branches and may provide insights into the preferential directionality of electron transfer.

@article{ekspong_solar-driven_2021,
	title = {Solar-{Driven} {Water} {Splitting} at 13.8\% {Solar}-to-{Hydrogen} {Efficiency} by an {Earth}-{Abundant} {Electrolyzer}},
	volume = {9},
	url = {https://doi.org/10.1021/acssuschemeng.1c03565},
	doi = {10.1021/acssuschemeng.1c03565},
	abstract = {We present the synthesis and characterization of an efficient and low cost solar-driven electrolyzer consisting of Earth-abundant materials. The trimetallic NiFeMo electrocatalyst takes the shape of nanometer-sized flakes anchored to a fully carbon-based current collector comprising a nitrogen-doped carbon nanotube network, which in turn is grown on a carbon fiber paper support. This catalyst electrode contains solely Earth-abundant materials, and the carbon fiber support renders it effective despite a low metal content. Notably, a bifunctional catalyst–electrode pair exhibits a low total overpotential of 450 mV to drive a full water-splitting reaction at a current density of 10 mA cm–2 and a measured hydrogen Faradaic efficiency of ∼100\%. We combine the catalyst–electrode pair with solution-processed perovskite solar cells to form a lightweight solar-driven water-splitting device with a high peak solar-to-fuel conversion efficiency of 13.8\%.},
	number = {42},
	urldate = {2024-10-16},
	journal = {ACS Sustainable Chemistry \& Engineering},
	publisher = {American Chemical Society},
	author = {Ekspong, Joakim and Larsen, Christian and Stenberg, Jonas and Kwong, Wai Ling and Wang, Jia and Zhang, Jinbao and Johansson, Erik M. J. and Messinger, Johannes and Edman, Ludvig and Wågberg, Thomas},
	month = oct,
	year = {2021},
	pages = {14070--14078},
}

We present the synthesis and characterization of an efficient and low cost solar-driven electrolyzer consisting of Earth-abundant materials. The trimetallic NiFeMo electrocatalyst takes the shape of nanometer-sized flakes anchored to a fully carbon-based current collector comprising a nitrogen-doped carbon nanotube network, which in turn is grown on a carbon fiber paper support. This catalyst electrode contains solely Earth-abundant materials, and the carbon fiber support renders it effective despite a low metal content. Notably, a bifunctional catalyst–electrode pair exhibits a low total overpotential of 450 mV to drive a full water-splitting reaction at a current density of 10 mA cm–2 and a measured hydrogen Faradaic efficiency of ∼100%. We combine the catalyst–electrode pair with solution-processed perovskite solar cells to form a lightweight solar-driven water-splitting device with a high peak solar-to-fuel conversion efficiency of 13.8%.

@article{hussein_structural_2021,
	title = {Structural dynamics in the water and proton channels of photosystem {II} during the {S2} to {S3} transition},
	volume = {12},
	copyright = {2021 The Author(s)},
	issn = {2041-1723},
	url = {https://www.nature.com/articles/s41467-021-26781-z},
	doi = {10.1038/s41467-021-26781-z},
	abstract = {Light-driven oxidation of water to molecular oxygen is catalyzed by the oxygen-evolving complex (OEC) in Photosystem II (PS II). This multi-electron, multi-proton catalysis requires the transport of two water molecules to and four protons from the OEC. A high-resolution 1.89 Å structure obtained by averaging all the S states and refining the data of various time points during the S2 to S3 transition has provided better visualization of the potential pathways for substrate water insertion and proton release. Our results indicate that the O1 channel is the likely water intake pathway, and the Cl1 channel is the likely proton release pathway based on the structural rearrangements of water molecules and amino acid side chains along these channels. In particular in the Cl1 channel, we suggest that residue D1-E65 serves as a gate for proton transport by minimizing the back reaction. The results show that the water oxidation reaction at the OEC is well coordinated with the amino acid side chains and the H-bonding network over the entire length of the channels, which is essential in shuttling substrate waters and protons.},
	language = {en},
	number = {1},
	urldate = {2024-10-16},
	journal = {Nature Communications},
	publisher = {Nature Publishing Group},
	author = {Hussein, Rana and Ibrahim, Mohamed and Bhowmick, Asmit and Simon, Philipp S. and Chatterjee, Ruchira and Lassalle, Louise and Doyle, Margaret and Bogacz, Isabel and Kim, In-Sik and Cheah, Mun Hon and Gul, Sheraz and de Lichtenberg, Casper and Chernev, Petko and Pham, Cindy C. and Young, Iris D. and Carbajo, Sergio and Fuller, Franklin D. and Alonso-Mori, Roberto and Batyuk, Alex and Sutherlin, Kyle D. and Brewster, Aaron S. and Bolotovsky, Robert and Mendez, Derek and Holton, James M. and Moriarty, Nigel W. and Adams, Paul D. and Bergmann, Uwe and Sauter, Nicholas K. and Dobbek, Holger and Messinger, Johannes and Zouni, Athina and Kern, Jan and Yachandra, Vittal K. and Yano, Junko},
	month = nov,
	year = {2021},
	keywords = {Bioenergetics, Structural biology, X-ray crystallography},
	pages = {6531},
}

Light-driven oxidation of water to molecular oxygen is catalyzed by the oxygen-evolving complex (OEC) in Photosystem II (PS II). This multi-electron, multi-proton catalysis requires the transport of two water molecules to and four protons from the OEC. A high-resolution 1.89 Å structure obtained by averaging all the S states and refining the data of various time points during the S2 to S3 transition has provided better visualization of the potential pathways for substrate water insertion and proton release. Our results indicate that the O1 channel is the likely water intake pathway, and the Cl1 channel is the likely proton release pathway based on the structural rearrangements of water molecules and amino acid side chains along these channels. In particular in the Cl1 channel, we suggest that residue D1-E65 serves as a gate for proton transport by minimizing the back reaction. The results show that the water oxidation reaction at the OEC is well coordinated with the amino acid side chains and the H-bonding network over the entire length of the channels, which is essential in shuttling substrate waters and protons.

@article{de_lichtenberg_d1-v185n_2021,
	title = {The {D1}-{V185N} mutation alters substrate water exchange by stabilizing alternative structures of the {Mn4Ca}-cluster in photosystem {II}},
	volume = {1862},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272820301699},
	doi = {10.1016/j.bbabio.2020.148319},
	abstract = {In photosynthesis, the oxygen-evolving complex (OEC) of the pigment-protein complex photosystem II (PSII) orchestrates the oxidation of water. Introduction of the V185N mutation into the D1 protein was previously reported to drastically slow O2-release and strongly perturb the water network surrounding the Mn4Ca cluster. Employing time-resolved membrane inlet mass spectrometry, we measured here the H218O/H216O-exchange kinetics of the fast (Wf) and slow (Ws) exchanging substrate waters bound in the S1, S2 and S3 states to the Mn4Ca cluster of PSII core complexes isolated from wild type and D1-V185N strains of Synechocystis sp. PCC 6803. We found that the rate of exchange for Ws was increased in the S1 and S2 states, while both Wf and Ws exchange rates were decreased in the S3 state. Additionally, we used EPR spectroscopy to characterize the Mn4Ca cluster and its interaction with the redox active D1-Tyr161 (YZ). In the S2 state, we observed a greatly diminished multiline signal in the V185N-PSII that could be recovered by addition of ammonia. The split signal in the S1 state was not affected, while the split signal in the S3 state was absent in the D1-V185N mutant. These findings are rationalized by the proposal that the N185 residue stabilizes the binding of an additional water-derived ligand at the Mn1 site of the Mn4Ca cluster via hydrogen bonding. Implications for the sites of substrate water binding are discussed.},
	number = {1},
	urldate = {2024-10-16},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {de Lichtenberg, Casper and Avramov, Anton P. and Zhang, Minquan and Mamedov, Fikret and Burnap, Robert L. and Messinger, Johannes},
	month = jan,
	year = {2021},
	keywords = {EPR, Manganese cluster, O-O bond formation, Photosystem II, Substrate water exchange, Water oxidation},
	pages = {148319},
}

In photosynthesis, the oxygen-evolving complex (OEC) of the pigment-protein complex photosystem II (PSII) orchestrates the oxidation of water. Introduction of the V185N mutation into the D1 protein was previously reported to drastically slow O2-release and strongly perturb the water network surrounding the Mn4Ca cluster. Employing time-resolved membrane inlet mass spectrometry, we measured here the H218O/H216O-exchange kinetics of the fast (Wf) and slow (Ws) exchanging substrate waters bound in the S1, S2 and S3 states to the Mn4Ca cluster of PSII core complexes isolated from wild type and D1-V185N strains of Synechocystis sp. PCC 6803. We found that the rate of exchange for Ws was increased in the S1 and S2 states, while both Wf and Ws exchange rates were decreased in the S3 state. Additionally, we used EPR spectroscopy to characterize the Mn4Ca cluster and its interaction with the redox active D1-Tyr161 (YZ). In the S2 state, we observed a greatly diminished multiline signal in the V185N-PSII that could be recovered by addition of ammonia. The split signal in the S1 state was not affected, while the split signal in the S3 state was absent in the D1-V185N mutant. These findings are rationalized by the proposal that the N185 residue stabilizes the binding of an additional water-derived ligand at the Mn1 site of the Mn4Ca cluster via hydrogen bonding. Implications for the sites of substrate water binding are discussed.

@article{lichtenberg_exchange_2021,
	title = {The exchange of the fast substrate water in the {S2} state of photosystem {II} is limited by diffusion of bulk water through channels – implications for the water oxidation mechanism},
	volume = {12},
	issn = {2041-6539},
	url = {https://pubs.rsc.org/en/content/articlelanding/2021/sc/d1sc02265b},
	doi = {10.1039/D1SC02265B},
	abstract = {The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn4CaO5 cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the ‘slow exchanging substrate water (Ws)’, while, in the S2 state, the terminal water ligands W2 and W3 are both discussed as the ‘fast exchanging substrate water (Wf)’. A critical point for the assignment of Wf is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn4CaO5 cluster. In this study, we measured the rates of H216O/H218O substrate water exchange in the S2 and S3 states of PSII core complexes from wild-type (WT) Synechocystis sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access via the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of Wf and Ws were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn4CaO5 cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that Wf exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S2 state, namely the indifference of the rate of Wf exchange towards Ca/Sr substitution.},
	language = {en},
	number = {38},
	urldate = {2024-10-16},
	journal = {Chemical Science},
	publisher = {The Royal Society of Chemistry},
	author = {Lichtenberg, Casper de and Kim, Christopher J. and Chernev, Petko and Debus, Richard J. and Messinger, Johannes},
	month = oct,
	year = {2021},
	pages = {12763--12775},
}

The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn4CaO5 cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the ‘slow exchanging substrate water (Ws)’, while, in the S2 state, the terminal water ligands W2 and W3 are both discussed as the ‘fast exchanging substrate water (Wf)’. A critical point for the assignment of Wf is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn4CaO5 cluster. In this study, we measured the rates of H216O/H218O substrate water exchange in the S2 and S3 states of PSII core complexes from wild-type (WT) Synechocystis sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access via the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of Wf and Ws were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn4CaO5 cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that Wf exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S2 state, namely the indifference of the rate of Wf exchange towards Ca/Sr substitution.

@article{cheah_assessment_2020,
	title = {Assessment of the manganese cluster’s oxidation state via photoactivation of photosystem {II} microcrystals},
	volume = {117},
	issn = {0027-8424, 1091-6490},
	url = {http://www.pnas.org/lookup/doi/10.1073/pnas.1915879117},
	doi = {10/gjd35c},
	abstract = {Knowledge of the manganese oxidation states of the oxygen-evolving Mn
              4
              CaO
              5
              cluster in photosystem II (PSII) is crucial toward understanding the mechanism of biological water oxidation. There is a 4 decade long debate on this topic that historically originates from the observation of a multiline electron paramagnetic resonance (EPR) signal with effective total spin of S = 1/2 in the singly oxidized S
              2
              state of this cluster. This signal implies an overall oxidation state of either Mn(III)
              3
              Mn(IV) or Mn(III)Mn(IV)
              3
              for the S
              2
              state. These 2 competing assignments are commonly known as “low oxidation (LO)” and “high oxidation (HO)” models of the Mn
              4
              CaO
              5
              cluster. Recent advanced EPR and Mn K-edge X-ray spectroscopy studies converge upon the HO model. However, doubts about these assignments have been voiced, fueled especially by studies counting the number of flash-driven electron removals required for the assembly of an active Mn
              4
              CaO
              5
              cluster starting from Mn(II) and Mn-free PSII. This process, known as photoactivation, appeared to support the LO model since the first oxygen is reported to evolve already after 7 flashes. In this study, we improved the quantum yield and sensitivity of the photoactivation experiment by employing PSII microcrystals that retained all protein subunits after complete manganese removal and by oxygen detection via a custom built thin-layer cell connected to a membrane inlet mass spectrometer. We demonstrate that 9 flashes by a nanosecond laser are required for the production of the first oxygen, which proves that the HO model provides the correct description of the Mn
              4
              CaO
              5
              cluster’s oxidation states.},
	language = {en},
	number = {1},
	urldate = {2021-06-07},
	journal = {Proceedings of the National Academy of Sciences},
	author = {Cheah, Mun Hon and Zhang, Miao and Shevela, Dmitry and Mamedov, Fikret and Zouni, Athina and Messinger, Johannes},
	month = jan,
	year = {2020},
	pages = {141--145},
}

Knowledge of the manganese oxidation states of the oxygen-evolving Mn 4 CaO 5 cluster in photosystem II (PSII) is crucial toward understanding the mechanism of biological water oxidation. There is a 4 decade long debate on this topic that historically originates from the observation of a multiline electron paramagnetic resonance (EPR) signal with effective total spin of S = 1/2 in the singly oxidized S 2 state of this cluster. This signal implies an overall oxidation state of either Mn(III) 3 Mn(IV) or Mn(III)Mn(IV) 3 for the S 2 state. These 2 competing assignments are commonly known as “low oxidation (LO)” and “high oxidation (HO)” models of the Mn 4 CaO 5 cluster. Recent advanced EPR and Mn K-edge X-ray spectroscopy studies converge upon the HO model. However, doubts about these assignments have been voiced, fueled especially by studies counting the number of flash-driven electron removals required for the assembly of an active Mn 4 CaO 5 cluster starting from Mn(II) and Mn-free PSII. This process, known as photoactivation, appeared to support the LO model since the first oxygen is reported to evolve already after 7 flashes. In this study, we improved the quantum yield and sensitivity of the photoactivation experiment by employing PSII microcrystals that retained all protein subunits after complete manganese removal and by oxygen detection via a custom built thin-layer cell connected to a membrane inlet mass spectrometer. We demonstrate that 9 flashes by a nanosecond laser are required for the production of the first oxygen, which proves that the HO model provides the correct description of the Mn 4 CaO 5 cluster’s oxidation states.

@article{shevela_bicarbonate-mediated_2020,
	title = {Bicarbonate-{Mediated} {CO2} {Formation} on {Both} {Sides} of {Photosystem} {II}},
	volume = {59},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/acs.biochem.0c00208},
	doi = {10.1021/acs.biochem.0c00208},
	abstract = {The effect of bicarbonate (HCO3–) on photosystem II (PSII) activity was discovered in the 1950s, but only recently have its molecular mechanisms begun to be clarified. Two chemical mechanisms have been proposed. One is for the electron-donor side, in which mobile HCO3– enhances and possibly regulates water oxidation by acting as proton acceptor, after which it dissociates into CO2 and H2O. The other is for the electron-acceptor side, in which (i) reduction of the QA quinone leads to the release of HCO3– from its binding site on the non-heme iron and (ii) the Em potential of the QA/QA•– couple increases when HCO3– dissociates. This suggested a protective/regulatory role of HCO3– as it is known that increasing the Em of QA decreases the extent of back-reaction-associated photodamage. Here we demonstrate, using plant thylakoids, that time-resolved membrane-inlet mass spectrometry together with 13C isotope labeling of HCO3– allows donor- and acceptor-side formation of CO2 by PSII to be demonstrated and distinguished, which opens the door for future studies of the importance of both mechanisms under in vivo conditions.},
	number = {26},
	urldate = {2024-10-16},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Shevela, Dmitry and Do, Hoang-Nguyen and Fantuzzi, Andrea and Rutherford, A. William and Messinger, Johannes},
	month = jul,
	year = {2020},
	pages = {2442--2449},
}

The effect of bicarbonate (HCO3–) on photosystem II (PSII) activity was discovered in the 1950s, but only recently have its molecular mechanisms begun to be clarified. Two chemical mechanisms have been proposed. One is for the electron-donor side, in which mobile HCO3– enhances and possibly regulates water oxidation by acting as proton acceptor, after which it dissociates into CO2 and H2O. The other is for the electron-acceptor side, in which (i) reduction of the QA quinone leads to the release of HCO3– from its binding site on the non-heme iron and (ii) the Em potential of the QA/QA•– couple increases when HCO3– dissociates. This suggested a protective/regulatory role of HCO3– as it is known that increasing the Em of QA decreases the extent of back-reaction-associated photodamage. Here we demonstrate, using plant thylakoids, that time-resolved membrane-inlet mass spectrometry together with 13C isotope labeling of HCO3– allows donor- and acceptor-side formation of CO2 by PSII to be demonstrated and distinguished, which opens the door for future studies of the importance of both mechanisms under in vivo conditions.

@article{kawde_more_2020,
	title = {More than protection: the function of {TiO} $_{\textrm{2}}$ interlayers in hematite functionalized {Si} photoanodes},
	volume = {22},
	issn = {1463-9076, 1463-9084},
	shorttitle = {More than protection},
	url = {http://xlink.rsc.org/?DOI=D0CP04280C},
	doi = {10/gjdpf7},
	abstract = {Signature of performance-enhancing oxygen vacancies in the mesoporous TiO
              2
               interlayer of a hematite functionalized Si microwire photoanode revealed by hard energy X-ray spectroscopy.
            
          , 
            
              Worldwide significant efforts are ongoing to develop devices that store solar energy as fuels. In one such approach, solar energy is absorbed by semiconductors and utilized directly by catalysts at their surfaces to split water into H
              2
              and O
              2
              . To protect the semiconductors in these photo-electrochemical cells (PEC) from corrosion, frequently thin TiO
              2
              interlayers are applied. Employing a well-performing photoanode comprised of 1-D n-Si microwires (MWs) covered with a mesoporous (mp) TiO
              2
              interlayer fabricated by solution processing and functionalized with α-Fe
              2
              O
              3
              nanorods, we studied here the function of this TiO
              2
              interlayer by high-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy, along with X-ray emission spectroscopy (XES) and standard characterization techniques. Our data reveal that the TiO
              2
              interlayer not only protects the n-Si MW surface from corrosion, but that it also acts as a template for the hydrothermal growth of α-Fe
              2
              O
              3
              nanorods and improves the photocatalytic efficiency. We show that the latter effect correlates with the presence of stable oxygen vacancies at the interface between mp-TiO
              2
              and α-Fe
              2
              O
              3
              , which act as electron traps and thereby substantially reduce the charge recombination rate at the hematite surface.},
	language = {en},
	number = {48},
	urldate = {2021-06-07},
	journal = {Physical Chemistry Chemical Physics},
	author = {Kawde, Anurag and Annamalai, Alagappan and Sellstedt, Anita and Uhlig, Jens and Wågberg, Thomas and Glatzel, Pieter and Messinger, Johannes},
	year = {2020},
	pages = {28459--28467},
}

Signature of performance-enhancing oxygen vacancies in the mesoporous TiO 2  interlayer of a hematite functionalized Si microwire photoanode revealed by hard energy X-ray spectroscopy. , Worldwide significant efforts are ongoing to develop devices that store solar energy as fuels. In one such approach, solar energy is absorbed by semiconductors and utilized directly by catalysts at their surfaces to split water into H 2 and O 2 . To protect the semiconductors in these photo-electrochemical cells (PEC) from corrosion, frequently thin TiO 2 interlayers are applied. Employing a well-performing photoanode comprised of 1-D n-Si microwires (MWs) covered with a mesoporous (mp) TiO 2 interlayer fabricated by solution processing and functionalized with α-Fe 2 O 3 nanorods, we studied here the function of this TiO 2 interlayer by high-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy, along with X-ray emission spectroscopy (XES) and standard characterization techniques. Our data reveal that the TiO 2 interlayer not only protects the n-Si MW surface from corrosion, but that it also acts as a template for the hydrothermal growth of α-Fe 2 O 3 nanorods and improves the photocatalytic efficiency. We show that the latter effect correlates with the presence of stable oxygen vacancies at the interface between mp-TiO 2 and α-Fe 2 O 3 , which act as electron traps and thereby substantially reduce the charge recombination rate at the hematite surface.

@article{boniolo_spin_2020,
	title = {Spin transition in a ferrous chloride complex supported by a pentapyridine ligand},
	volume = {56},
	issn = {1364-548X},
	url = {https://pubs.rsc.org/en/content/articlelanding/2020/cc/c9cc09630b},
	doi = {10.1039/C9CC09630B},
	abstract = {Ferrous chloride complexes [FeIILxCl] commonly attain a high-spin state independently of the supporting ligand(s) and temperature. Herein, we present the first report of a complete spin crossover with T1/2 = 80 K in [FeII(Py5OH)Cl]+ (Py5OH = pyridine-2,6-diylbis[di(pyridin-2-yl)methanol]). Both spin forms of the complex are analyzed by X-ray spectroscopy and DFT calculations.},
	language = {en},
	number = {18},
	urldate = {2024-10-16},
	journal = {Chemical Communications},
	publisher = {The Royal Society of Chemistry},
	author = {Boniolo, Manuel and Shylin, Sergii I. and Chernev, Petko and Cheah, Mun Hon and Heizmann, Philipp A. and Huang, Ping and Salhi, Nessima and Hossain, Kamal and Thapper, Anders and Lundberg, Marcus and Messinger, Johannes},
	month = mar,
	year = {2020},
	pages = {2703--2706},
}

Ferrous chloride complexes [FeIILxCl] commonly attain a high-spin state independently of the supporting ligand(s) and temperature. Herein, we present the first report of a complete spin crossover with T1/2 = 80 K in [FeII(Py5OH)Cl]+ (Py5OH = pyridine-2,6-diylbis[di(pyridin-2-yl)methanol]). Both spin forms of the complex are analyzed by X-ray spectroscopy and DFT calculations.

@article{lichtenberg_substrate_2020,
	title = {Substrate water exchange in the {S2} state of photosystem {II} is dependent on the conformation of the {Mn4Ca} cluster},
	volume = {22},
	issn = {1463-9084},
	url = {https://pubs.rsc.org/en/content/articlelanding/2020/cp/d0cp01380c},
	doi = {10.1039/D0CP01380C},
	abstract = {In photosynthesis, dioxygen formation from water is catalyzed by the oxygen evolving complex (OEC) in Photosystem II (PSII) that harbours the Mn4Ca cluster. During catalysis, the OEC cycles through five redox states, S0 to S4. In the S2 state, the Mn4Ca cluster can exist in two conformations, which are signified by the low-spin (LS) g = 2 EPR multiline signal and the high-spin (HS) g = 4.1 EPR signal. Here, we employed time-resolved membrane inlet mass spectrometry to measure the kinetics of H218O/H216O exchange between bulk water and the two substrate waters bound at the Mn4Ca cluster in the SLS2, SHS2, and the S3 states in both Ca-PSII and Sr-PSII core complexes from T. elongatus. We found that the slowly exchanging substrate water exchanges 10 times faster in the SHS2 than in the SLS2 state, and that the SLS2 → SHS2 conversion has at physiological temperature an activation barrier of 17 ± 1 kcal mol−1. Of the presently suggested SHS2 models, our findings are best in agreement with a water exchange pathway involving a SHS2 state that has an open cubane structure with a hydroxide bound between Ca and Mn1. We also show that water exchange in the S3 state is governed by a different equilibrium than in S2, and that the exchange of the fast substrate water in the S2 state is unaffected by Ca/Sr substitution. These findings support that (i) O5 is the slowly exchanging substrate water, with W2 being the only other option, and (ii) either W2 or W3 is the fast exchanging substrate. The three remaining possibilities for O–O bond formation in PSII are discussed.},
	language = {en},
	number = {23},
	urldate = {2024-10-16},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {The Royal Society of Chemistry},
	author = {Lichtenberg, Casper de and Messinger, Johannes},
	month = jun,
	year = {2020},
	pages = {12894--12908},
}

In photosynthesis, dioxygen formation from water is catalyzed by the oxygen evolving complex (OEC) in Photosystem II (PSII) that harbours the Mn4Ca cluster. During catalysis, the OEC cycles through five redox states, S0 to S4. In the S2 state, the Mn4Ca cluster can exist in two conformations, which are signified by the low-spin (LS) g = 2 EPR multiline signal and the high-spin (HS) g = 4.1 EPR signal. Here, we employed time-resolved membrane inlet mass spectrometry to measure the kinetics of H218O/H216O exchange between bulk water and the two substrate waters bound at the Mn4Ca cluster in the SLS2, SHS2, and the S3 states in both Ca-PSII and Sr-PSII core complexes from T. elongatus. We found that the slowly exchanging substrate water exchanges 10 times faster in the SHS2 than in the SLS2 state, and that the SLS2 → SHS2 conversion has at physiological temperature an activation barrier of 17 ± 1 kcal mol−1. Of the presently suggested SHS2 models, our findings are best in agreement with a water exchange pathway involving a SHS2 state that has an open cubane structure with a hydroxide bound between Ca and Mn1. We also show that water exchange in the S3 state is governed by a different equilibrium than in S2, and that the exchange of the fast substrate water in the S2 state is unaffected by Ca/Sr substitution. These findings support that (i) O5 is the slowly exchanging substrate water, with W2 being the only other option, and (ii) either W2 or W3 is the fast exchanging substrate. The three remaining possibilities for O–O bond formation in PSII are discussed.

@article{magnuson_toward_2020,
	title = {Toward {Sustainable} {H2} {Production}: {Linking} {Hydrogenase} with {Photosynthesis}},
	volume = {4},
	issn = {2542-4351},
	shorttitle = {Toward {Sustainable} {H2} {Production}},
	url = {https://www.sciencedirect.com/science/article/pii/S2542435120302324},
	doi = {10.1016/j.joule.2020.05.014},
	abstract = {Molecular hydrogen, H2, is an energy carrier that is increasing in popularity as an alternative fuel. Presently, the major part of commercially available H2 is produced from fossil resources. To make H2 a true contender to fossil fuels, a sustainable production via water splitting by renewable electricity in combination with earth-abundant catalysts or by photosynthetic microorganisms is required.},
	number = {6},
	urldate = {2024-10-16},
	journal = {Joule},
	author = {Magnuson, Ann and Mamedov, Fikret and Messinger, Johannes},
	month = jun,
	year = {2020},
	pages = {1157--1159},
}

Molecular hydrogen, H2, is an energy carrier that is increasing in popularity as an alternative fuel. Presently, the major part of commercially available H2 is produced from fossil resources. To make H2 a true contender to fossil fuels, a sustainable production via water splitting by renewable electricity in combination with earth-abundant catalysts or by photosynthetic microorganisms is required.

@article{ibrahim_untangling_2020,
	title = {Untangling the sequence of events during the {S2} → {S3} transition in photosystem {II} and implications for the water oxidation mechanism},
	volume = {117},
	url = {https://www.pnas.org/doi/10.1073/pnas.2000529117},
	doi = {10.1073/pnas.2000529117},
	abstract = {In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S1, S2, S3, and S0, showing that a water molecule is inserted during the S2 → S3 transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O2 formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S2 → S3 transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, QA and QB, are observed. At the donor site, tyrosine YZ and His190 H-bonded to it move by 50 µs after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of OX(H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a “water wheel”-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (τ of ∼350 µs) during the S2 → S3 transition mirrors the appearance of OX electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated.},
	number = {23},
	urldate = {2024-10-16},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Ibrahim, Mohamed and Fransson, Thomas and Chatterjee, Ruchira and Cheah, Mun Hon and Hussein, Rana and Lassalle, Louise and Sutherlin, Kyle D. and Young, Iris D. and Fuller, Franklin D. and Gul, Sheraz and Kim, In-Sik and Simon, Philipp S. and de Lichtenberg, Casper and Chernev, Petko and Bogacz, Isabel and Pham, Cindy C. and Orville, Allen M. and Saichek, Nicholas and Northen, Trent and Batyuk, Alexander and Carbajo, Sergio and Alonso-Mori, Roberto and Tono, Kensuke and Owada, Shigeki and Bhowmick, Asmit and Bolotovsky, Robert and Mendez, Derek and Moriarty, Nigel W. and Holton, James M. and Dobbek, Holger and Brewster, Aaron S. and Adams, Paul D. and Sauter, Nicholas K. and Bergmann, Uwe and Zouni, Athina and Messinger, Johannes and Kern, Jan and Yachandra, Vittal K. and Yano, Junko},
	month = jun,
	year = {2020},
	pages = {12624--12635},
}

In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S1, S2, S3, and S0, showing that a water molecule is inserted during the S2 → S3 transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O2 formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S2 → S3 transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, QA and QB, are observed. At the donor site, tyrosine YZ and His190 H-bonded to it move by 50 µs after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of OX(H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a “water wheel”-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (τ of ∼350 µs) during the S2 → S3 transition mirrors the appearance of OX electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated.

@article{kosourov_water_2020,
	title = {Water oxidation by photosystem {II} is the primary source of electrons for sustained {H2} photoproduction in nutrient-replete green algae},
	volume = {117},
	url = {https://www.pnas.org/doi/10.1073/pnas.2009210117},
	doi = {10.1073/pnas.2009210117},
	abstract = {The unicellular green alga Chlamydomonas reinhardtii is capable of photosynthetic H2 production. H2 evolution occurs under anaerobic conditions and is difficult to sustain due to 1) competition between [FeFe]-hydrogenase (H2ase), the key enzyme responsible for H2 metabolism in algae, and the Calvin–Benson–Bassham (CBB) cycle for photosynthetic reductants and 2) inactivation of H2ase by O2 coevolved in photosynthesis. Recently, we achieved sustainable H2 photoproduction by shifting algae from continuous illumination to a train of short (1 s) light pulses, interrupted by longer (9 s) dark periods. This illumination regime prevents activation of the CBB cycle and redirects photosynthetic electrons to H2ase. Employing membrane-inlet mass spectrometry and 
H218O
, we now present clear evidence that efficient H2 photoproduction in pulse-illuminated algae depends primarily on direct water biophotolysis, where water oxidation at the donor side of photosystem II (PSII) provides electrons for the reduction of protons by H2ase downstream of photosystem I. This occurs exclusively in the absence of CO2 fixation, while with the activation of the CBB cycle by longer (8 s) light pulses the H2 photoproduction ceases and instead a slow overall H2 uptake is observed. We also demonstrate that the loss of PSII activity in DCMU-treated algae or in PSII-deficient mutant cells can be partly compensated for by the indirect (PSII-independent) H2 photoproduction pathway, but only for a short ({\textless}1 h) period. Thus, PSII activity is indispensable for a sustained process, where it is responsible for more than 92\% of the final H2 yield.},
	number = {47},
	urldate = {2024-10-16},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Kosourov, Sergey and Nagy, Valéria and Shevela, Dmitry and Jokel, Martina and Messinger, Johannes and Allahverdiyeva, Yagut},
	month = nov,
	year = {2020},
	pages = {29629--29636},
}

The unicellular green alga Chlamydomonas reinhardtii is capable of photosynthetic H2 production. H2 evolution occurs under anaerobic conditions and is difficult to sustain due to 1) competition between [FeFe]-hydrogenase (H2ase), the key enzyme responsible for H2 metabolism in algae, and the Calvin–Benson–Bassham (CBB) cycle for photosynthetic reductants and 2) inactivation of H2ase by O2 coevolved in photosynthesis. Recently, we achieved sustainable H2 photoproduction by shifting algae from continuous illumination to a train of short (1 s) light pulses, interrupted by longer (9 s) dark periods. This illumination regime prevents activation of the CBB cycle and redirects photosynthetic electrons to H2ase. Employing membrane-inlet mass spectrometry and H218O , we now present clear evidence that efficient H2 photoproduction in pulse-illuminated algae depends primarily on direct water biophotolysis, where water oxidation at the donor side of photosystem II (PSII) provides electrons for the reduction of protons by H2ase downstream of photosystem I. This occurs exclusively in the absence of CO2 fixation, while with the activation of the CBB cycle by longer (8 s) light pulses the H2 photoproduction ceases and instead a slow overall H2 uptake is observed. We also demonstrate that the loss of PSII activity in DCMU-treated algae or in PSII-deficient mutant cells can be partly compensated for by the indirect (PSII-independent) H2 photoproduction pathway, but only for a short (\textless1 h) period. Thus, PSII activity is indispensable for a sustained process, where it is responsible for more than 92% of the final H2 yield.

@article{kawde_microstructured_2019,
	title = {A microstructured p-{Si} photocathode outcompetes {Pt} as a counter electrode to hematite in photoelectrochemical water splitting},
	volume = {48},
	issn = {1477-9226, 1477-9234},
	url = {http://xlink.rsc.org/?DOI=C8DT03653E},
	doi = {10.1039/C8DT03653E},
	abstract = {Herein we demonstrate that an earth-abundant semiconductor photocathode (p-Si/TiO
              2
              /NiO
              x
              ) out-competes rare and expensive Pt as counter electrode to Fe-oxide for overall photoelectrochemical water splitting.
            
          , 
            
              Herein, we communicate about an Earth-abundant semiconductor photocathode (p-Si/TiO
              2
              /NiO
              x
              ) as an alternative for the rare and expensive Pt as a counter electrode for overall photoelectrochemical water splitting. The proposed photoelectrochemical (PEC) water-splitting device mimics the “Z”-scheme observed in natural photosynthesis by combining two photoelectrodes in a parallel-illumination mode. A nearly 60\% increase in the photocurrent density (
              J
              ph
              ) for pristine α-Fe
              2
              O
              3
              and a 77\% increase in the applied bias photocurrent efficiency (ABPE) were achieved by replacing the conventionally used Pt cathode with an efficient, cost effective p-Si/TiO
              2
              /NiO
              x
              photocathode under parallel illumination. The resulting photocurrent density of 1.26 mA cm
              −2
              at 1.23
              V
              RHE
              represents a new record performance for hydrothermally grown pristine α-Fe
              2
              O
              3
              nanorod photoanodes in combination with a photocathode, which opens the prospect for further improvement by doping α-Fe
              2
              O
              3
              or by its decoration with co-catalysts. Electrochemical impedance spectroscopy measurements suggest that this significant performance increase is due to the enhancement of the space-charge field in α-Fe
              2
              O
              3
              .},
	language = {en},
	number = {4},
	urldate = {2021-06-07},
	journal = {Dalton Transactions},
	author = {Kawde, Anurag and Annamalai, Alagappan and Sellstedt, Anita and Glatzel, Pieter and Wågberg, Thomas and Messinger, Johannes},
	year = {2019},
	pages = {1166--1170},
}

Herein we demonstrate that an earth-abundant semiconductor photocathode (p-Si/TiO 2 /NiO x ) out-competes rare and expensive Pt as counter electrode to Fe-oxide for overall photoelectrochemical water splitting. , Herein, we communicate about an Earth-abundant semiconductor photocathode (p-Si/TiO 2 /NiO x ) as an alternative for the rare and expensive Pt as a counter electrode for overall photoelectrochemical water splitting. The proposed photoelectrochemical (PEC) water-splitting device mimics the “Z”-scheme observed in natural photosynthesis by combining two photoelectrodes in a parallel-illumination mode. A nearly 60% increase in the photocurrent density ( J ph ) for pristine α-Fe 2 O 3 and a 77% increase in the applied bias photocurrent efficiency (ABPE) were achieved by replacing the conventionally used Pt cathode with an efficient, cost effective p-Si/TiO 2 /NiO x photocathode under parallel illumination. The resulting photocurrent density of 1.26 mA cm −2 at 1.23 V RHE represents a new record performance for hydrothermally grown pristine α-Fe 2 O 3 nanorod photoanodes in combination with a photocathode, which opens the prospect for further improvement by doping α-Fe 2 O 3 or by its decoration with co-catalysts. Electrochemical impedance spectroscopy measurements suggest that this significant performance increase is due to the enhancement of the space-charge field in α-Fe 2 O 3 .

@article{kwong_cobalt-doped_2019,
	title = {Cobalt-doped hematite thin films for electrocatalytic water oxidation in highly acidic media},
	volume = {55},
	issn = {1364-548X},
	url = {https://pubs.rsc.org/en/content/articlelanding/2019/cc/c9cc01369e},
	doi = {10.1039/C9CC01369E},
	abstract = {Earth-abundant cobalt-doped hematite thin-film electrocatalysts were explored for acidic water oxidation. The strategically doped hematite produced a stable geometric current density of 10 mA cm−2 for up to 50 h at pH 0.3, as a result of Co-enhanced intrinsic catalytic activity and charge transport properties across the film matrix.},
	language = {en},
	number = {34},
	urldate = {2024-10-16},
	journal = {Chemical Communications},
	publisher = {The Royal Society of Chemistry},
	author = {Kwong, Wai Ling and Lee, Cheng Choo and Shchukarev, Andrey and Messinger, Johannes},
	month = apr,
	year = {2019},
	pages = {5017--5020},
}

Earth-abundant cobalt-doped hematite thin-film electrocatalysts were explored for acidic water oxidation. The strategically doped hematite produced a stable geometric current density of 10 mA cm−2 for up to 50 h at pH 0.3, as a result of Co-enhanced intrinsic catalytic activity and charge transport properties across the film matrix.

@article{chrysina_five-coordinate_2019,
	title = {Five-coordinate {MnIV} intermediate in the activation of nature’s water splitting cofactor},
	volume = {116},
	url = {https://www.pnas.org/doi/10.1073/pnas.1817526116},
	doi = {10.1073/pnas.1817526116},
	abstract = {Nature’s water splitting cofactor passes through a series of catalytic intermediates (S0-S4) before O-O bond formation and O2 release. In the second last transition (S2 to S3) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all MnIV form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S4. How cofactor activation proceeds remains an open question. Here, we report a so far not described intermediate (S3') in which cofactor oxidation has occurred without water insertion. This intermediate can be trapped in a significant fraction of centers ({\textgreater}50\%) in (i) chemical-modified cofactors in which Ca2+ is exchanged with Sr2+; the Mn4O5Sr cofactor remains active, but the S2-S3 and S3-S0 transitions are slower than for the Mn4O5Ca cofactor; and (ii) upon addition of 3\% vol/vol methanol; methanol is thought to act as a substrate water analog. The S3' electron paramagnetic resonance (EPR) signal is significantly broader than the untreated S3 signal (2.5 T vs. 1.5 T), indicating the cofactor still contains a 5-coordinate Mn ion, as seen in the preceding S2 state. Magnetic double resonance data extend these findings revealing the electronic connectivity of the S3' cofactor is similar to the high spin form of the preceding S2 state, which contains a cuboidal Mn3O4Ca unit tethered to an external, 5-coordinate Mn ion (Mn4). These results demonstrate that cofactor oxidation regulates water molecule insertion via binding to Mn4. The interaction of ammonia with the cofactor is also discussed.},
	number = {34},
	urldate = {2024-10-16},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Chrysina, Maria and Heyno, Eiri and Kutin, Yury and Reus, Michael and Nilsson, Håkan and Nowaczyk, Marc M. and DeBeer, Serena and Neese, Frank and Messinger, Johannes and Lubitz, Wolfgang and Cox, Nicholas},
	month = aug,
	year = {2019},
	pages = {16841--16846},
}

Nature’s water splitting cofactor passes through a series of catalytic intermediates (S0-S4) before O-O bond formation and O2 release. In the second last transition (S2 to S3) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all MnIV form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S4. How cofactor activation proceeds remains an open question. Here, we report a so far not described intermediate (S3') in which cofactor oxidation has occurred without water insertion. This intermediate can be trapped in a significant fraction of centers (\textgreater50%) in (i) chemical-modified cofactors in which Ca2+ is exchanged with Sr2+; the Mn4O5Sr cofactor remains active, but the S2-S3 and S3-S0 transitions are slower than for the Mn4O5Ca cofactor; and (ii) upon addition of 3% vol/vol methanol; methanol is thought to act as a substrate water analog. The S3' electron paramagnetic resonance (EPR) signal is significantly broader than the untreated S3 signal (2.5 T vs. 1.5 T), indicating the cofactor still contains a 5-coordinate Mn ion, as seen in the preceding S2 state. Magnetic double resonance data extend these findings revealing the electronic connectivity of the S3' cofactor is similar to the high spin form of the preceding S2 state, which contains a cuboidal Mn3O4Ca unit tethered to an external, 5-coordinate Mn ion (Mn4). These results demonstrate that cofactor oxidation regulates water molecule insertion via binding to Mn4. The interaction of ammonia with the cofactor is also discussed.

@article{chatterjee_structural_2019,
	title = {Structural isomers of the {S2} state in photosystem {II}: do they exist at room temperature and are they important for function?},
	volume = {166},
	copyright = {© 2019 Scandinavian Plant Physiology Society},
	issn = {1399-3054},
	shorttitle = {Structural isomers of the {S2} state in photosystem {II}},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/ppl.12947},
	doi = {10.1111/ppl.12947},
	abstract = {In nature, an oxo-bridged Mn4CaO5 cluster embedded in photosystem II (PSII), a membrane-bound multi-subunit pigment protein complex, catalyzes the water oxidation reaction that is driven by light-induced charge separations in the reaction center of PSII. The Mn4CaO5 cluster accumulates four oxidizing equivalents to enable the four-electron four-proton catalysis of two water molecules to one dioxygen molecule and cycles through five intermediate S-states, S0 – S4 in the Kok cycle. One important question related to the catalytic mechanism of the oxygen-evolving complex (OEC) that remains is, whether structural isomers are present in some of the intermediate S-states and if such equilibria are essential for the mechanism of the O-O bond formation. Here we compare results from electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) obtained at cryogenic temperatures for the S2 state of PSII with structural data collected of the S1, S2 and S3 states by serial crystallography at neutral pH (∼6.5) using an X-ray free electron laser at room temperature. While the cryogenic data show the presence of at least two structural forms of the S2 state, the room temperature crystallography data can be well-described by just one S2 structure. We discuss the deviating results and outline experimental strategies for clarifying this mechanistically important question.},
	language = {en},
	number = {1},
	urldate = {2024-10-16},
	journal = {Physiologia Plantarum},
	author = {Chatterjee, Ruchira and Lassalle, Louise and Gul, Sheraz and Fuller, Franklin D. and Young, Iris D. and Ibrahim, Mohamed and de Lichtenberg, Casper and Cheah, Mun Hon and Zouni, Athina and Messinger, Johannes and Yachandra, Vittal K. and Kern, Jan and Yano, Junko},
	year = {2019},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1111/ppl.12947},
	pages = {60--72},
}

In nature, an oxo-bridged Mn4CaO5 cluster embedded in photosystem II (PSII), a membrane-bound multi-subunit pigment protein complex, catalyzes the water oxidation reaction that is driven by light-induced charge separations in the reaction center of PSII. The Mn4CaO5 cluster accumulates four oxidizing equivalents to enable the four-electron four-proton catalysis of two water molecules to one dioxygen molecule and cycles through five intermediate S-states, S0 – S4 in the Kok cycle. One important question related to the catalytic mechanism of the oxygen-evolving complex (OEC) that remains is, whether structural isomers are present in some of the intermediate S-states and if such equilibria are essential for the mechanism of the O-O bond formation. Here we compare results from electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) obtained at cryogenic temperatures for the S2 state of PSII with structural data collected of the S1, S2 and S3 states by serial crystallography at neutral pH (∼6.5) using an X-ray free electron laser at room temperature. While the cryogenic data show the presence of at least two structural forms of the S2 state, the room temperature crystallography data can be well-described by just one S2 structure. We discuss the deviating results and outline experimental strategies for clarifying this mechanistically important question.

@article{conlan_thomas_2019,
	title = {Thomas {John} {Wydrzynski} (8 {July} 1947–16 {March} 2018)},
	volume = {140},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-018-0606-9},
	doi = {10.1007/s11120-018-0606-9},
	abstract = {With this Tribute, we remember and honor Thomas John (Tom) Wydrzynski. Tom was a highly innovative, independent and committed researcher, who had, early in his career, defined his life-long research goal. He was committed to understand how Photosystem II produces molecular oxygen from water, using the energy of sunlight, and to apply this knowledge towards making artificial systems. In this tribute, we summarize his research journey, which involved working on ‘soft money’ in several laboratories around the world for many years, as well as his research achievements. We also reflect upon his approach to life, science and student supervision, as we perceive it. Tom was not only a thoughtful scientist that inspired many to enter this field of research, but also a wonderful supervisor and friend, who is deeply missed (see footnote*).},
	language = {en},
	number = {3},
	urldate = {2024-10-16},
	journal = {Photosynthesis Research},
	author = {Conlan, Brendon and {Govindjee} and Messinger, Johannes},
	month = jun,
	year = {2019},
	keywords = {Artificial photosynthesis, Chloride, Manganese, Photosystem II, Water oxidation},
	pages = {253--261},
}

With this Tribute, we remember and honor Thomas John (Tom) Wydrzynski. Tom was a highly innovative, independent and committed researcher, who had, early in his career, defined his life-long research goal. He was committed to understand how Photosystem II produces molecular oxygen from water, using the energy of sunlight, and to apply this knowledge towards making artificial systems. In this tribute, we summarize his research journey, which involved working on ‘soft money’ in several laboratories around the world for many years, as well as his research achievements. We also reflect upon his approach to life, science and student supervision, as we perceive it. Tom was not only a thoughtful scientist that inspired many to enter this field of research, but also a wonderful supervisor and friend, who is deeply missed (see footnote*).

@article{pham_unequal_2019,
	title = {Unequal misses during the flash-induced advancement of photosystem {II}: effects of the {S} state and acceptor side cycles},
	volume = {139},
	issn = {1573-5079},
	shorttitle = {Unequal misses during the flash-induced advancement of photosystem {II}},
	url = {https://doi.org/10.1007/s11120-018-0574-0},
	doi = {10.1007/s11120-018-0574-0},
	abstract = {Photosynthetic water oxidation is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII). This process is energetically driven by light-induced charge separation in the reaction center of PSII, which leads to a stepwise accumulation of oxidizing equivalents in the OEC (Si states, i = 0–4) resulting in O2 evolution after each fourth flash, and to the reduction of plastoquinone to plastoquinol on the acceptor side of PSII. However, the Si-state advancement is not perfect, which according to the Kok model is described by miss-hits (misses). These may be caused by redox equilibria or kinetic limitations on the donor (OEC) or the acceptor side. In this study, we investigate the effects of individual S state transitions and of the quinone acceptor side on the miss parameter by analyzing the flash-induced oxygen evolution patterns and the S2, S3 and S0 state lifetimes in thylakoid samples of the extremophilic red alga Cyanidioschyzon merolae. The data are analyzed employing a global fit analysis and the results are compared to the data obtained previously for spinach thylakoids. These two organisms were selected, because the redox potential of QA/QA− in PSII is significantly less negative in C. merolae (Em = − 104 mV) than in spinach (Em = − 163 mV). This significant difference in redox potential was expected to allow the disentanglement of acceptor and donor side effects on the miss parameter. Our data indicate that, at slightly acidic and neutral pH values, the Em of QA−/QA plays only a minor role for the miss parameter. By contrast, the increased energy gap for the backward electron transfer from QA− to Pheo slows down the charge recombination reaction with the S3 and S2 states considerably. In addition, our data support the concept that the S2 → S3 transition is the least efficient step during the oxidation of water to molecular oxygen in the Kok cycle of PSII.},
	language = {en},
	number = {1},
	urldate = {2024-10-16},
	journal = {Photosynthesis Research},
	author = {Pham, Long Vo and Janna Olmos, Julian David and Chernev, Petko and Kargul, Joanna and Messinger, Johannes},
	month = mar,
	year = {2019},
	keywords = {Cyanidioschyzon merolae, Flash-induced oxygen oscillation pattern (FIOP), Mechanism of water oxidation, Photosynthesis, Photosystem II, Unequal miss parameter},
	pages = {93--106},
}

Photosynthetic water oxidation is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII). This process is energetically driven by light-induced charge separation in the reaction center of PSII, which leads to a stepwise accumulation of oxidizing equivalents in the OEC (Si states, i = 0–4) resulting in O2 evolution after each fourth flash, and to the reduction of plastoquinone to plastoquinol on the acceptor side of PSII. However, the Si-state advancement is not perfect, which according to the Kok model is described by miss-hits (misses). These may be caused by redox equilibria or kinetic limitations on the donor (OEC) or the acceptor side. In this study, we investigate the effects of individual S state transitions and of the quinone acceptor side on the miss parameter by analyzing the flash-induced oxygen evolution patterns and the S2, S3 and S0 state lifetimes in thylakoid samples of the extremophilic red alga Cyanidioschyzon merolae. The data are analyzed employing a global fit analysis and the results are compared to the data obtained previously for spinach thylakoids. These two organisms were selected, because the redox potential of QA/QA− in PSII is significantly less negative in C. merolae (Em = − 104 mV) than in spinach (Em = − 163 mV). This significant difference in redox potential was expected to allow the disentanglement of acceptor and donor side effects on the miss parameter. Our data indicate that, at slightly acidic and neutral pH values, the Em of QA−/QA plays only a minor role for the miss parameter. By contrast, the increased energy gap for the backward electron transfer from QA− to Pheo slows down the charge recombination reaction with the S3 and S2 states considerably. In addition, our data support the concept that the S2 → S3 transition is the least efficient step during the oxidation of water to molecular oxygen in the Kok cycle of PSII.

@article{govindjee_we_2019,
	title = {We remember those who left us in the recent past},
	volume = {166},
	copyright = {© 2018 Scandinavian Plant Physiology Society},
	issn = {1399-3054},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/ppl.12859},
	doi = {10.1111/ppl.12859},
	language = {en},
	number = {1},
	urldate = {2024-10-16},
	journal = {Physiologia Plantarum},
	author = {{Govindjee} and Messinger, Johannes},
	year = {2019},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1111/ppl.12859},
	pages = {7--11},
}

@article{chatterjee_xanes_2019,
	title = {{XANES} and {EXAFS} of dilute solutions of transition metals at {XFELs}},
	volume = {26},
	issn = {1600-5775},
	url = {https://journals.iucr.org/s/issues/2019/05/00/ig5072/},
	doi = {10.1107/S1600577519007550},
	abstract = {This work has demonstrated that X-ray absorption spectroscopy (XAS), both Mn XANES and EXAFS, of solutions with millimolar concentrations of metal is possible using the femtosecond X-ray pulses from XFELs. Mn XAS data were collected using two different sample delivery methods, a Rayleigh jet and a drop-on-demand setup, with varying concentrations of Mn. Here, a new method for normalization of XAS spectra based on solvent scattering that is compatible with data collection from a highly variable pulsed source is described. The measured XANES and EXAFS spectra of such dilute solution samples are in good agreement with data collected at synchrotron sources using traditional scanning protocols. The procedures described here will enable XFEL-based XAS on dilute biological samples, especially metalloproteins, with low sample consumption. Details of the experimental setup and data analysis methods used in this XANES and EXAFS study are presented. This method will also benefit XAS performed at high-repetition-rate XFELs such as the European XFEL, LCLS-II and LCLS-II-HE.},
	language = {en},
	number = {5},
	urldate = {2024-10-16},
	journal = {Journal of Synchrotron Radiation},
	publisher = {International Union of Crystallography},
	author = {Chatterjee, R. and Weninger, C. and Loukianov, A. and Gul, S. and Fuller, F. D. and Cheah, M. H. and Fransson, T. and Pham, C. C. and Nelson, S. and Song, S. and Britz, A. and Messinger, J. and Bergmann, U. and Alonso-Mori, R. and Yachandra, V. K. and Kern, J. and Yano, J.},
	month = sep,
	year = {2019},
	pages = {1716--1724},
}

This work has demonstrated that X-ray absorption spectroscopy (XAS), both Mn XANES and EXAFS, of solutions with millimolar concentrations of metal is possible using the femtosecond X-ray pulses from XFELs. Mn XAS data were collected using two different sample delivery methods, a Rayleigh jet and a drop-on-demand setup, with varying concentrations of Mn. Here, a new method for normalization of XAS spectra based on solvent scattering that is compatible with data collection from a highly variable pulsed source is described. The measured XANES and EXAFS spectra of such dilute solution samples are in good agreement with data collected at synchrotron sources using traditional scanning protocols. The procedures described here will enable XFEL-based XAS on dilute biological samples, especially metalloproteins, with low sample consumption. Details of the experimental setup and data analysis methods used in this XANES and EXAFS study are presented. This method will also benefit XAS performed at high-repetition-rate XFELs such as the European XFEL, LCLS-II and LCLS-II-HE.

@article{shevela_birth_2019,
	title = {‘{Birth} defects’ of photosystem {II} make it highly susceptible to photodamage during chloroplast biogenesis},
	volume = {166},
	copyright = {© 2019 Scandinavian Plant Physiology Society},
	issn = {1399-3054},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/ppl.12932},
	doi = {10.1111/ppl.12932},
	abstract = {High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water-oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane-inlet mass spectrometry and O2-polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these ‘PSII birth defects’ in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de-etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O2-polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, QB-inhibitor binding, and thermoluminescence studies indicate that the decline of the high-light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability QA− → QB during de-etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer-range energy transfer.},
	language = {en},
	number = {1},
	urldate = {2024-10-16},
	journal = {Physiologia Plantarum},
	author = {Shevela, Dmitry and Ananyev, Gennady and Vatland, Ann K. and Arnold, Janine and Mamedov, Fikret and Eichacker, Lutz A. and Dismukes, G. Charles and Messinger, Johannes},
	year = {2019},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1111/ppl.12932},
	pages = {165--180},
}

High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water-oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane-inlet mass spectrometry and O2-polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these ‘PSII birth defects’ in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de-etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O2-polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, QB-inhibitor binding, and thermoluminescence studies indicate that the decline of the high-light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability QA− → QB during de-etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer-range energy transfer.

@article{messinger_artificial_2018,
	title = {Artificial photosynthesis – from sunlight to fuels and valuable products for a sustainable future},
	volume = {2},
	issn = {2398-4902},
	url = {https://pubs.rsc.org/en/content/articlelanding/2018/se/c8se90049c},
	doi = {10.1039/C8SE90049C},
	abstract = {A graphical abstract is available for this content},
	language = {en},
	number = {9},
	urldate = {2024-12-10},
	journal = {Sustainable Energy \& Fuels},
	publisher = {The Royal Society of Chemistry},
	author = {Messinger, Johannes and Ishitani, Osamu and Wang, Dunwei},
	month = aug,
	year = {2018},
	pages = {1891--1892},
}

A graphical abstract is available for this content

@article{kwong_high-performance_2018,
	title = {High-performance iron ({III}) oxide electrocatalyst for water oxidation in strongly acidic media},
	volume = {365},
	issn = {0021-9517},
	url = {https://www.sciencedirect.com/science/article/pii/S0021951718302409},
	doi = {10.1016/j.jcat.2018.06.018},
	abstract = {Stable and efficient oxygen evolution reaction (OER) catalysts for the oxidation of water to dioxygen in highly acidic media are currently limited to expensive noble metal (Ir and Ru) oxides since presently known OER catalysts made of inexpensive earth-abundant materials generally suffer anodic corrosion at low pH. In this study, we report that a mixed-polymorph film comprising maghemite and hematite, prepared using spray pyrolysis deposition followed by low-temperature annealing, showed a sustained OER rate ({\textgreater}24 h) corresponding to a current density of 10 mA cm−2 at an initial overpotential of 650 mV, with a Tafel slope of only 56 mV dec−1 and near-100\% Faradaic efficiency in 0.5 M H2SO4 (pH 0.3). This performance is remarkable, since iron (III) oxide films comprising only maghemite were found to exhibit a comparable intrinsic activity, but considerably lower stability for OER, while films of pure hematite were OER-inactive. These results are explained by the differences in the polymorph crystal structures, which cause different electrical conductivity and surface interactions with water molecules and protons. Our findings not only reveal the potential of iron (III) oxide as acid-stable OER catalyst, but also highlight the important yet hitherto largely unexplored effect of crystal polymorphism on electrocatalytic OER performance.},
	urldate = {2024-12-10},
	journal = {Journal of Catalysis},
	author = {Kwong, Wai Ling and Lee, Cheng Choo and Shchukarev, Andrey and Björn, Erik and Messinger, Johannes},
	month = sep,
	year = {2018},
	keywords = {Acidic electrolyte, Artificial photosynthesis, Iron oxide, Oxygen evolution reaction, Water oxidation},
	pages = {29--35},
}

Stable and efficient oxygen evolution reaction (OER) catalysts for the oxidation of water to dioxygen in highly acidic media are currently limited to expensive noble metal (Ir and Ru) oxides since presently known OER catalysts made of inexpensive earth-abundant materials generally suffer anodic corrosion at low pH. In this study, we report that a mixed-polymorph film comprising maghemite and hematite, prepared using spray pyrolysis deposition followed by low-temperature annealing, showed a sustained OER rate (\textgreater24 h) corresponding to a current density of 10 mA cm−2 at an initial overpotential of 650 mV, with a Tafel slope of only 56 mV dec−1 and near-100% Faradaic efficiency in 0.5 M H2SO4 (pH 0.3). This performance is remarkable, since iron (III) oxide films comprising only maghemite were found to exhibit a comparable intrinsic activity, but considerably lower stability for OER, while films of pure hematite were OER-inactive. These results are explained by the differences in the polymorph crystal structures, which cause different electrical conductivity and surface interactions with water molecules and protons. Our findings not only reveal the potential of iron (III) oxide as acid-stable OER catalyst, but also highlight the important yet hitherto largely unexplored effect of crystal polymorphism on electrocatalytic OER performance.

@incollection{shevela_liquid-phase_2018,
	address = {New York, NY},
	title = {Liquid-{Phase} {Measurements} of {Photosynthetic} {Oxygen} {Evolution}},
	isbn = {978-1-4939-7786-4},
	url = {https://doi.org/10.1007/978-1-4939-7786-4_11},
	doi = {10.1007/978-1-4939-7786-4_11},
	abstract = {This chapter compares two different techniques for monitoring photosynthetic O2 production: the widespread Clark-type O2 electrode and the more sophisticated membrane inlet mass spectrometry (MIMS) technique. We describe how a simple membrane inlet for MIMS can be made out of a commercial Clark-type cell, and outline the advantages and drawbacks of the two techniques to guide researchers in deciding which method to use. Protocols and examples are given for measuring O2 evolution rates and for determining the number of chlorophyll molecules per active photosystem II reaction center.},
	language = {en},
	urldate = {2024-12-10},
	booktitle = {Photosynthesis: {Methods} and {Protocols}},
	publisher = {Springer},
	author = {Shevela, Dmitriy and Schröder, Wolfgang P. and Messinger, Johannes},
	editor = {Covshoff, Sarah},
	year = {2018},
	keywords = {Clark-type electrode, Membrane inlet mass spectrometry, O2 evolution, Photosynthetic water oxidation, Photosystem II},
	pages = {197--211},
}

This chapter compares two different techniques for monitoring photosynthetic O2 production: the widespread Clark-type O2 electrode and the more sophisticated membrane inlet mass spectrometry (MIMS) technique. We describe how a simple membrane inlet for MIMS can be made out of a commercial Clark-type cell, and outline the advantages and drawbacks of the two techniques to guide researchers in deciding which method to use. Protocols and examples are given for measuring O2 evolution rates and for determining the number of chlorophyll molecules per active photosystem II reaction center.

@article{kawde_photo-electrochemical_2018,
	title = {Photo-electrochemical hydrogen production from neutral phosphate buffer and seawater using micro-structured p-{Si} photo-electrodes functionalized by solution-based methods},
	volume = {2},
	issn = {2398-4902},
	url = {http://xlink.rsc.org/?DOI=C8SE00291F},
	doi = {10.1039/C8SE00291F},
	abstract = {Micro-structured p-Si/TiO
              2
              /NiO
              x
              allows for efficient photoelectrochemical H
              2
              production from seawater.
            
          , 
            
              Solar fuels such as H
              2
              generated from sunlight and seawater using earth-abundant materials are expected to be a crucial component of a next generation renewable energy mix. We herein report a systematic analysis of the photo-electrochemical performance of TiO
              2
              coated, microstructured p-Si photo-electrodes (p-Si/TiO
              2
              ) that were functionalized with CoO
              x
              and NiO
              x
              for H
              2
              generation. These photocathodes were synthesized from commercial p-Si wafers employing wet chemical methods. In neutral phosphate buffer and standard 1 sun illumination, the p-Si/TiO
              2
              /NiO
              x
              photoelectrode showed a photocurrent density of −1.48 mA cm
              −2
              at zero bias (0 V
              RHE
              ), which was three times and 15 times better than the photocurrent densities of p-Si/TiO
              2
              /CoO
              x
              and p-Si/TiO
              2
              , respectively. No decline in activity was observed over a five hour test period, yielding a Faradaic efficiency of 96\% for H
              2
              production. Based on the electrochemical characterizations and the high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) and emission spectroscopy measurements performed at the Ti Kα
              1
              fluorescence line, the superior performance of the p-Si/TiO
              2
              /NiO
              x
              photoelectrode was attributed to improved charge transfer properties induced by the NiO
              x
              coating on the protective TiO
              2
              layer, in combination with a higher catalytic activity of NiO
              x
              for H
              2
              -evolution. Moreover, we report here an excellent photo-electrochemical performance of p-Si/TiO
              2
              /NiO
              x
              photoelectrode in corrosive artificial seawater (pH 8.4) with an unprecedented photocurrent density of 10 mA cm
              −2
              at an applied potential of −0.7 V
              RHE
              , and of 20 mA cm
              −2
              at −0.9 V
              RHE
              . The applied bias photon-to-current conversion efficiency (ABPE) at −0.7 V
              RHE
              and 10 mA cm
              −2
              was found to be 5.1\%.},
	language = {en},
	number = {10},
	urldate = {2021-06-07},
	journal = {Sustainable Energy \& Fuels},
	author = {Kawde, Anurag and Annamalai, Alagappan and Amidani, Lucia and Boniolo, Manuel and Kwong, Wai Ling and Sellstedt, Anita and Glatzel, Pieter and Wågberg, Thomas and Messinger, Johannes},
	year = {2018},
	pages = {2215--2223},
}

Micro-structured p-Si/TiO 2 /NiO x allows for efficient photoelectrochemical H 2 production from seawater. , Solar fuels such as H 2 generated from sunlight and seawater using earth-abundant materials are expected to be a crucial component of a next generation renewable energy mix. We herein report a systematic analysis of the photo-electrochemical performance of TiO 2 coated, microstructured p-Si photo-electrodes (p-Si/TiO 2 ) that were functionalized with CoO x and NiO x for H 2 generation. These photocathodes were synthesized from commercial p-Si wafers employing wet chemical methods. In neutral phosphate buffer and standard 1 sun illumination, the p-Si/TiO 2 /NiO x photoelectrode showed a photocurrent density of −1.48 mA cm −2 at zero bias (0 V RHE ), which was three times and 15 times better than the photocurrent densities of p-Si/TiO 2 /CoO x and p-Si/TiO 2 , respectively. No decline in activity was observed over a five hour test period, yielding a Faradaic efficiency of 96% for H 2 production. Based on the electrochemical characterizations and the high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) and emission spectroscopy measurements performed at the Ti Kα 1 fluorescence line, the superior performance of the p-Si/TiO 2 /NiO x photoelectrode was attributed to improved charge transfer properties induced by the NiO x coating on the protective TiO 2 layer, in combination with a higher catalytic activity of NiO x for H 2 -evolution. Moreover, we report here an excellent photo-electrochemical performance of p-Si/TiO 2 /NiO x photoelectrode in corrosive artificial seawater (pH 8.4) with an unprecedented photocurrent density of 10 mA cm −2 at an applied potential of −0.7 V RHE , and of 20 mA cm −2 at −0.9 V RHE . The applied bias photon-to-current conversion efficiency (ABPE) at −0.7 V RHE and 10 mA cm −2 was found to be 5.1%.

@article{tikhonov_quantification_2018,
	title = {Quantification of bound bicarbonate in photosystem {II}},
	volume = {56},
	issn = {1573-9058},
	url = {https://doi.org/10.1007/s11099-017-0758-4},
	doi = {10.1007/s11099-017-0758-4},
	abstract = {In this study, we presented a new approach for quantification of bicarbonate (HCO3−) molecules bound to PSII. Our method, which is based on a combination of membrane-inlet mass spectrometry (MIMS) and 18O-labelling, excludes the possibility of “non-accounted” HCO3− by avoiding (1) the employment of formate for removal of HCO3− from PSII, and (2) the extremely low concentrations of HCO3−/CO2 during online MIMS measurements. By equilibration of PSII sample to ambient CO2 concentration of dissolved CO2/HCO3−, the method ensures that all physiological binding sites are saturated before analysis. With this approach, we determined that in spinach PSII membrane fragments 1.1 ± 0.1 HCO3− are bound per PSII reaction center, while none was bound to isolated PsbO protein. Our present results confirmed that PSII binds one HCO3− molecule as ligand to the non-heme iron of PSII, while unbound HCO3− optimizes the water-splitting reactions by acting as a mobile proton shuttle.},
	language = {en},
	number = {1},
	urldate = {2024-12-10},
	journal = {Photosynthetica},
	author = {Tikhonov, K. and Shevela, D. and Klimov, V. V. and Messinger, J.},
	month = mar,
	year = {2018},
	keywords = {Mn-stabilizing protein, hydrogen carbonate, inorganic carbon, mass spectrometry, non-heme iron, oxygen-evolving complex},
	pages = {210--216},
}

In this study, we presented a new approach for quantification of bicarbonate (HCO3−) molecules bound to PSII. Our method, which is based on a combination of membrane-inlet mass spectrometry (MIMS) and 18O-labelling, excludes the possibility of “non-accounted” HCO3− by avoiding (1) the employment of formate for removal of HCO3− from PSII, and (2) the extremely low concentrations of HCO3−/CO2 during online MIMS measurements. By equilibration of PSII sample to ambient CO2 concentration of dissolved CO2/HCO3−, the method ensures that all physiological binding sites are saturated before analysis. With this approach, we determined that in spinach PSII membrane fragments 1.1 ± 0.1 HCO3− are bound per PSII reaction center, while none was bound to isolated PsbO protein. Our present results confirmed that PSII binds one HCO3− molecule as ligand to the non-heme iron of PSII, while unbound HCO3− optimizes the water-splitting reactions by acting as a mobile proton shuttle.

@article{kern_structures_2018,
	title = {Structures of the intermediates of {Kok}’s photosynthetic water oxidation clock},
	volume = {563},
	issn = {0028-0836, 1476-4687},
	url = {http://www.nature.com/articles/s41586-018-0681-2},
	doi = {10.1038/s41586-018-0681-2},
	language = {en},
	number = {7731},
	urldate = {2021-06-07},
	journal = {Nature},
	author = {Kern, Jan and Chatterjee, Ruchira and Young, Iris D. and Fuller, Franklin D. and Lassalle, Louise and Ibrahim, Mohamed and Gul, Sheraz and Fransson, Thomas and Brewster, Aaron S. and Alonso-Mori, Roberto and Hussein, Rana and Zhang, Miao and Douthit, Lacey and de Lichtenberg, Casper and Cheah, Mun Hon and Shevela, Dmitry and Wersig, Julia and Seuffert, Ina and Sokaras, Dimosthenis and Pastor, Ernest and Weninger, Clemens and Kroll, Thomas and Sierra, Raymond G. and Aller, Pierre and Butryn, Agata and Orville, Allen M. and Liang, Mengning and Batyuk, Alexander and Koglin, Jason E. and Carbajo, Sergio and Boutet, Sébastien and Moriarty, Nigel W. and Holton, James M. and Dobbek, Holger and Adams, Paul D. and Bergmann, Uwe and Sauter, Nicholas K. and Zouni, Athina and Messinger, Johannes and Yano, Junko and Yachandra, Vittal K.},
	month = nov,
	year = {2018},
	pages = {421--425},
}

@article{kwong_cationic_2017,
	title = {Cationic {Vacancy} {Defects} in {Iron} {Phosphide}: {A} {Promising} {Route} toward {Efficient} and {Stable} {Hydrogen} {Evolution} by {Electrochemical} {Water} {Splitting}},
	volume = {10},
	copyright = {© 2017 The Authors. Published by Wiley-VCH Verlag GmbH \& Co. KGaA.},
	issn = {1864-564X},
	shorttitle = {Cationic {Vacancy} {Defects} in {Iron} {Phosphide}},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/cssc.201701565},
	doi = {10.1002/cssc.201701565},
	abstract = {Engineering the electronic properties of transition metal phosphides has shown great effectiveness in improving their intrinsic catalytic activity for the hydrogen evolution reaction (HER) in water splitting applications. Herein, we report for the first time, the creation of Fe vacancies as an approach to modulate the electronic structure of iron phosphide (FeP). The Fe vacancies were produced by chemical leaching of Mg that was introduced into FeP as “sacrificial dopant”. The obtained Fevacancy-rich FeP nanoparticulate films, which were deposited on Ti foil, show excellent HER activity compared to pristine FeP and Mg-doped FeP, achieving a current density of 10 mA cm−2 at overpotentials of 108 mV in 1 m KOH and 65 mV in 0.5 m H2SO4, with a near-100 \% Faradaic efficiency. Our theoretical and experimental analyses reveal that the improved HER activity originates from the presence of Fe vacancies, which lead to a synergistic modulation of the structural and electronic properties that result in a near-optimal hydrogen adsorption free energy and enhanced proton trapping. The success in catalytic improvement through the introduction of cationic vacancy defects has not only demonstrated the potential of Fe-vacancy-rich FeP as highly efficient, earth abundant HER catalyst, but also opens up an exciting pathway for activating other promising catalysts for electrochemical water splitting.},
	language = {en},
	number = {22},
	urldate = {2024-12-10},
	journal = {ChemSusChem},
	author = {Kwong, Wai Ling and Gracia-Espino, Eduardo and Lee, Cheng Choo and Sandström, Robin and Wågberg, Thomas and Messinger, Johannes},
	year = {2017},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.201701565},
	keywords = {artificial photosynthesis, cation vacancy, iron phosphide, sacrificial dopant, solar fuels},
	pages = {4544--4551},
}

Engineering the electronic properties of transition metal phosphides has shown great effectiveness in improving their intrinsic catalytic activity for the hydrogen evolution reaction (HER) in water splitting applications. Herein, we report for the first time, the creation of Fe vacancies as an approach to modulate the electronic structure of iron phosphide (FeP). The Fe vacancies were produced by chemical leaching of Mg that was introduced into FeP as “sacrificial dopant”. The obtained Fevacancy-rich FeP nanoparticulate films, which were deposited on Ti foil, show excellent HER activity compared to pristine FeP and Mg-doped FeP, achieving a current density of 10 mA cm−2 at overpotentials of 108 mV in 1 m KOH and 65 mV in 0.5 m H2SO4, with a near-100 % Faradaic efficiency. Our theoretical and experimental analyses reveal that the improved HER activity originates from the presence of Fe vacancies, which lead to a synergistic modulation of the structural and electronic properties that result in a near-optimal hydrogen adsorption free energy and enhanced proton trapping. The success in catalytic improvement through the introduction of cationic vacancy defects has not only demonstrated the potential of Fe-vacancy-rich FeP as highly efficient, earth abundant HER catalyst, but also opens up an exciting pathway for activating other promising catalysts for electrochemical water splitting.

@article{fuller_drop--demand_2017,
	title = {Drop-on-demand sample delivery for studying biocatalysts in action at {X}-ray free-electron lasers},
	volume = {14},
	copyright = {2017 Springer Nature America, Inc.},
	issn = {1548-7105},
	url = {https://www.nature.com/articles/nmeth.4195},
	doi = {10.1038/nmeth.4195},
	abstract = {A robust acoustic droplet ejection–drop-on-tape method delivers samples to an X-ray free-electron laser source for combined serial femtosecond crystallography and X-ray emission spectroscopy analysis, providing detailed insights into macromolecular reaction dynamics.},
	language = {en},
	number = {4},
	urldate = {2024-12-10},
	journal = {Nature Methods},
	publisher = {Nature Publishing Group},
	author = {Fuller, Franklin D. and Gul, Sheraz and Chatterjee, Ruchira and Burgie, E. Sethe and Young, Iris D. and Lebrette, Hugo and Srinivas, Vivek and Brewster, Aaron S. and Michels-Clark, Tara and Clinger, Jonathan A. and Andi, Babak and Ibrahim, Mohamed and Pastor, Ernest and de Lichtenberg, Casper and Hussein, Rana and Pollock, Christopher J. and Zhang, Miao and Stan, Claudiu A. and Kroll, Thomas and Fransson, Thomas and Weninger, Clemens and Kubin, Markus and Aller, Pierre and Lassalle, Louise and Bräuer, Philipp and Miller, Mitchell D. and Amin, Muhamed and Koroidov, Sergey and Roessler, Christian G. and Allaire, Marc and Sierra, Raymond G. and Docker, Peter T. and Glownia, James M. and Nelson, Silke and Koglin, Jason E. and Zhu, Diling and Chollet, Matthieu and Song, Sanghoon and Lemke, Henrik and Liang, Mengning and Sokaras, Dimosthenis and Alonso-Mori, Roberto and Zouni, Athina and Messinger, Johannes and Bergmann, Uwe and Boal, Amie K. and Bollinger, J. Martin and Krebs, Carsten and Högbom, Martin and Phillips, George N. and Vierstra, Richard D. and Sauter, Nicholas K. and Orville, Allen M. and Kern, Jan and Yachandra, Vittal K. and Yano, Junko},
	month = apr,
	year = {2017},
	keywords = {Biocatalysis, Biophysical methods, Enzymes, Molecular biophysics, Nanocrystallography},
	pages = {443--449},
}

A robust acoustic droplet ejection–drop-on-tape method delivers samples to an X-ray free-electron laser source for combined serial femtosecond crystallography and X-ray emission spectroscopy analysis, providing detailed insights into macromolecular reaction dynamics.

@article{melder_electrocatalytic_2017,
	title = {Electrocatalytic {Water} {Oxidation} by {MnO}/{C}: {In} {Situ} {Catalyst} {Formation}, {Carbon} {Substrate} {Variations}, and {Direct} {O2}/{CO2} {Monitoring} by {Membrane}-{Inlet} {Mass} {Spectrometry}},
	volume = {10},
	copyright = {© 2017 Wiley-VCH Verlag GmbH \& Co. KGaA, Weinheim},
	issn = {1864-564X},
	shorttitle = {Electrocatalytic {Water} {Oxidation} by {MnO}/{C}},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/cssc.201701383},
	doi = {10.1002/cssc.201701383},
	abstract = {Layers of amorphous manganese oxides were directly formed on the surfaces of different carbon materials by exposing the carbon to aqueous solutions of permanganate (MnO4−) followed by sintering at 100–400 °C. During electrochemical measurements in neutral aqueous buffer, nearly all of the MnOx/C electrodes show significant oxidation currents at potentials relevant for the oxygen evolution reaction (OER). However, by combining electrolysis with product detection by using mass spectrometry, it was found that these currents were only strictly linked to water oxidation if MnOx was deposited on graphitic carbon materials (faradaic O2 yields {\textgreater}90 \%). On the contrary, supports containing sp3-C were found to be unsuitable as the OER is accompanied by carbon corrosion to CO2. Thus, choosing the “right” carbon material is crucial for the preparation of stable and efficient MnOx/C anodes for water oxidation catalysis. For MnOx on graphitic substrates, current densities of {\textgreater}1 mA cm−2 at η=540 mV could be maintained for at least 16 h of continuous operation at pH 7 (very good values for electrodes containing only abundant elements such as C, O, and Mn) and post-operando measurements proved the integrity of both the catalyst coating and the underlying carbon at OER conditions.},
	language = {en},
	number = {22},
	urldate = {2024-12-10},
	journal = {ChemSusChem},
	author = {Melder, Jens and Kwong, Wai Ling and Shevela, Dmitriy and Messinger, Johannes and Kurz, Philipp},
	year = {2017},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.201701383},
	keywords = {carbon materials, electrocatalysis, manganese, mass spectrometry, oxides},
	pages = {4491--4502},
}

Layers of amorphous manganese oxides were directly formed on the surfaces of different carbon materials by exposing the carbon to aqueous solutions of permanganate (MnO4−) followed by sintering at 100–400 °C. During electrochemical measurements in neutral aqueous buffer, nearly all of the MnOx/C electrodes show significant oxidation currents at potentials relevant for the oxygen evolution reaction (OER). However, by combining electrolysis with product detection by using mass spectrometry, it was found that these currents were only strictly linked to water oxidation if MnOx was deposited on graphitic carbon materials (faradaic O2 yields \textgreater90 %). On the contrary, supports containing sp3-C were found to be unsuitable as the OER is accompanied by carbon corrosion to CO2. Thus, choosing the “right” carbon material is crucial for the preparation of stable and efficient MnOx/C anodes for water oxidation catalysis. For MnOx on graphitic substrates, current densities of \textgreater1 mA cm−2 at η=540 mV could be maintained for at least 16 h of continuous operation at pH 7 (very good values for electrodes containing only abundant elements such as C, O, and Mn) and post-operando measurements proved the integrity of both the catalyst coating and the underlying carbon at OER conditions.

@article{kwong_scalable_2017,
	title = {Scalable {Two}-{Step} {Synthesis} of {Nickel}–{Iron} {Phosphide} {Electrodes} for {Stable} and {Efficient} {Electrocatalytic} {Hydrogen} {Evolution}},
	volume = {121},
	issn = {1932-7447},
	url = {https://doi.org/10.1021/acs.jpcc.6b09050},
	doi = {10.1021/acs.jpcc.6b09050},
	abstract = {The development of efficient, durable, and inexpensive hydrogen evolution electrodes remains a key challenge for realizing a sustainable H2 fuel production via electrocatalytic water splitting. Herein, nickel–iron phosphide porous films with precisely controlled metal content were synthesized on Ti foil using a simple and scalable two-step strategy of spray-pyrolysis deposition followed by low-temperature phosphidation. The nickel–iron phosphide of an optimized Ni:Fe ratio of 1:4 demonstrated excellent overall catalytic activity for hydrogen evolution reaction (HER) in 0.5 M H2SO4, achieving current densities of −10 and −30 mA cm–2 at overpotentials of 101 and 123 mV, respectively, with a Tafel slope of 43 mV dec–1. Detailed analysis obtained by X-ray diffraction, electron microscopy, electrochemistry, and X-ray photoelectron spectroscopy revealed that the superior overall HER activity of nickel–iron phosphide as compared to nickel phosphide and iron phosphide was a combined effect of differences in the morphology (real surface area) and the intrinsic catalytic properties (electronic structure). Together with a long-term stability and a near-100\% Faradaic efficiency, the nickel–iron phosphide electrodes produced in this study provide blueprints for large-scale H2 production.},
	number = {1},
	urldate = {2024-12-10},
	journal = {The Journal of Physical Chemistry C},
	publisher = {American Chemical Society},
	author = {Kwong, Wai Ling and Lee, Cheng Choo and Messinger, Johannes},
	month = jan,
	year = {2017},
	pages = {284--292},
}

The development of efficient, durable, and inexpensive hydrogen evolution electrodes remains a key challenge for realizing a sustainable H2 fuel production via electrocatalytic water splitting. Herein, nickel–iron phosphide porous films with precisely controlled metal content were synthesized on Ti foil using a simple and scalable two-step strategy of spray-pyrolysis deposition followed by low-temperature phosphidation. The nickel–iron phosphide of an optimized Ni:Fe ratio of 1:4 demonstrated excellent overall catalytic activity for hydrogen evolution reaction (HER) in 0.5 M H2SO4, achieving current densities of −10 and −30 mA cm–2 at overpotentials of 101 and 123 mV, respectively, with a Tafel slope of 43 mV dec–1. Detailed analysis obtained by X-ray diffraction, electron microscopy, electrochemistry, and X-ray photoelectron spectroscopy revealed that the superior overall HER activity of nickel–iron phosphide as compared to nickel phosphide and iron phosphide was a combined effect of differences in the morphology (real surface area) and the intrinsic catalytic properties (electronic structure). Together with a long-term stability and a near-100% Faradaic efficiency, the nickel–iron phosphide electrodes produced in this study provide blueprints for large-scale H2 production.

@article{kubin_soft_2017,
	title = {Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers},
	volume = {4},
	issn = {2329-7778},
	url = {https://doi.org/10.1063/1.4986627},
	doi = {10.1063/1.4986627},
	abstract = {X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6–15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn \< 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions.},
	number = {5},
	urldate = {2024-12-10},
	journal = {Structural Dynamics},
	author = {Kubin, Markus and Kern, Jan and Gul, Sheraz and Kroll, Thomas and Chatterjee, Ruchira and Löchel, Heike and Fuller, Franklin D. and Sierra, Raymond G. and Quevedo, Wilson and Weniger, Christian and Rehanek, Jens and Firsov, Anatoly and Laksmono, Hartawan and Weninger, Clemens and Alonso-Mori, Roberto and Nordlund, Dennis L. and Lassalle-Kaiser, Benedikt and Glownia, James M. and Krzywinski, Jacek and Moeller, Stefan and Turner, Joshua J. and Minitti, Michael P. and Dakovski, Georgi L. and Koroidov, Sergey and Kawde, Anurag and Kanady, Jacob S. and Tsui, Emily Y. and Suseno, Sandy and Han, Zhiji and Hill, Ethan and Taguchi, Taketo and Borovik, Andrew S. and Agapie, Theodor and Messinger, Johannes and Erko, Alexei and Föhlisch, Alexander and Bergmann, Uwe and Mitzner, Rolf and Yachandra, Vittal K. and Yano, Junko and Wernet, Philippe},
	month = sep,
	year = {2017},
	pages = {054307},
}

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6–15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions.

@article{christianson_tumor_2017,
	title = {Tumor antigen glycosaminoglycan modification regulates antibody-drug conjugate delivery and cytotoxicity},
	volume = {8},
	issn = {1949-2553},
	url = {https://www.oncotarget.com/article/16921/text/},
	doi = {10.18632/oncotarget.16921},
	abstract = {https://doi.org/10.18632/oncotarget.16921 Helena C. Christianson, Julien A. Menard, Vineesh Indira Chandran, Erika Bourseau-Guilmain, Dmitry Shevela, Jon Lidfeldt, Ann-Sofie Månsson, Silvia...},
	language = {en},
	number = {40},
	urldate = {2024-12-10},
	journal = {Oncotarget},
	publisher = {Impact Journals},
	author = {Christianson, Helena C. and Menard, Julien A. and Chandran, Vineesh Indira and Bourseau-Guilmain, Erika and Shevela, Dmitry and Lidfeldt, Jon and Månsson, Ann-Sofie and Pastorekova, Silvia and Messinger, Johannes and Belting, Mattias},
	month = apr,
	year = {2017},
	pages = {66960--66974},
}

https://doi.org/10.18632/oncotarget.16921 Helena C. Christianson, Julien A. Menard, Vineesh Indira Chandran, Erika Bourseau-Guilmain, Dmitry Shevela, Jon Lidfeldt, Ann-Sofie Månsson, Silvia...

@article{shevela_biogenesis_2016,
	title = {Biogenesis of water splitting by photosystem {II} during de-etiolation of barley ({Hordeum} vulgare {L}.)},
	volume = {39},
	copyright = {© 2016 John Wiley \& Sons Ltd},
	issn = {1365-3040},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/pce.12719},
	doi = {10.1111/pce.12719},
	abstract = {Etioplasts lack thylakoid membranes and photosystem complexes. Light triggers differentiation of etioplasts into mature chloroplasts, and photosystem complexes assemble in parallel with thylakoid membrane development. Plastids isolated at various time points of de-etiolation are ideal to study the kinetic biogenesis of photosystem complexes during chloroplast development. Here, we investigated the chronology of photosystem II (PSII) biogenesis by monitoring assembly status of chlorophyll-binding protein complexes and development of water splitting via O2 production in plastids (etiochloroplasts) isolated during de-etiolation of barley (Hordeum vulgare L.). Assembly of PSII monomers, dimers and complexes binding outer light-harvesting antenna [PSII-light-harvesting complex II (LHCII) supercomplexes] was identified after 1, 2 and 4 h of de-etiolation, respectively. Water splitting was detected in parallel with assembly of PSII monomers, and its development correlated with an increase of bound Mn in the samples. After 4 h of de-etiolation, etiochloroplasts revealed the same water-splitting efficiency as mature chloroplasts. We conclude that the capability of PSII to split water during de-etiolation precedes assembly of the PSII-LHCII supercomplexes. Taken together, data show a rapid establishment of water-splitting activity during etioplast-to-chloroplast transition and emphasize that assembly of the functional water-splitting site of PSII is not the rate-limiting step in the formation of photoactive thylakoid membranes.},
	language = {en},
	number = {7},
	urldate = {2024-12-10},
	journal = {Plant, Cell \& Environment},
	author = {Shevela, Dmitriy and Arnold, Janine and Reisinger, Veronika and Berends, Hans-Martin and Kmiec, Karol and Koroidov, Sergey and Bue, Ann Kristin and Messinger, Johannes and Eichacker, Lutz A.},
	year = {2016},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1111/pce.12719},
	keywords = {chloroplast biogenesis, oxygen evolution, oxygen-evolving complex, photosystem II assembly},
	pages = {1524--1536},
}

Etioplasts lack thylakoid membranes and photosystem complexes. Light triggers differentiation of etioplasts into mature chloroplasts, and photosystem complexes assemble in parallel with thylakoid membrane development. Plastids isolated at various time points of de-etiolation are ideal to study the kinetic biogenesis of photosystem complexes during chloroplast development. Here, we investigated the chronology of photosystem II (PSII) biogenesis by monitoring assembly status of chlorophyll-binding protein complexes and development of water splitting via O2 production in plastids (etiochloroplasts) isolated during de-etiolation of barley (Hordeum vulgare L.). Assembly of PSII monomers, dimers and complexes binding outer light-harvesting antenna [PSII-light-harvesting complex II (LHCII) supercomplexes] was identified after 1, 2 and 4 h of de-etiolation, respectively. Water splitting was detected in parallel with assembly of PSII monomers, and its development correlated with an increase of bound Mn in the samples. After 4 h of de-etiolation, etiochloroplasts revealed the same water-splitting efficiency as mature chloroplasts. We conclude that the capability of PSII to split water during de-etiolation precedes assembly of the PSII-LHCII supercomplexes. Taken together, data show a rapid establishment of water-splitting activity during etioplast-to-chloroplast transition and emphasize that assembly of the functional water-splitting site of PSII is not the rate-limiting step in the formation of photoactive thylakoid membranes.

@article{nilsson_estimation_2016,
	title = {Estimation of the driving force for dioxygen formation in photosynthesis},
	volume = {1857},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272815001991},
	doi = {10.1016/j.bbabio.2015.09.011},
	abstract = {Photosynthetic water oxidation to molecular oxygen is carried out by photosystem II (PSII) over a reaction cycle involving four photochemical steps that drive the oxygen-evolving complex through five redox states Si (i=0,…, 4). For understanding the catalytic strategy of biological water oxidation it is important to elucidate the energetic landscape of PSII and in particular that of the final S4→S0 transition. In this short-lived chemical step the four oxidizing equivalents accumulated in the preceding photochemical events are used up to form molecular oxygen, two protons are released and at least one substrate water molecule binds to the Mn4CaO5 cluster. In this study we probed the probability to form S4 from S0 and O2 by incubating YD-less PSII in the S0 state for 2–3days in the presence of 18O2 and H216O. The absence of any measurable 16,18O2 formation by water-exchange in the S4 state suggests that the S4 state is hardly ever populated. On the basis of a detailed analysis we determined that the equilibrium constant K of the S4→S0 transition is larger than 1.0×107 so that this step is highly exergonic. We argue that this finding is consistent with current knowledge of the energetics of the S0 to S4 reactions, and that the high exergonicity is required for the kinetic efficiency of PSII.},
	number = {1},
	urldate = {2024-12-10},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Nilsson, Håkan and Cournac, Laurent and Rappaport, Fabrice and Messinger, Johannes and Lavergne, Jérôme},
	month = jan,
	year = {2016},
	keywords = {Equilibrium constant for S→S transition, Oxygen-evolving complex (OEC), Photosystem II, Water-oxidizing complex (WOC)},
	pages = {23--33},
}

Photosynthetic water oxidation to molecular oxygen is carried out by photosystem II (PSII) over a reaction cycle involving four photochemical steps that drive the oxygen-evolving complex through five redox states Si (i=0,…, 4). For understanding the catalytic strategy of biological water oxidation it is important to elucidate the energetic landscape of PSII and in particular that of the final S4→S0 transition. In this short-lived chemical step the four oxidizing equivalents accumulated in the preceding photochemical events are used up to form molecular oxygen, two protons are released and at least one substrate water molecule binds to the Mn4CaO5 cluster. In this study we probed the probability to form S4 from S0 and O2 by incubating YD-less PSII in the S0 state for 2–3days in the presence of 18O2 and H216O. The absence of any measurable 16,18O2 formation by water-exchange in the S4 state suggests that the S4 state is hardly ever populated. On the basis of a detailed analysis we determined that the equilibrium constant K of the S4→S0 transition is larger than 1.0×107 so that this step is highly exergonic. We argue that this finding is consistent with current knowledge of the energetics of the S0 to S4 reactions, and that the high exergonicity is required for the kinetic efficiency of PSII.

@article{siggel_gernot_2016,
	title = {Gernot {Renger} (1937–2013): his life, {Max}-{Volmer} {Laboratory}, and photosynthesis research},
	volume = {129},
	issn = {1573-5079},
	shorttitle = {Gernot {Renger} (1937–2013)},
	url = {https://doi.org/10.1007/s11120-016-0280-8},
	doi = {10.1007/s11120-016-0280-8},
	abstract = {Gernot Renger (October 23, 1937–January 12, 2013), one of the leading biophysicists in the field of photosynthesis research, studied and worked at the Max-Volmer-Institute (MVI) of the Technische Universität Berlin, Germany, for more than 50 years, and thus witnessed the rise and decline of photosynthesis research at this institute, which at its prime was one of the leading centers in this field. We present a tribute to Gernot Renger’s work and life in the context of the history of photosynthesis research of that period, with special focus on the MVI. Gernot will be remembered for his thought-provoking questions and his boundless enthusiasm for science.},
	language = {en},
	number = {2},
	urldate = {2024-12-10},
	journal = {Photosynthesis Research},
	author = {Siggel, Ulrich and Schmitt, Franz-Josef and Messinger, Johannes},
	month = aug,
	year = {2016},
	keywords = {ADRY agent, Horst T. Witt, Max-Volmer-Institute, Mechanism of water splitting, Oxygen evolving complex, Photosystem II, Water oxidizing complex (WOC)},
	pages = {109--127},
}

Gernot Renger (October 23, 1937–January 12, 2013), one of the leading biophysicists in the field of photosynthesis research, studied and worked at the Max-Volmer-Institute (MVI) of the Technische Universität Berlin, Germany, for more than 50 years, and thus witnessed the rise and decline of photosynthesis research at this institute, which at its prime was one of the leading centers in this field. We present a tribute to Gernot Renger’s work and life in the context of the history of photosynthesis research of that period, with special focus on the MVI. Gernot will be remembered for his thought-provoking questions and his boundless enthusiasm for science.

@article{sharifi_maghemite_2016,
	title = {Maghemite nanorods anchored on a {3D} nitrogen-doped carbon nanotubes substrate as scalable direct electrode for water oxidation},
	volume = {41},
	issn = {0360-3199},
	url = {https://www.sciencedirect.com/science/article/pii/S0360319915027433},
	doi = {10.1016/j.ijhydene.2015.11.165},
	abstract = {A hybrid catalyst 3D electrode for electrochemical water oxidation to molecular oxygen is presented. The electrode comprises needle shaped maghemite nanorods firmly anchored to nitrogen doped carbon nanotubes, which in turn are grown on a conducting carbon paper that acts as efficient current collector. In 0.1 M KOH this hybrid electrode reaches a current density of 1 mA/cm2 (geometric surface) at an overpotential of 362 mV performing high chronoamperometric stability. The electrochemical attributes point toward efficient catalytic processes at the surface of the maghemite nanorods, and demonstrate a very high surface area of the 3D electrode, as well as a firm anchoring of each active component enabling an efficient charge transport from the surface of the maghemite rods to the carbon paper current collector. The latter property also explains the good stability of our hybrid electrode compared to transition metal oxides deposited on conducting support such as fluorine doped tin oxide. These results introduce maghemite as efficient, stable and earth abundant oxygen evolution reaction catalyst, and provide insight into key issues for obtaining practical electrodes for oxygen evolution reaction, which are compatible with large scale production processes.},
	number = {1},
	urldate = {2024-12-10},
	journal = {International Journal of Hydrogen Energy},
	author = {Sharifi, Tiva and Kwong, Wai Ling and Berends, Hans-Martin and Larsen, Christian and Messinger, Johannes and Wågberg, Thomas},
	month = jan,
	year = {2016},
	keywords = {3D electrode, Hybrid catalyst, Maghemite, Nitrogen-doped carbon nanotubes, Water oxidation},
	pages = {69--78},
}

A hybrid catalyst 3D electrode for electrochemical water oxidation to molecular oxygen is presented. The electrode comprises needle shaped maghemite nanorods firmly anchored to nitrogen doped carbon nanotubes, which in turn are grown on a conducting carbon paper that acts as efficient current collector. In 0.1 M KOH this hybrid electrode reaches a current density of 1 mA/cm2 (geometric surface) at an overpotential of 362 mV performing high chronoamperometric stability. The electrochemical attributes point toward efficient catalytic processes at the surface of the maghemite nanorods, and demonstrate a very high surface area of the 3D electrode, as well as a firm anchoring of each active component enabling an efficient charge transport from the surface of the maghemite rods to the carbon paper current collector. The latter property also explains the good stability of our hybrid electrode compared to transition metal oxides deposited on conducting support such as fluorine doped tin oxide. These results introduce maghemite as efficient, stable and earth abundant oxygen evolution reaction catalyst, and provide insight into key issues for obtaining practical electrodes for oxygen evolution reaction, which are compatible with large scale production processes.

@article{sauter_no_2016,
	title = {No observable conformational changes in {PSII}},
	volume = {533},
	copyright = {2016 Springer Nature Limited},
	issn = {1476-4687},
	url = {https://www.nature.com/articles/nature17983},
	doi = {10.1038/nature17983},
	language = {en},
	number = {7603},
	urldate = {2024-12-10},
	journal = {Nature},
	publisher = {Nature Publishing Group},
	author = {Sauter, Nicholas K. and Echols, Nathaniel and Adams, Paul D. and Zwart, Petrus H. and Kern, Jan and Brewster, Aaron S. and Koroidov, Sergey and Alonso-Mori, Roberto and Zouni, Athina and Messinger, Johannes and Bergmann, Uwe and Yano, Junko and Yachandra, Vittal K.},
	month = may,
	year = {2016},
	keywords = {Photosystem II, X-ray crystallography},
	pages = {E1--E2},
}

@article{pham_probing_2016,
	title = {Probing {S}-state advancements and recombination pathways in photosystem {II} with a global fit program for flash-induced oxygen evolution pattern},
	volume = {1857},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272816300603},
	doi = {10.1016/j.bbabio.2016.03.013},
	abstract = {The oxygen-evolving complex (OEC) in photosystem II catalyzes the oxidation of water to molecular oxygen. Four decades ago, measurements of flash-induced oxygen evolution have shown that the OEC steps through oxidation states S0, S1, S2, S3 and S4 before O2 is released and the S0 state is reformed. The light-induced transitions between these states involve misses and double hits. While it is widely accepted that the miss parameter is S state dependent and may be further modulated by the oxidation state of the acceptor side, the traditional way of analyzing each flash-induced oxygen evolution pattern (FIOP) individually did not allow using enough free parameters to thoroughly test this proposal. Furthermore, this approach does not allow assessing whether the presently known recombination processes in photosystem II fully explain all measured oxygen yields during Si state lifetime measurements. Here we present a global fit program that simultaneously fits all flash-induced oxygen yields of a standard FIOP (2Hz flash frequency) and of 11–18 FIOPs each obtained while probing the S0, S2 and S3 state lifetimes in spinach thylakoids at neutral pH. This comprehensive data treatment demonstrates the presence of a very slow phase of S2 decay, in addition to the commonly discussed fast and slow reduction of S2 by YD and QB−, respectively. Our data support previous suggestions that the S0→S1 and S1→S2 transitions involve low or no misses, while high misses occur in the S2→S3 or S3→S0 transitions.},
	number = {6},
	urldate = {2024-12-10},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Pham, Long Vo and Messinger, Johannes},
	month = jun,
	year = {2016},
	keywords = {Flash induced oxygen evolution pattern (FIOP), Kok model, Oxygen (O) evolution, Photosynthesis, Photosystem 2 (PSII), Water oxidation},
	pages = {848--859},
}

The oxygen-evolving complex (OEC) in photosystem II catalyzes the oxidation of water to molecular oxygen. Four decades ago, measurements of flash-induced oxygen evolution have shown that the OEC steps through oxidation states S0, S1, S2, S3 and S4 before O2 is released and the S0 state is reformed. The light-induced transitions between these states involve misses and double hits. While it is widely accepted that the miss parameter is S state dependent and may be further modulated by the oxidation state of the acceptor side, the traditional way of analyzing each flash-induced oxygen evolution pattern (FIOP) individually did not allow using enough free parameters to thoroughly test this proposal. Furthermore, this approach does not allow assessing whether the presently known recombination processes in photosystem II fully explain all measured oxygen yields during Si state lifetime measurements. Here we present a global fit program that simultaneously fits all flash-induced oxygen yields of a standard FIOP (2Hz flash frequency) and of 11–18 FIOPs each obtained while probing the S0, S2 and S3 state lifetimes in spinach thylakoids at neutral pH. This comprehensive data treatment demonstrates the presence of a very slow phase of S2 decay, in addition to the commonly discussed fast and slow reduction of S2 by YD and QB−, respectively. Our data support previous suggestions that the S0→S1 and S1→S2 transitions involve low or no misses, while high misses occur in the S2→S3 or S3→S0 transitions.

@article{young_structure_2016,
	title = {Structure of photosystem {II} and substrate binding at room temperature},
	volume = {540},
	copyright = {2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.},
	issn = {1476-4687},
	url = {https://www.nature.com/articles/nature20161},
	doi = {10.1038/nature20161},
	abstract = {The structures of three intermediate states of photosystem II, which is crucial for photosynthesis, have been solved at room temperature, shedding new light on this process.},
	language = {en},
	number = {7633},
	urldate = {2024-12-10},
	journal = {Nature},
	publisher = {Nature Publishing Group},
	author = {Young, Iris D. and Ibrahim, Mohamed and Chatterjee, Ruchira and Gul, Sheraz and Fuller, Franklin D. and Koroidov, Sergey and Brewster, Aaron S. and Tran, Rosalie and Alonso-Mori, Roberto and Kroll, Thomas and Michels-Clark, Tara and Laksmono, Hartawan and Sierra, Raymond G. and Stan, Claudiu A. and Hussein, Rana and Zhang, Miao and Douthit, Lacey and Kubin, Markus and de Lichtenberg, Casper and Vo Pham, Long and Nilsson, Håkan and Cheah, Mun Hon and Shevela, Dmitriy and Saracini, Claudio and Bean, Mackenzie A. and Seuffert, Ina and Sokaras, Dimosthenis and Weng, Tsu-Chien and Pastor, Ernest and Weninger, Clemens and Fransson, Thomas and Lassalle, Louise and Bräuer, Philipp and Aller, Pierre and Docker, Peter T. and Andi, Babak and Orville, Allen M. and Glownia, James M. and Nelson, Silke and Sikorski, Marcin and Zhu, Diling and Hunter, Mark S. and Lane, Thomas J. and Aquila, Andy and Koglin, Jason E. and Robinson, Joseph and Liang, Mengning and Boutet, Sébastien and Lyubimov, Artem Y. and Uervirojnangkoorn, Monarin and Moriarty, Nigel W. and Liebschner, Dorothee and Afonine, Pavel V. and Waterman, David G. and Evans, Gwyndaf and Wernet, Philippe and Dobbek, Holger and Weis, William I. and Brunger, Axel T. and Zwart, Petrus H. and Adams, Paul D. and Zouni, Athina and Messinger, Johannes and Bergmann, Uwe and Sauter, Nicholas K. and Kern, Jan and Yachandra, Vittal K. and Yano, Junko},
	month = dec,
	year = {2016},
	keywords = {Bioenergetics, Biophysical chemistry, Nanocrystallography, Photosystem II},
	pages = {453--457},
}

The structures of three intermediate states of photosystem II, which is crucial for photosynthesis, have been solved at room temperature, shedding new light on this process.

@article{sharifi_toward_2016,
	title = {Toward a {Low}-{Cost} {Artificial} {Leaf}: {Driving} {Carbon}-{Based} and {Bifunctional} {Catalyst} {Electrodes} with {Solution}-{Processed} {Perovskite} {Photovoltaics}},
	volume = {6},
	copyright = {© 2016 The Authors. Published by WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
	issn = {1614-6840},
	shorttitle = {Toward a {Low}-{Cost} {Artificial} {Leaf}},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.201600738},
	doi = {10.1002/aenm.201600738},
	abstract = {Molecular hydrogen can be generated renewably by water splitting with an “artificial-leaf device��, which essentially comprises two electrocatalyst electrodes immersed in water and powered by photovoltaics. Ideally, this device should operate efficiently and be fabricated with cost-efficient means using earth-abundant materials. Here, a lightweight electrocatalyst electrode, comprising large surface-area NiCo2O4 nanorods that are firmly anchored onto a carbon–paper current collector via a dense network of nitrogen-doped carbon nanotubes is presented. This electrocatalyst electrode is bifunctional in that it can efficiently operate as both anode and cathode in the same alkaline solution, as quantified by a delivered current density of 10 mA cm−2 at an overpotential of 400 mV for each of the oxygen and hydrogen evolution reactions. By driving two such identical electrodes with a solution-processed thin-film perovskite photovoltaic assembly, a wired artificial-leaf device is obtained that features a Faradaic H2 evolution efficiency of 100\%, and a solar-to-hydrogen conversion efficiency of 6.2\%. A detailed cost analysis is presented, which implies that the material-payback time of this device is of the order of 100 days.},
	language = {en},
	number = {20},
	urldate = {2024-12-10},
	journal = {Advanced Energy Materials},
	author = {Sharifi, Tiva and Larsen, Christian and Wang, Jia and Kwong, Wai Ling and Gracia-Espino, Eduardo and Mercier, Guillaume and Messinger, Johannes and Wågberg, Thomas and Edman, Ludvig},
	year = {2016},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.201600738},
	keywords = {artificial-leaf devices, bifunctional electrocatalyst, carbon paper, nitrogen-doped carbon nanotubes, perovskite photovoltaics},
	pages = {1600738},
}

Molecular hydrogen can be generated renewably by water splitting with an “artificial-leaf device��, which essentially comprises two electrocatalyst electrodes immersed in water and powered by photovoltaics. Ideally, this device should operate efficiently and be fabricated with cost-efficient means using earth-abundant materials. Here, a lightweight electrocatalyst electrode, comprising large surface-area NiCo2O4 nanorods that are firmly anchored onto a carbon–paper current collector via a dense network of nitrogen-doped carbon nanotubes is presented. This electrocatalyst electrode is bifunctional in that it can efficiently operate as both anode and cathode in the same alkaline solution, as quantified by a delivered current density of 10 mA cm−2 at an overpotential of 400 mV for each of the oxygen and hydrogen evolution reactions. By driving two such identical electrodes with a solution-processed thin-film perovskite photovoltaic assembly, a wired artificial-leaf device is obtained that features a Faradaic H2 evolution efficiency of 100%, and a solar-to-hydrogen conversion efficiency of 6.2%. A detailed cost analysis is presented, which implies that the material-payback time of this device is of the order of 100 days.

@article{alonso-mori_towards_2016,
	title = {Towards characterization of photo-excited electron transfer and catalysis in natural and artificial systems using {XFELs}},
	volume = {194},
	issn = {1364-5498},
	url = {https://pubs.rsc.org/en/content/articlelanding/2016/fd/c6fd00084c},
	doi = {10.1039/C6FD00084C},
	abstract = {The ultra-bright femtosecond X-ray pulses provided by X-ray Free Electron Lasers (XFELs) open capabilities for studying the structure and dynamics of a wide variety of biological and inorganic systems beyond what is possible at synchrotron sources. Although the structure and chemistry at the catalytic sites have been studied intensively in both biological and inorganic systems, a full understanding of the atomic-scale chemistry requires new approaches beyond the steady state X-ray crystallography and X-ray spectroscopy at cryogenic temperatures. Following the dynamic changes in the geometric and electronic structure at ambient conditions, while overcoming X-ray damage to the redox active catalytic center, is key for deriving reaction mechanisms. Such studies become possible by using the intense and ultra-short femtosecond X-ray pulses from an XFEL, where sample is probed before it is damaged. We have developed methodology for simultaneously collecting X-ray diffraction data and X-ray emission spectra, using an energy dispersive spectrometer, at ambient conditions, and used this approach to study the room temperature structure and intermediate states of the photosynthetic water oxidizing metallo-protein, photosystem II. Moreover, we have also used this setup to simultaneously collect the X-ray emission spectra from multiple metals to follow the ultrafast dynamics of light-induced charge transfer between multiple metal sites. A Mn–Ti containing system was studied at an XFEL to demonstrate the efficacy and potential of this method.},
	language = {en},
	number = {0},
	urldate = {2024-12-10},
	journal = {Faraday Discussions},
	publisher = {The Royal Society of Chemistry},
	author = {Alonso-Mori, R. and Asa, K. and Bergmann, U. and Brewster, A. S. and Chatterjee, R. and Cooper, J. K. and Frei, H. M. and Fuller, F. D. and Goggins, E. and Gul, S. and Fukuzawa, H. and Iablonskyi, D. and Ibrahim, M. and Katayama, T. and Kroll, T. and Kumagai, Y. and McClure, B. A. and Messinger, J. and Motomura, K. and Nagaya, K. and Nishiyama, T. and Saracini, C. and Sato, Y. and Sauter, N. K. and Sokaras, D. and Takanashi, T. and Togashi, T. and Ueda, K. and Weare, W. W. and Weng, T.-C. and Yabashi, M. and Yachandra, V. K. and Young, I. D. and Zouni, A. and Kern, J. F. and Yano, J.},
	month = dec,
	year = {2016},
	pages = {621--638},
}

The ultra-bright femtosecond X-ray pulses provided by X-ray Free Electron Lasers (XFELs) open capabilities for studying the structure and dynamics of a wide variety of biological and inorganic systems beyond what is possible at synchrotron sources. Although the structure and chemistry at the catalytic sites have been studied intensively in both biological and inorganic systems, a full understanding of the atomic-scale chemistry requires new approaches beyond the steady state X-ray crystallography and X-ray spectroscopy at cryogenic temperatures. Following the dynamic changes in the geometric and electronic structure at ambient conditions, while overcoming X-ray damage to the redox active catalytic center, is key for deriving reaction mechanisms. Such studies become possible by using the intense and ultra-short femtosecond X-ray pulses from an XFEL, where sample is probed before it is damaged. We have developed methodology for simultaneously collecting X-ray diffraction data and X-ray emission spectra, using an energy dispersive spectrometer, at ambient conditions, and used this approach to study the room temperature structure and intermediate states of the photosynthetic water oxidizing metallo-protein, photosystem II. Moreover, we have also used this setup to simultaneously collect the X-ray emission spectra from multiple metals to follow the ultrafast dynamics of light-induced charge transfer between multiple metal sites. A Mn–Ti containing system was studied at an XFEL to demonstrate the efficacy and potential of this method.

@article{kwong_transparent_2016,
	title = {Transparent {Nanoparticulate} {FeOOH} {Improves} the {Performance} of a {WO3} {Photoanode} in a {Tandem} {Water}-{Splitting} {Device}},
	volume = {120},
	issn = {1932-7447},
	url = {https://doi.org/10.1021/acs.jpcc.6b02432},
	doi = {10.1021/acs.jpcc.6b02432},
	abstract = {Oxygen evolution catalysts (OEC) are often employed on the surface of photoactive, semiconducting photoanodes to boost their kinetics and stability during photoelectrochemical water oxidation. However, the necessity of using optically transparent OEC to avoid parasitic light absorption by the OEC under front-side illumination is often neglected. Here, we show that furnishing the surface of a WO3 photoanode with suitable loading of FeOOH as a transparent OEC improved the photocurrent density by 300\% at 1 V versus RHE and the initial photocurrent-to-O2 Faradaic efficiency from ∼70 to ∼100\%. The data from the photovoltammetry, electrochemical impedance, and gas evolution measurements show that these improvements were a combined result of reduced hole-transfer resistance for water oxidation, minimized surface recombination of charge carriers, and improved stability against photocorrosion of WO3. We demonstrate the utility of transparent FeOOH-coated WO3 in a solar-powered, tandem water-splitting device by combining it with a double-junction Si solar cell and a Ni–Mo hydrogen evolution catalyst. This device performed at a solar-to-hydrogen conversion efficiency of 1.8\% in near-neutral K2SO4 electrolyte.},
	number = {20},
	urldate = {2024-12-10},
	journal = {The Journal of Physical Chemistry C},
	publisher = {American Chemical Society},
	author = {Kwong, Wai Ling and Lee, Cheng Choo and Messinger, Johannes},
	month = may,
	year = {2016},
	pages = {10941--10950},
}

Oxygen evolution catalysts (OEC) are often employed on the surface of photoactive, semiconducting photoanodes to boost their kinetics and stability during photoelectrochemical water oxidation. However, the necessity of using optically transparent OEC to avoid parasitic light absorption by the OEC under front-side illumination is often neglected. Here, we show that furnishing the surface of a WO3 photoanode with suitable loading of FeOOH as a transparent OEC improved the photocurrent density by 300% at 1 V versus RHE and the initial photocurrent-to-O2 Faradaic efficiency from ∼70 to ∼100%. The data from the photovoltammetry, electrochemical impedance, and gas evolution measurements show that these improvements were a combined result of reduced hole-transfer resistance for water oxidation, minimized surface recombination of charge carriers, and improved stability against photocorrosion of WO3. We demonstrate the utility of transparent FeOOH-coated WO3 in a solar-powered, tandem water-splitting device by combining it with a double-junction Si solar cell and a Ni–Mo hydrogen evolution catalyst. This device performed at a solar-to-hydrogen conversion efficiency of 1.8% in near-neutral K2SO4 electrolyte.

@article{benlloch_crystal_2015,
	title = {Crystal structure and functional characterization of photosystem {II}-associated carbonic anhydrase {CAH3} in {Chlamydomonas} reinhardtii},
	volume = {167},
	issn = {1532-2548 (Electronic) 0032-0889 (Linking)},
	url = {https://www.ncbi.nlm.nih.gov/pubmed/25617045},
	doi = {10.1104/pp.114.253591},
	abstract = {In oxygenic photosynthesis, light energy is stored in the form of chemical energy by converting CO2 and water into carbohydrates. The light-driven oxidation of water that provides the electrons and protons for the subsequent CO2 fixation takes place in photosystem II (PSII). Recent studies show that in higher plants, HCO3 (-) increases PSII activity by acting as a mobile acceptor of the protons produced by PSII. In the green alga Chlamydomonas reinhardtii, a luminal carbonic anhydrase, CrCAH3, was suggested to improve proton removal from PSII, possibly by rapid reformation of HCO3 (-) from CO2. In this study, we investigated the interplay between PSII and CrCAH3 by membrane inlet mass spectrometry and x-ray crystallography. Membrane inlet mass spectrometry measurements showed that CrCAH3 was most active at the slightly acidic pH values prevalent in the thylakoid lumen under illumination. Two crystal structures of CrCAH3 in complex with either acetazolamide or phosphate ions were determined at 2.6- and 2.7-A resolution, respectively. CrCAH3 is a dimer at pH 4.1 that is stabilized by swapping of the N-terminal arms, a feature not previously observed in alpha-type carbonic anhydrases. The structure contains a disulfide bond, and redox titration of CrCAH3 function with dithiothreitol suggested a possible redox regulation of the enzyme. The stimulating effect of CrCAH3 and CO2/HCO3 (-) on PSII activity was demonstrated by comparing the flash-induced oxygen evolution pattern of wild-type and CrCAH3-less PSII preparations. We showed that CrCAH3 has unique structural features that allow this enzyme to maximize PSII activity at low pH and CO2 concentration.},
	language = {en},
	number = {3},
	urldate = {2021-06-07},
	journal = {Plant Physiol},
	author = {Benlloch, R. and Shevela, D. and Hainzl, T. and Grundstrom, C. and Shutova, T. and Messinger, J. and Samuelsson, G. and Sauer-Eriksson, A. E.},
	month = mar,
	year = {2015},
	note = {Edition: 2015/01/27},
	keywords = {Carbonic Anhydrase Inhibitors/pharmacology, Carbonic Anhydrases/*chemistry/*metabolism, Catalytic Domain, Chlamydomonas reinhardtii/*enzymology, Crystallography, X-Ray, Cysteine/metabolism, Disulfides/metabolism, Hydrogen-Ion Concentration, Mass Spectrometry, Mutation, Oxidation-Reduction/drug effects, Oxygen/metabolism, Photosystem II Protein Complex/*metabolism, Protein Structure, Secondary},
	pages = {950--62},
}

In oxygenic photosynthesis, light energy is stored in the form of chemical energy by converting CO2 and water into carbohydrates. The light-driven oxidation of water that provides the electrons and protons for the subsequent CO2 fixation takes place in photosystem II (PSII). Recent studies show that in higher plants, HCO3 (-) increases PSII activity by acting as a mobile acceptor of the protons produced by PSII. In the green alga Chlamydomonas reinhardtii, a luminal carbonic anhydrase, CrCAH3, was suggested to improve proton removal from PSII, possibly by rapid reformation of HCO3 (-) from CO2. In this study, we investigated the interplay between PSII and CrCAH3 by membrane inlet mass spectrometry and x-ray crystallography. Membrane inlet mass spectrometry measurements showed that CrCAH3 was most active at the slightly acidic pH values prevalent in the thylakoid lumen under illumination. Two crystal structures of CrCAH3 in complex with either acetazolamide or phosphate ions were determined at 2.6- and 2.7-A resolution, respectively. CrCAH3 is a dimer at pH 4.1 that is stabilized by swapping of the N-terminal arms, a feature not previously observed in alpha-type carbonic anhydrases. The structure contains a disulfide bond, and redox titration of CrCAH3 function with dithiothreitol suggested a possible redox regulation of the enzyme. The stimulating effect of CrCAH3 and CO2/HCO3 (-) on PSII activity was demonstrated by comparing the flash-induced oxygen evolution pattern of wild-type and CrCAH3-less PSII preparations. We showed that CrCAH3 has unique structural features that allow this enzyme to maximize PSII activity at low pH and CO2 concentration.

@article{koroidov_first_2015,
	title = {First turnover analysis of water-oxidation catalyzed by {Co}-oxide nanoparticles},
	volume = {8},
	issn = {1754-5706},
	url = {https://pubs.rsc.org/en/content/articlelanding/2015/ee/c5ee00700c},
	doi = {10.1039/C5EE00700C},
	abstract = {Co-oxides are promising water oxidation catalysts for artificial photosynthesis devices. Presently, several different proposals exist for how they catalyze O2 formation from water. Knowledge about this process at molecular detail will be required for their further improvement. Here we present time-resolved 18O-labelling isotope-ratio membrane-inlet mass spectrometry (MIMS) experiments to study the mechanism of water oxidation in Co/methylenediphosphonate (Co/M2P) oxide nanoparticles using [Ru(bpy)3]3+ (bpy = 2,2′-bipyridine) as chemical oxidant. We show that 16O–Co/M2P-oxide nanoparticles produce 16O2 during their first turnover after simultaneous addition of H218O and [Ru(bpy)3]3+, while sequential addition with a delay of 3 s yields oxygen reflecting bulk water 18O-enrichment. This result is interpreted to show that the O–O bond formation in Co/M2P-oxide nanoparticles occurs via intramolecular oxygen coupling between two terminal Co–OHn ligands that are readily exchangeable with bulk water in the resting state of the catalyst. Importantly, our data allow the determination of the number of catalytic sites within this amorphous nanoparticular material, to calculate the TOF per catalytic site and to derive the number of holes needed for the production of the first O2 molecule per catalytic site. We propose that the mechanism of O–O bond formation during bulk catalysis in amorphous Co-oxides may differ from that taking place at the surface of crystalline materials.},
	language = {en},
	number = {8},
	urldate = {2024-12-10},
	journal = {Energy \& Environmental Science},
	publisher = {The Royal Society of Chemistry},
	author = {Koroidov, Sergey and Anderlund, Magnus F. and Styring, Stenbjörn and Thapper, Anders and Messinger, Johannes},
	month = jul,
	year = {2015},
	pages = {2492--2503},
}

Co-oxides are promising water oxidation catalysts for artificial photosynthesis devices. Presently, several different proposals exist for how they catalyze O2 formation from water. Knowledge about this process at molecular detail will be required for their further improvement. Here we present time-resolved 18O-labelling isotope-ratio membrane-inlet mass spectrometry (MIMS) experiments to study the mechanism of water oxidation in Co/methylenediphosphonate (Co/M2P) oxide nanoparticles using [Ru(bpy)3]3+ (bpy = 2,2′-bipyridine) as chemical oxidant. We show that 16O–Co/M2P-oxide nanoparticles produce 16O2 during their first turnover after simultaneous addition of H218O and [Ru(bpy)3]3+, while sequential addition with a delay of 3 s yields oxygen reflecting bulk water 18O-enrichment. This result is interpreted to show that the O–O bond formation in Co/M2P-oxide nanoparticles occurs via intramolecular oxygen coupling between two terminal Co–OHn ligands that are readily exchangeable with bulk water in the resting state of the catalyst. Importantly, our data allow the determination of the number of catalytic sites within this amorphous nanoparticular material, to calculate the TOF per catalytic site and to derive the number of holes needed for the production of the first O2 molecule per catalytic site. We propose that the mechanism of O–O bond formation during bulk catalysis in amorphous Co-oxides may differ from that taking place at the surface of crystalline materials.

@incollection{yano_light-dependent_2015,
	address = {Cham},
	title = {Light-{Dependent} {Production} of {Dioxygen} in {Photosynthesis}},
	isbn = {978-3-319-12415-5},
	url = {https://doi.org/10.1007/978-3-319-12415-5_2},
	doi = {10.1007/978-3-319-12415-5_2},
	abstract = {Oxygen, that supports all aerobic life, is abundant in the atmosphere because of its constant regeneration by photosynthetic water oxidation, which is catalyzed by a Mn4CaO5 cluster in photosystem II (PS II), a multi subunit membrane protein complex. X-ray and other spectroscopy studies of the electronic and geometric structure of the Mn4CaO5 cluster as it advances through the intermediate states have been important for understanding the mechanism of water oxidation. The results and interpretations, especially from X-ray spectroscopy studies, regarding the geometric and electronic structure and the changes as the system proceeds through the catalytic cycle will be summarized in this review. This review will also include newer methodologies in time-resolved X-ray diffraction and spectroscopy that have become available since the commissioning of the X-ray free electron laser (XFEL) and are being applied to study the oxygen-evolving complex (OEC). The femtosecond X-ray pulses of the XFEL allows us to outrun X-ray damage at room temperature, and the time-evolution of the photo-induced reaction can be probed using a visible laser-pump followed by the X-ray-probe pulse. XFELs can be used to simultaneously determine the light-induced protein dynamics using crystallography and the local chemistry that occurs at the catalytic center using X-ray spectroscopy under functional conditions. Membrane inlet mass spectrometry has been important for providing direct information about the exchange of substrate water molecules, which has a direct bearing on the mechanism of water oxidation. Moreover, it has been indispensable for the time-resolved X-ray diffraction and spectroscopy studies and will be briefly reviewed in this chapter. Given the role of PS II in maintaining life in the biosphere and the future vision of a renewable energy economy, understanding the structure and mechanism of the photosynthetic water oxidation catalyst is an important goal for the future.},
	language = {en},
	urldate = {2024-12-10},
	booktitle = {Sustaining {Life} on {Planet} {Earth}: {Metalloenzymes} {Mastering} {Dioxygen} and {Other} {Chewy} {Gases}},
	publisher = {Springer International Publishing},
	author = {Yano, Junko and Kern, Jan and Yachandra, Vittal K. and Nilsson, Håkan and Koroidov, Sergey and Messinger, Johannes},
	editor = {Kroneck, Peter M. H and Sosa Torres, Martha E.},
	year = {2015},
	pages = {13--43},
}

Oxygen, that supports all aerobic life, is abundant in the atmosphere because of its constant regeneration by photosynthetic water oxidation, which is catalyzed by a Mn4CaO5 cluster in photosystem II (PS II), a multi subunit membrane protein complex. X-ray and other spectroscopy studies of the electronic and geometric structure of the Mn4CaO5 cluster as it advances through the intermediate states have been important for understanding the mechanism of water oxidation. The results and interpretations, especially from X-ray spectroscopy studies, regarding the geometric and electronic structure and the changes as the system proceeds through the catalytic cycle will be summarized in this review. This review will also include newer methodologies in time-resolved X-ray diffraction and spectroscopy that have become available since the commissioning of the X-ray free electron laser (XFEL) and are being applied to study the oxygen-evolving complex (OEC). The femtosecond X-ray pulses of the XFEL allows us to outrun X-ray damage at room temperature, and the time-evolution of the photo-induced reaction can be probed using a visible laser-pump followed by the X-ray-probe pulse. XFELs can be used to simultaneously determine the light-induced protein dynamics using crystallography and the local chemistry that occurs at the catalytic center using X-ray spectroscopy under functional conditions. Membrane inlet mass spectrometry has been important for providing direct information about the exchange of substrate water molecules, which has a direct bearing on the mechanism of water oxidation. Moreover, it has been indispensable for the time-resolved X-ray diffraction and spectroscopy studies and will be briefly reviewed in this chapter. Given the role of PS II in maintaining life in the biosphere and the future vision of a renewable energy economy, understanding the structure and mechanism of the photosynthetic water oxidation catalyst is an important goal for the future.

@article{krewald_metal_2015,
	title = {Metal oxidation states in biological water splitting},
	volume = {6},
	issn = {2041-6539},
	url = {https://pubs.rsc.org/en/content/articlelanding/2015/sc/c4sc03720k},
	doi = {10.1039/C4SC03720K},
	abstract = {A central question in biological water splitting concerns the oxidation states of the manganese ions that comprise the oxygen-evolving complex of photosystem II. Understanding the nature and order of oxidation events that occur during the catalytic cycle of five Si states (i = 0–4) is of fundamental importance both for the natural system and for artificial water oxidation catalysts. Despite the widespread adoption of the so-called “high-valent scheme”—where, for example, the Mn oxidation states in the S2 state are assigned as III, IV, IV, IV—the competing “low-valent scheme” that differs by a total of two metal unpaired electrons (i.e. III, III, III, IV in the S2 state) is favored by several recent studies for the biological catalyst. The question of the correct oxidation state assignment is addressed here by a detailed computational comparison of the two schemes using a common structural platform and theoretical approach. Models based on crystallographic constraints were constructed for all conceivable oxidation state assignments in the four (semi)stable S states of the oxygen evolving complex, sampling various protonation levels and patterns to ensure comprehensive coverage. The models are evaluated with respect to their geometric, energetic, electronic, and spectroscopic properties against available experimental EXAFS, XFEL-XRD, EPR, ENDOR and Mn K pre-edge XANES data. New 2.5 K 55Mn ENDOR data of the S2 state are also reported. Our results conclusively show that the entire S state phenomenology can only be accommodated within the high-valent scheme by adopting a single motif and protonation pattern that progresses smoothly from S0 (III, III, III, IV) to S3 (IV, IV, IV, IV), satisfying all experimental constraints and reproducing all observables. By contrast, it was impossible to construct a consistent cycle based on the low-valent scheme for all S states. Instead, the low-valent models developed here may provide new insight into the over-reduced S states and the states involved in the assembly of the catalytically active water oxidizing cluster.},
	language = {en},
	number = {3},
	urldate = {2024-12-10},
	journal = {Chemical Science},
	publisher = {The Royal Society of Chemistry},
	author = {Krewald, Vera and Retegan, Marius and Cox, Nicholas and Messinger, Johannes and Lubitz, Wolfgang and DeBeer, Serena and Neese, Frank and Pantazis, Dimitrios A.},
	month = feb,
	year = {2015},
	pages = {1676--1695},
}

A central question in biological water splitting concerns the oxidation states of the manganese ions that comprise the oxygen-evolving complex of photosystem II. Understanding the nature and order of oxidation events that occur during the catalytic cycle of five Si states (i = 0–4) is of fundamental importance both for the natural system and for artificial water oxidation catalysts. Despite the widespread adoption of the so-called “high-valent scheme”—where, for example, the Mn oxidation states in the S2 state are assigned as III, IV, IV, IV—the competing “low-valent scheme” that differs by a total of two metal unpaired electrons (i.e. III, III, III, IV in the S2 state) is favored by several recent studies for the biological catalyst. The question of the correct oxidation state assignment is addressed here by a detailed computational comparison of the two schemes using a common structural platform and theoretical approach. Models based on crystallographic constraints were constructed for all conceivable oxidation state assignments in the four (semi)stable S states of the oxygen evolving complex, sampling various protonation levels and patterns to ensure comprehensive coverage. The models are evaluated with respect to their geometric, energetic, electronic, and spectroscopic properties against available experimental EXAFS, XFEL-XRD, EPR, ENDOR and Mn K pre-edge XANES data. New 2.5 K 55Mn ENDOR data of the S2 state are also reported. Our results conclusively show that the entire S state phenomenology can only be accommodated within the high-valent scheme by adopting a single motif and protonation pattern that progresses smoothly from S0 (III, III, III, IV) to S3 (IV, IV, IV, IV), satisfying all experimental constraints and reproducing all observables. By contrast, it was impossible to construct a consistent cycle based on the low-valent scheme for all S states. Instead, the low-valent models developed here may provide new insight into the over-reduced S states and the states involved in the assembly of the catalytically active water oxidizing cluster.

@article{hattne_accurate_2014,
	title = {Accurate macromolecular structures using minimal measurements from {X}-ray free-electron lasers},
	volume = {11},
	copyright = {2014 Springer Nature America, Inc.},
	issn = {1548-7105},
	url = {https://www.nature.com/articles/nmeth.2887},
	doi = {10.1038/nmeth.2887},
	abstract = {A computational approach and software tool, cctbx.xfel, enables the determination of accurate macromolecular structure factors using a relatively small number of serial femtosecond crystallography diffraction snapshots.},
	language = {en},
	number = {5},
	urldate = {2024-12-10},
	journal = {Nature Methods},
	publisher = {Nature Publishing Group},
	author = {Hattne, Johan and Echols, Nathaniel and Tran, Rosalie and Kern, Jan and Gildea, Richard J. and Brewster, Aaron S. and Alonso-Mori, Roberto and Glöckner, Carina and Hellmich, Julia and Laksmono, Hartawan and Sierra, Raymond G. and Lassalle-Kaiser, Benedikt and Lampe, Alyssa and Han, Guangye and Gul, Sheraz and DiFiore, Dörte and Milathianaki, Despina and Fry, Alan R. and Miahnahri, Alan and White, William E. and Schafer, Donald W. and Seibert, M. Marvin and Koglin, Jason E. and Sokaras, Dimosthenis and Weng, Tsu-Chien and Sellberg, Jonas and Latimer, Matthew J. and Glatzel, Pieter and Zwart, Petrus H. and Grosse-Kunstleve, Ralf W. and Bogan, Michael J. and Messerschmidt, Marc and Williams, Garth J. and Boutet, Sébastien and Messinger, Johannes and Zouni, Athina and Yano, Junko and Bergmann, Uwe and Yachandra, Vittal K. and Adams, Paul D. and Sauter, Nicholas K.},
	month = may,
	year = {2014},
	keywords = {Nanocrystallography, Protein analysis, Proteins, Software},
	pages = {545--548},
}

A computational approach and software tool, cctbx.xfel, enables the determination of accurate macromolecular structure factors using a relatively small number of serial femtosecond crystallography diffraction snapshots.

@article{arafa_dinuclear_2014,
	title = {Dinuclear manganese complexes for water oxidation: evaluation of electronic effects and catalytic activity},
	volume = {16},
	issn = {1463-9084},
	shorttitle = {Dinuclear manganese complexes for water oxidation},
	url = {https://pubs.rsc.org/en/content/articlelanding/2014/cp/c3cp54800g},
	doi = {10.1039/C3CP54800G},
	abstract = {During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O2 and solar fuels, such as H2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn2II,III catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O–O bond formation.},
	language = {en},
	number = {24},
	urldate = {2024-12-10},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {The Royal Society of Chemistry},
	author = {Arafa, Wael A. A. and Kärkäs, Markus D. and Lee, Bao-Lin and Åkermark, Torbjörn and Liao, Rong-Zhen and Berends, Hans-Martin and Messinger, Johannes and Siegbahn, Per E. M. and Åkermark, Björn},
	month = may,
	year = {2014},
	pages = {11950--11964},
}

During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O2 and solar fuels, such as H2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn2II,III catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O–O bond formation.

@article{pham_electrochemically_2014,
	series = {Photosynthesis {Research} for {Sustainability}: {Keys} to {Produce} {Clean} {Energy}},
	title = {Electrochemically produced hydrogen peroxide affects {Joliot}-type oxygen-evolution measurements of photosystem {II}},
	volume = {1837},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272814000152},
	doi = {10.1016/j.bbabio.2014.01.013},
	abstract = {The main technique employed to characterize the efficiency of water-splitting in photosynthetic preparations in terms of miss and double hit parameters and for the determination of Si (i=2,3,0) state lifetimes is the measurement of flash-induced oxygen oscillation pattern on bare platinum (Joliot-type) electrodes. We demonstrate here that this technique is not innocent. Polarization of the electrode against an Ag/AgCl electrode leads to a time-dependent formation of hydrogen peroxide by two-electron reduction of dissolved oxygen continuously supplied by the flow buffer. While the miss and double hit parameters are almost unaffected by H2O2, a time dependent reduction of S1 to S−1 occurs over a time period of 20min. The S1 reduction can be largely prevented by adding catalase or by removing O2 from the flow buffer with N2. Importantly, we demonstrate that even at the shortest possible polarization times (40s in our set up) the S2 and S0 decays are significantly accelerated by the side reaction with H2O2. The removal of hydrogen peroxide leads to unperturbed S2 state data that reveal three instead of the traditionally reported two phases of decay. In addition, even under the best conditions (catalase+N2; 40s polarization) about 4\% of S−1 state is observed in well dark-adapted samples, likely indicating limitations of the equal fit approach. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.},
	number = {9},
	urldate = {2024-12-10},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Pham, Long Vo and Messinger, Johannes},
	month = sep,
	year = {2014},
	keywords = {Hydrogen peroxide (HO), Manganese, Oxygen evolving complex (OEC), Photosystem II (PSII), Water oxidation},
	pages = {1411--1416},
}

The main technique employed to characterize the efficiency of water-splitting in photosynthetic preparations in terms of miss and double hit parameters and for the determination of Si (i=2,3,0) state lifetimes is the measurement of flash-induced oxygen oscillation pattern on bare platinum (Joliot-type) electrodes. We demonstrate here that this technique is not innocent. Polarization of the electrode against an Ag/AgCl electrode leads to a time-dependent formation of hydrogen peroxide by two-electron reduction of dissolved oxygen continuously supplied by the flow buffer. While the miss and double hit parameters are almost unaffected by H2O2, a time dependent reduction of S1 to S−1 occurs over a time period of 20min. The S1 reduction can be largely prevented by adding catalase or by removing O2 from the flow buffer with N2. Importantly, we demonstrate that even at the shortest possible polarization times (40s in our set up) the S2 and S0 decays are significantly accelerated by the side reaction with H2O2. The removal of hydrogen peroxide leads to unperturbed S2 state data that reveal three instead of the traditionally reported two phases of decay. In addition, even under the best conditions (catalase+N2; 40s polarization) about 4% of S−1 state is observed in well dark-adapted samples, likely indicating limitations of the equal fit approach. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.

@article{han_hydration_2014,
	title = {Hydration of the oxygen-evolving complex of photosystem {II} probed in the dark-stable {S1} state using proton {NMR} dispersion profiles},
	volume = {16},
	issn = {1463-9084},
	url = {https://pubs.rsc.org/en/content/articlelanding/2014/cp/c3cp55232b},
	doi = {10.1039/C3CP55232B},
	abstract = {The hydration of the oxygen-evolving complex (OEC) was characterized in the dark stable S1 state of photosystem II using water R1(ω) NMR dispersion (NMRD) profiles. The R1(ω) NMRD profiles were recorded over a frequency range from 0.01 MHz to 40 MHz for both intact and Mn-depleted photosystem II core complexes from Thermosynechococcus vulcanus (T. vulcanus). The intact-minus-(Mn)-depleted difference NMRD profiles show a characteristic dispersion from approximately 0.03 MHz to 1 MHz, which is interpreted on the basis of the Solomon–Bloembergen–Morgan (SBM) and the slow motion theories as being due to a paramagnetic enhanced relaxation (PRE) of water protons. Both theories are qualitatively consistent with the ST = 1, g = 4.9 paramagnetic state previously described for the S1 state of the OEC; however, an alternative explanation involving the loss of a separate class of long-lived internal waters due to the Mn-depletion procedure can presently not be ruled out. Using a point-dipole approximation the PRE-NMRD effect can be described as being caused by 1–2 water molecules that are located about 10 Å away from the spin center of the Mn4CaO5 cluster in the OEC. The application of the SBM theory to the dispersion observed for PSII in the S1 state is questionable, because the parameters extracted do not fulfil the presupposed perturbation criterion. In contrast, the slow motion theory gives a consistent picture indicating that the water molecules are in fast chemical exchange with the bulk (τw {\textless} 1 μs). The modulation of the zero-field splitting (ZFS) interaction suggests a (restricted) reorientation/structural equilibrium of the Mn4CaO5 cluster with a characteristic time constant of τZFS = 0.6–0.9 μs.},
	language = {en},
	number = {24},
	urldate = {2024-12-10},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {The Royal Society of Chemistry},
	author = {Han, Guangye and Huang, Yang and Koua, Faisal Hammad Mekky and Shen, Jian-Ren and Westlund, Per-Olof and Messinger, Johannes},
	month = may,
	year = {2014},
	pages = {11924--11935},
}

The hydration of the oxygen-evolving complex (OEC) was characterized in the dark stable S1 state of photosystem II using water R1(ω) NMR dispersion (NMRD) profiles. The R1(ω) NMRD profiles were recorded over a frequency range from 0.01 MHz to 40 MHz for both intact and Mn-depleted photosystem II core complexes from Thermosynechococcus vulcanus (T. vulcanus). The intact-minus-(Mn)-depleted difference NMRD profiles show a characteristic dispersion from approximately 0.03 MHz to 1 MHz, which is interpreted on the basis of the Solomon–Bloembergen–Morgan (SBM) and the slow motion theories as being due to a paramagnetic enhanced relaxation (PRE) of water protons. Both theories are qualitatively consistent with the ST = 1, g = 4.9 paramagnetic state previously described for the S1 state of the OEC; however, an alternative explanation involving the loss of a separate class of long-lived internal waters due to the Mn-depletion procedure can presently not be ruled out. Using a point-dipole approximation the PRE-NMRD effect can be described as being caused by 1–2 water molecules that are located about 10 Å away from the spin center of the Mn4CaO5 cluster in the OEC. The application of the SBM theory to the dispersion observed for PSII in the S1 state is questionable, because the parameters extracted do not fulfil the presupposed perturbation criterion. In contrast, the slow motion theory gives a consistent picture indicating that the water molecules are in fast chemical exchange with the bulk (τw \textless 1 μs). The modulation of the zero-field splitting (ZFS) interaction suggests a (restricted) reorientation/structural equilibrium of the Mn4CaO5 cluster with a characteristic time constant of τZFS = 0.6–0.9 μs.

@article{liang_improving_2014,
	title = {Improving {BiVO4} photoanodes for solar water splitting through surface passivation},
	volume = {16},
	issn = {1463-9084},
	url = {https://pubs.rsc.org/en/content/articlelanding/2014/cp/c4cp00674g},
	doi = {10.1039/C4CP00674G},
	abstract = {BiVO4 has shown great potential as a semiconductor photoanode for solar water splitting. Significant improvements made during recent years allowed researchers to obtain a photocurrent density of up to 4.0 mA cm−2 (AM1.5 sunlight illumination, 1.23 VRHE bias). For further improvements of the BiVO4 photoelectrodes, a deep understanding of the processes occurring at the BiVO4–H2O interface is crucial. Employing an electrochemical loading and removal process of NiOx, we show here that carrier recombination at this interface strongly affects the photocurrents. The removal of NiOx species by electrochemical treatment in a phosphate electrolyte leads to significantly increased photocurrents for BiVO4 photoelectrodes. At a bias of 1.23 VRHE, the Incident Photon-to-Current Efficiency (IPCE) at 450 nm reaches 43\% for the passivated BiVO4 electrode under back side illumination. A model incorporating heterogeneity of NiOx centers on the BiVO4 surface (OER catalytic centers, recombination centers, and passivation centers) is proposed to explain this improved performance.},
	language = {en},
	number = {24},
	urldate = {2024-12-10},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {The Royal Society of Chemistry},
	author = {Liang, Yongqi and Messinger, Johannes},
	month = may,
	year = {2014},
	pages = {12014--12020},
}

BiVO4 has shown great potential as a semiconductor photoanode for solar water splitting. Significant improvements made during recent years allowed researchers to obtain a photocurrent density of up to 4.0 mA cm−2 (AM1.5 sunlight illumination, 1.23 VRHE bias). For further improvements of the BiVO4 photoelectrodes, a deep understanding of the processes occurring at the BiVO4–H2O interface is crucial. Employing an electrochemical loading and removal process of NiOx, we show here that carrier recombination at this interface strongly affects the photocurrents. The removal of NiOx species by electrochemical treatment in a phosphate electrolyte leads to significantly increased photocurrents for BiVO4 photoelectrodes. At a bias of 1.23 VRHE, the Incident Photon-to-Current Efficiency (IPCE) at 450 nm reaches 43% for the passivated BiVO4 electrode under back side illumination. A model incorporating heterogeneity of NiOx centers on the BiVO4 surface (OER catalytic centers, recombination centers, and passivation centers) is proposed to explain this improved performance.

@article{koroidov_mobile_2014,
	title = {Mobile hydrogen carbonate acts as proton acceptor in photosynthetic water oxidation},
	volume = {111},
	issn = {0027-8424, 1091-6490},
	url = {http://www.pnas.org/cgi/doi/10.1073/pnas.1323277111},
	doi = {10/f2z4cv},
	language = {en},
	number = {17},
	urldate = {2021-06-08},
	journal = {Proceedings of the National Academy of Sciences},
	author = {Koroidov, S. and Shevela, D. and Shutova, T. and Samuelsson, G. and Messinger, J.},
	month = apr,
	year = {2014},
	pages = {6299--6304},
}

@article{messinger_photosynthesis_2014,
	title = {Photosynthesis: from natural to artificial},
	volume = {16},
	issn = {1463-9084},
	shorttitle = {Photosynthesis},
	url = {https://pubs.rsc.org/en/content/articlelanding/2014/cp/c4cp90053g},
	doi = {10.1039/C4CP90053G},
	abstract = {A graphical abstract is available for this content},
	language = {en},
	number = {24},
	urldate = {2024-12-10},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {The Royal Society of Chemistry},
	author = {Messinger, Johannes and Lubitz, Wolfgang and Shen, Jian-Ren},
	month = may,
	year = {2014},
	pages = {11810--11811},
}

A graphical abstract is available for this content

@article{nilsson_substrate_2014,
	title = {Substrate water exchange in photosystem {II} core complexes of the extremophilic red alga \textit{{Cyanidioschyzon} merolae}},
	volume = {1837},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272814001078},
	doi = {10.1016/j.bbabio.2014.04.001},
	abstract = {The binding affinity of the two substrate–water molecules to the water-oxidizing Mn4CaO5 catalyst in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae was studied in the S2 and S3 states by the exchange of bound 16O-substrate against 18O-labeled water. The rate of this exchange was detected via the membrane-inlet mass spectrometric analysis of flash-induced oxygen evolution. For both redox states a fast and slow phase of water-exchange was resolved at the mixed labeled m/z 34 mass peak: kf=52±8s−1 and ks=1.9±0.3s−1 in the S2 state, and kf=42±2s−1 and kslow=1.2±0.3s−1 in S3, respectively. Overall these exchange rates are similar to those observed previously with preparations of other organisms. The most remarkable finding is a significantly slower exchange at the fast substrate–water site in the S2 state, which confirms beyond doubt that both substrate–water molecules are already bound in the S2 state. This leads to a very small change of the affinity for both the fast and the slowly exchanging substrates during the S2→S3 transition. Implications for recent models for water-oxidation are briefly discussed.},
	number = {8},
	urldate = {2024-12-10},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Nilsson, Håkan and Krupnik, Tomasz and Kargul, Joanna and Messinger, Johannes},
	month = aug,
	year = {2014},
	keywords = {Membrane-inlet mass spectrometry, Oxygen evolution, Photosystem II, Substrate–water exchange, Water oxidation},
	pages = {1257--1262},
}

The binding affinity of the two substrate–water molecules to the water-oxidizing Mn4CaO5 catalyst in photosystem II core complexes of the extremophilic red alga Cyanidioschyzon merolae was studied in the S2 and S3 states by the exchange of bound 16O-substrate against 18O-labeled water. The rate of this exchange was detected via the membrane-inlet mass spectrometric analysis of flash-induced oxygen evolution. For both redox states a fast and slow phase of water-exchange was resolved at the mixed labeled m/z 34 mass peak: kf=52±8s−1 and ks=1.9±0.3s−1 in the S2 state, and kf=42±2s−1 and kslow=1.2±0.3s−1 in S3, respectively. Overall these exchange rates are similar to those observed previously with preparations of other organisms. The most remarkable finding is a significantly slower exchange at the fast substrate–water site in the S2 state, which confirms beyond doubt that both substrate–water molecules are already bound in the S2 state. This leads to a very small change of the affinity for both the fast and the slowly exchanging substrates during the S2→S3 transition. Implications for recent models for water-oxidation are briefly discussed.

@article{nilsson_substratewater_2014,
	title = {Substrate–water exchange in photosystem {II} is arrested before dioxygen formation},
	volume = {5},
	copyright = {2014 The Author(s)},
	issn = {2041-1723},
	url = {https://www.nature.com/articles/ncomms5305},
	doi = {10.1038/ncomms5305},
	abstract = {Light-driven oxidation of water into dioxygen, catalysed by the oxygen-evolving complex (OEC) in photosystem II, is essential for life on Earth and provides the blueprint for devices for producing fuel from sunlight. Although the structure of the OEC is known at atomic level for its dark-stable state, the mechanism by which water is oxidized remains unsettled. Important mechanistic information was gained in the past two decades by mass spectrometric studies of the H218O/H216O substrate–water exchange in the four (semi) stable redox states of the OEC. However, until now such data were not attainable in the transient states formed immediately before the O–O bond formation. Using modified photosystem II complexes displaying up to 40-fold slower O2 production rates, we show here that in the transient state the substrate–water exchange is dramatically slowed as compared with the earlier S states. This further constrains the possible sites for substrate–water binding in photosystem II.},
	language = {en},
	number = {1},
	urldate = {2024-12-10},
	journal = {Nature Communications},
	publisher = {Nature Publishing Group},
	author = {Nilsson, Håkan and Rappaport, Fabrice and Boussac, Alain and Messinger, Johannes},
	month = jul,
	year = {2014},
	keywords = {Bioenergetics, Photosystem II},
	pages = {4305},
}

Light-driven oxidation of water into dioxygen, catalysed by the oxygen-evolving complex (OEC) in photosystem II, is essential for life on Earth and provides the blueprint for devices for producing fuel from sunlight. Although the structure of the OEC is known at atomic level for its dark-stable state, the mechanism by which water is oxidized remains unsettled. Important mechanistic information was gained in the past two decades by mass spectrometric studies of the H218O/H216O substrate–water exchange in the four (semi) stable redox states of the OEC. However, until now such data were not attainable in the transient states formed immediately before the O–O bond formation. Using modified photosystem II complexes displaying up to 40-fold slower O2 production rates, we show here that in the transient state the substrate–water exchange is dramatically slowed as compared with the earlier S states. This further constrains the possible sites for substrate–water binding in photosystem II.

@article{kern_taking_2014,
	title = {Taking snapshots of photosynthetic water oxidation using femtosecond {X}-ray diffraction and spectroscopy},
	volume = {5},
	copyright = {2014 Springer Nature Limited},
	issn = {2041-1723},
	url = {https://www.nature.com/articles/ncomms5371},
	doi = {10.1038/ncomms5371},
	abstract = {The dioxygen we breathe is formed by light-induced oxidation of water in photosystem II. O2 formation takes place at a catalytic manganese cluster within milliseconds after the photosystem II reaction centre is excited by three single-turnover flashes. Here we present combined X-ray emission spectra and diffraction data of 2-flash (2F) and 3-flash (3F) photosystem II samples, and of a transient 3F’ state (250 μs after the third flash), collected under functional conditions using an X-ray free electron laser. The spectra show that the initial O–O bond formation, coupled to Mn reduction, does not yet occur within 250 μs after the third flash. Diffraction data of all states studied exhibit an anomalous scattering signal from Mn but show no significant structural changes at the present resolution of 4.5 Å. This study represents the initial frames in a molecular movie of the structural changes during the catalytic reaction in photosystem II.},
	language = {en},
	number = {1},
	urldate = {2024-12-10},
	journal = {Nature Communications},
	publisher = {Nature Publishing Group},
	author = {Kern, Jan and Tran, Rosalie and Alonso-Mori, Roberto and Koroidov, Sergey and Echols, Nathaniel and Hattne, Johan and Ibrahim, Mohamed and Gul, Sheraz and Laksmono, Hartawan and Sierra, Raymond G. and Gildea, Richard J. and Han, Guangye and Hellmich, Julia and Lassalle-Kaiser, Benedikt and Chatterjee, Ruchira and Brewster, Aaron S. and Stan, Claudiu A. and Glöckner, Carina and Lampe, Alyssa and DiFiore, Dörte and Milathianaki, Despina and Fry, Alan R. and Seibert, M. Marvin and Koglin, Jason E. and Gallo, Erik and Uhlig, Jens and Sokaras, Dimosthenis and Weng, Tsu-Chien and Zwart, Petrus H. and Skinner, David E. and Bogan, Michael J. and Messerschmidt, Marc and Glatzel, Pieter and Williams, Garth J. and Boutet, Sébastien and Adams, Paul D. and Zouni, Athina and Messinger, Johannes and Sauter, Nicholas K. and Bergmann, Uwe and Yano, Junko and Yachandra, Vittal K.},
	month = jul,
	year = {2014},
	keywords = {Biophysics, Optical spectroscopy, Photosynthesis, X-ray diffraction},
	pages = {4371},
}

The dioxygen we breathe is formed by light-induced oxidation of water in photosystem II. O2 formation takes place at a catalytic manganese cluster within milliseconds after the photosystem II reaction centre is excited by three single-turnover flashes. Here we present combined X-ray emission spectra and diffraction data of 2-flash (2F) and 3-flash (3F) photosystem II samples, and of a transient 3F’ state (250 μs after the third flash), collected under functional conditions using an X-ray free electron laser. The spectra show that the initial O–O bond formation, coupled to Mn reduction, does not yet occur within 250 μs after the third flash. Diffraction data of all states studied exhibit an anomalous scattering signal from Mn but show no significant structural changes at the present resolution of 4.5 Å. This study represents the initial frames in a molecular movie of the structural changes during the catalytic reaction in photosystem II.

@article{tran_mn4ca_2014,
	title = {The {Mn4Ca} photosynthetic water-oxidation catalyst studied by simultaneous {X}-ray spectroscopy and crystallography using an {X}-ray free-electron laser},
	volume = {369},
	url = {https://royalsocietypublishing.org/doi/10.1098/rstb.2013.0324},
	doi = {10.1098/rstb.2013.0324},
	abstract = {The structure of photosystem II and the catalytic intermediate states of the Mn4CaO5 cluster involved in water oxidation have been studied intensively over the past several years. An understanding of the sequential chemistry of light absorption and the mechanism of water oxidation, however, requires a new approach beyond the conventional steady-state crystallography and X-ray spectroscopy at cryogenic temperatures. In this report, we present the preliminary progress using an X-ray free-electron laser to determine simultaneously the light-induced protein dynamics via crystallography and the local chemistry that occurs at the catalytic centre using X-ray spectroscopy under functional conditions at room temperature.},
	number = {1647},
	urldate = {2024-12-10},
	journal = {Philosophical Transactions of the Royal Society B: Biological Sciences},
	publisher = {Royal Society},
	author = {Tran, Rosalie and Kern, Jan and Hattne, Johan and Koroidov, Sergey and Hellmich, Julia and Alonso-Mori, Roberto and Sauter, Nicholas K. and Bergmann, Uwe and Messinger, Johannes and Zouni, Athina and Yano, Junko and Yachandra, Vittal K.},
	month = jul,
	year = {2014},
	keywords = {X-ray crystallography, X-ray emission spectroscopy, X-ray free-electron laser, manganese, oxygen-evolving complex, photosystem II},
	pages = {20130324},
}

The structure of photosystem II and the catalytic intermediate states of the Mn4CaO5 cluster involved in water oxidation have been studied intensively over the past several years. An understanding of the sequential chemistry of light absorption and the mechanism of water oxidation, however, requires a new approach beyond the conventional steady-state crystallography and X-ray spectroscopy at cryogenic temperatures. In this report, we present the preliminary progress using an X-ray free-electron laser to determine simultaneously the light-induced protein dynamics via crystallography and the local chemistry that occurs at the catalytic centre using X-ray spectroscopy under functional conditions at room temperature.

@article{messinger_warwick_2014,
	title = {Warwick {Hillier}: a tribute},
	volume = {122},
	issn = {1573-5079},
	shorttitle = {Warwick {Hillier}},
	url = {https://doi.org/10.1007/s11120-014-0025-5},
	doi = {10.1007/s11120-014-0025-5},
	abstract = {Warwick Hillier (October 18, 1967–January 10, 2014) made seminal contributions to our understanding of photosynthetic water oxidation employing membrane inlet mass spectrometry and FTIR spectroscopy. This article offers a collection of historical perspectives on the scientific impact of Warwick Hillier’s work and tributes to the personal impact his life and ideas had on his collaborators and colleagues.},
	language = {en},
	number = {1},
	urldate = {2024-12-10},
	journal = {Photosynthesis Research},
	author = {Messinger, Johannes and Debus, Richard and Dismukes, G. Charles},
	month = oct,
	year = {2014},
	keywords = {Bicarbonate, FTIR, Isotopes, Mass spectrometry, Oxygenic photosynthesis, Photosystem II},
	pages = {1--11},
}

Warwick Hillier (October 18, 1967–January 10, 2014) made seminal contributions to our understanding of photosynthetic water oxidation employing membrane inlet mass spectrometry and FTIR spectroscopy. This article offers a collection of historical perspectives on the scientific impact of Warwick Hillier’s work and tributes to the personal impact his life and ideas had on his collaborators and colleagues.

@article{perez_navarro_ammonia_2013,
	title = {Ammonia binding to the oxygen-evolving complex of photosystem {II} identifies the solvent-exchangeable oxygen bridge (μ-oxo) of the manganese tetramer},
	volume = {110},
	url = {https://www.pnas.org/doi/10.1073/pnas.1304334110},
	doi = {10.1073/pnas.1304334110},
	abstract = {The assignment of the two substrate water sites of the tetra-manganese penta-oxygen calcium (Mn4O5Ca) cluster of photosystem II is essential for the elucidation of the mechanism of biological O-O bond formation and the subsequent design of bio-inspired water-splitting catalysts. We recently demonstrated using pulsed EPR spectroscopy that one of the five oxygen bridges (μ-oxo) exchanges unusually rapidly with bulk water and is thus a likely candidate for one of the substrates. Ammonia, a water analog, was previously shown to bind to the Mn4O5Ca cluster, potentially displacing a water/substrate ligand [Britt RD, et al. (1989) J Am Chem Soc 111(10):3522–3532]. Here we show by a combination of EPR and time-resolved membrane inlet mass spectrometry that the binding of ammonia perturbs the exchangeable μ-oxo bridge without drastically altering the binding/exchange kinetics of the two substrates. In combination with broken-symmetry density functional theory, our results show that (i) the exchangable μ-oxo bridge is O5 \{using the labeling of the current crystal structure [Umena Y, et al. (2011) Nature 473(7345):55–60]\}; (ii) ammonia displaces a water ligand to the outer manganese (MnA4-W1); and (iii) as W1 is trans to O5, ammonia binding elongates the MnA4-O5 bond, leading to the perturbation of the μ-oxo bridge resonance and to a small change in the water exchange rates. These experimental results support O-O bond formation between O5 and possibly an oxyl radical as proposed by Siegbahn and exclude W1 as the second substrate water.},
	number = {39},
	urldate = {2024-12-10},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Pérez Navarro, Montserrat and Ames, William M. and Nilsson, Håkan and Lohmiller, Thomas and Pantazis, Dimitrios A. and Rapatskiy, Leonid and Nowaczyk, Marc M. and Neese, Frank and Boussac, Alain and Messinger, Johannes and Lubitz, Wolfgang and Cox, Nicholas},
	month = sep,
	year = {2013},
	pages = {15561--15566},
}

The assignment of the two substrate water sites of the tetra-manganese penta-oxygen calcium (Mn4O5Ca) cluster of photosystem II is essential for the elucidation of the mechanism of biological O-O bond formation and the subsequent design of bio-inspired water-splitting catalysts. We recently demonstrated using pulsed EPR spectroscopy that one of the five oxygen bridges (μ-oxo) exchanges unusually rapidly with bulk water and is thus a likely candidate for one of the substrates. Ammonia, a water analog, was previously shown to bind to the Mn4O5Ca cluster, potentially displacing a water/substrate ligand [Britt RD, et al. (1989) J Am Chem Soc 111(10):3522–3532]. Here we show by a combination of EPR and time-resolved membrane inlet mass spectrometry that the binding of ammonia perturbs the exchangeable μ-oxo bridge without drastically altering the binding/exchange kinetics of the two substrates. In combination with broken-symmetry density functional theory, our results show that (i) the exchangable μ-oxo bridge is O5 \using the labeling of the current crystal structure [Umena Y, et al. (2011) Nature 473(7345):55–60]\; (ii) ammonia displaces a water ligand to the outer manganese (MnA4-W1); and (iii) as W1 is trans to O5, ammonia binding elongates the MnA4-O5 bond, leading to the perturbation of the μ-oxo bridge resonance and to a small change in the water exchange rates. These experimental results support O-O bond formation between O5 and possibly an oxyl radical as proposed by Siegbahn and exclude W1 as the second substrate water.

@article{faunce_artificial_2013,
	title = {Artificial photosynthesis as a frontier technology for energy sustainability},
	volume = {6},
	issn = {1754-5706},
	url = {https://pubs.rsc.org/en/content/articlelanding/2013/ee/c3ee40534f},
	doi = {10.1039/C3EE40534F},
	abstract = {A graphical abstract is available for this content},
	language = {en},
	number = {4},
	urldate = {2024-12-10},
	journal = {Energy \& Environmental Science},
	publisher = {The Royal Society of Chemistry},
	author = {Faunce, Thomas and Styring, Stenbjorn and Wasielewski, Michael R. and Brudvig, Gary W. and Rutherford, A. William and Messinger, Johannes and Lee, Adam F. and Hill, Craig L. and deGroot, Huub and Fontecave, Marc and MacFarlane, Doug R. and Hankamer, Ben and Nocera, Daniel G. and Tiede, David M. and Dau, Holger and Hillier, Warwick and Wang, Lianzhou and Amal, Rose},
	month = mar,
	year = {2013},
	pages = {1074--1076},
}

A graphical abstract is available for this content

@article{shevela_efficiency_2013,
	title = {Efficiency of photosynthetic water oxidation at ambient and depleted levels of inorganic carbon},
	volume = {117},
	issn = {0166-8595, 1573-5079},
	url = {http://link.springer.com/10.1007/s11120-013-9875-5},
	doi = {10/f2zpf2},
	language = {en},
	number = {1-3},
	urldate = {2021-06-08},
	journal = {Photosynthesis Research},
	author = {Shevela, Dmitriy and Nöring, Birgit and Koroidov, Sergey and Shutova, Tatiana and Samuelsson, Göran and Messinger, Johannes},
	month = nov,
	year = {2013},
	pages = {401--412},
}

@article{glatzel_electronic_2013,
	title = {Electronic {Structural} {Changes} of {Mn} in the {Oxygen}-{Evolving} {Complex} of {Photosystem} {II} during the {Catalytic} {Cycle}},
	volume = {52},
	issn = {0020-1669},
	url = {https://doi.org/10.1021/ic4005938},
	doi = {10.1021/ic4005938},
	abstract = {The oxygen-evolving complex (OEC) in photosystem II (PS II) was studied in the S0 through S3 states using 1s2p resonant inelastic X-ray scattering spectroscopy. The spectral changes of the OEC during the S-state transitions are subtle, indicating that the electrons are strongly delocalized throughout the cluster. The result suggests that, in addition to the Mn ions, ligands are also playing an important role in the redox reactions. A series of MnIV coordination complexes were compared, particularly with the PS II S3 state spectrum to understand its oxidation state. We find strong variations of the electronic structure within the series of MnIV model systems. The spectrum of the S3 state best resembles those of the MnIV complexes Mn3IVCa2 and saplnMn2IV(OH)2. The current result emphasizes that the assignment of formal oxidation states alone is not sufficient for understanding the detailed electronic structural changes that govern the catalytic reaction in the OEC.},
	number = {10},
	urldate = {2024-12-10},
	journal = {Inorganic Chemistry},
	publisher = {American Chemical Society},
	author = {Glatzel, Pieter and Schroeder, Henning and Pushkar, Yulia and Boron, Thaddeus III and Mukherjee, Shreya and Christou, George and Pecoraro, Vincent L. and Messinger, Johannes and Yachandra, Vittal K. and Bergmann, Uwe and Yano, Junko},
	month = may,
	year = {2013},
	pages = {5642--5644},
}

The oxygen-evolving complex (OEC) in photosystem II (PS II) was studied in the S0 through S3 states using 1s2p resonant inelastic X-ray scattering spectroscopy. The spectral changes of the OEC during the S-state transitions are subtle, indicating that the electrons are strongly delocalized throughout the cluster. The result suggests that, in addition to the Mn ions, ligands are also playing an important role in the redox reactions. A series of MnIV coordination complexes were compared, particularly with the PS II S3 state spectrum to understand its oxidation state. We find strong variations of the electronic structure within the series of MnIV model systems. The spectrum of the S3 state best resembles those of the MnIV complexes Mn3IVCa2 and saplnMn2IV(OH)2. The current result emphasizes that the assignment of formal oxidation states alone is not sufficient for understanding the detailed electronic structural changes that govern the catalytic reaction in the OEC.

@article{mitzner_l-edge_2013,
	title = {L-{Edge} {X}-ray {Absorption} {Spectroscopy} of {Dilute} {Systems} {Relevant} to {Metalloproteins} {Using} an {X}-ray {Free}-{Electron} {Laser}},
	volume = {4},
	url = {https://doi.org/10.1021/jz401837f},
	doi = {10.1021/jz401837f},
	abstract = {L-edge spectroscopy of 3d transition metals provides important electronic structure information and has been used in many fields. However, the use of this method for studying dilute aqueous systems, such as metalloenzymes, has not been prevalent because of severe radiation damage and the lack of suitable detection systems. Here we present spectra from a dilute Mn aqueous solution using a high-transmission zone-plate spectrometer at the Linac Coherent Light Source (LCLS). The spectrometer has been optimized for discriminating the Mn L-edge signal from the overwhelming O K-edge background that arises from water and protein itself, and the ultrashort LCLS X-ray pulses can outrun X-ray induced damage. We show that the deviations of the partial-fluorescence yield-detected spectra from the true absorption can be well modeled using the state-dependence of the fluorescence yield, and discuss implications for the application of our concept to biological samples.},
	number = {21},
	urldate = {2024-12-10},
	journal = {The Journal of Physical Chemistry Letters},
	publisher = {American Chemical Society},
	author = {Mitzner, Rolf and Rehanek, Jens and Kern, Jan and Gul, Sheraz and Hattne, Johan and Taguchi, Taketo and Alonso-Mori, Roberto and Tran, Rosalie and Weniger, Christian and Schröder, Henning and Quevedo, Wilson and Laksmono, Hartawan and Sierra, Raymond G. and Han, Guangye and Lassalle-Kaiser, Benedikt and Koroidov, Sergey and Kubicek, Katharina and Schreck, Simon and Kunnus, Kristjan and Brzhezinskaya, Maria and Firsov, Alexander and Minitti, Michael P. and Turner, Joshua J. and Moeller, Stefan and Sauter, Nicholas K. and Bogan, Michael J. and Nordlund, Dennis and Schlotter, William F. and Messinger, Johannes and Borovik, Andrew and Techert, Simone and de Groot, Frank M. F. and Föhlisch, Alexander and Erko, Alexei and Bergmann, Uwe and Yachandra, Vittal K. and Wernet, Philippe and Yano, Junko},
	month = nov,
	year = {2013},
	pages = {3641--3647},
}

L-edge spectroscopy of 3d transition metals provides important electronic structure information and has been used in many fields. However, the use of this method for studying dilute aqueous systems, such as metalloenzymes, has not been prevalent because of severe radiation damage and the lack of suitable detection systems. Here we present spectra from a dilute Mn aqueous solution using a high-transmission zone-plate spectrometer at the Linac Coherent Light Source (LCLS). The spectrometer has been optimized for discriminating the Mn L-edge signal from the overwhelming O K-edge background that arises from water and protein itself, and the ultrashort LCLS X-ray pulses can outrun X-ray induced damage. We show that the deviations of the partial-fluorescence yield-detected spectra from the true absorption can be well modeled using the state-dependence of the fluorescence yield, and discuss implications for the application of our concept to biological samples.

@article{cox_reflections_2013,
	title = {Reflections on substrate water and dioxygen formation},
	volume = {1827},
	issn = {00052728},
	url = {https://linkinghub.elsevier.com/retrieve/pii/S0005272813000170},
	doi = {10/f2zvnx},
	language = {en},
	number = {8-9},
	urldate = {2021-06-08},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Cox, Nicholas and Messinger, Johannes},
	month = aug,
	year = {2013},
	pages = {1020--1030},
}

@article{kern_simultaneous_2013,
	title = {Simultaneous {Femtosecond} {X}-ray {Spectroscopy} and {Diffraction} of {Photosystem} {II} at {Room} {Temperature}},
	volume = {340},
	url = {https://www.science.org/doi/10.1126/science.1234273},
	doi = {10.1126/science.1234273},
	abstract = {Intense femtosecond x-ray pulses produced at the Linac Coherent Light Source (LCLS) were used for simultaneous x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) of microcrystals of photosystem II (PS II) at room temperature. This method probes the overall protein structure and the electronic structure of the Mn4CaO5 cluster in the oxygen-evolving complex of PS II. XRD data are presented from both the dark state (S1) and the first illuminated state (S2) of PS II. Our simultaneous XRD-XES study shows that the PS II crystals are intact during our measurements at the LCLS, not only with respect to the structure of PS II, but also with regard to the electronic structure of the highly radiation-sensitive Mn4CaO5 cluster, opening new directions for future dynamics studies.},
	number = {6131},
	urldate = {2024-12-10},
	journal = {Science},
	publisher = {American Association for the Advancement of Science},
	author = {Kern, Jan and Alonso-Mori, Roberto and Tran, Rosalie and Hattne, Johan and Gildea, Richard J. and Echols, Nathaniel and Glöckner, Carina and Hellmich, Julia and Laksmono, Hartawan and Sierra, Raymond G. and Lassalle-Kaiser, Benedikt and Koroidov, Sergey and Lampe, Alyssa and Han, Guangye and Gul, Sheraz and DiFiore, Dörte and Milathianaki, Despina and Fry, Alan R. and Miahnahri, Alan and Schafer, Donald W. and Messerschmidt, Marc and Seibert, M. Marvin and Koglin, Jason E. and Sokaras, Dimosthenis and Weng, Tsu-Chien and Sellberg, Jonas and Latimer, Matthew J. and Grosse-Kunstleve, Ralf W. and Zwart, Petrus H. and White, William E. and Glatzel, Pieter and Adams, Paul D. and Bogan, Michael J. and Williams, Garth J. and Boutet, Sébastien and Messinger, Johannes and Zouni, Athina and Sauter, Nicholas K. and Yachandra, Vittal K. and Bergmann, Uwe and Yano, Junko},
	month = apr,
	year = {2013},
	pages = {491--495},
}

Intense femtosecond x-ray pulses produced at the Linac Coherent Light Source (LCLS) were used for simultaneous x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) of microcrystals of photosystem II (PS II) at room temperature. This method probes the overall protein structure and the electronic structure of the Mn4CaO5 cluster in the oxygen-evolving complex of PS II. XRD data are presented from both the dark state (S1) and the first illuminated state (S2) of PS II. Our simultaneous XRD-XES study shows that the PS II crystals are intact during our measurements at the LCLS, not only with respect to the structure of PS II, but also with regard to the electronic structure of the highly radiation-sensitive Mn4CaO5 cluster, opening new directions for future dynamics studies.

@article{shevela_studying_2013,
	title = {Studying the oxidation of water to molecular oxygen in photosynthetic and artificial systems by time-resolved membrane-inlet mass spectrometry},
	volume = {4},
	issn = {1664-462X},
	url = {https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2013.00473/full},
	doi = {10.3389/fpls.2013.00473},
	abstract = {{\textless}p{\textgreater}Monitoring isotopic compositions of gaseous products (e.g., H$_{\textrm{2}}$, O$_{\textrm{2}}$, and CO$_{\textrm{2}}$) by time-resolved isotope-ratio membrane-inlet mass spectrometry (TR-IR-MIMS) is widely used for kinetic and functional analyses in photosynthesis research. In particular, in combination with isotopic labeling, TR-MIMS became an essential and powerful research tool for the study of the mechanism of photosynthetic water-oxidation to molecular oxygen catalyzed by the water-oxidizing complex of photosystem II. Moreover, recently, the TR-MIMS and $^{\textrm{18}}$O-labeling approach was successfully applied for testing newly developed catalysts for artificial water-splitting and provided important insight about the mechanism and pathways of O$_{\textrm{2}}$ formation. In this mini-review we summarize these results and provide a brief introduction into key aspects of the TR-MIMS technique and its perspectives for future studies of the enigmatic water-splitting chemistry.{\textless}/p{\textgreater}},
	language = {English},
	urldate = {2024-12-10},
	journal = {Frontiers in Plant Science},
	publisher = {Frontiers},
	author = {Shevela, Dmitriy and Messinger, Johannes},
	month = nov,
	year = {2013},
	keywords = {Isotope-ratio membrane-inlet mass spectrometry, O2 evolution, isotope labelling, photosynthetic and artificial water-splitting, photosystem II, water-oxidizing complex},
}

\textlessp\textgreaterMonitoring isotopic compositions of gaseous products (e.g., H$_{\textrm{2}}$, O$_{\textrm{2}}$, and CO$_{\textrm{2}}$) by time-resolved isotope-ratio membrane-inlet mass spectrometry (TR-IR-MIMS) is widely used for kinetic and functional analyses in photosynthesis research. In particular, in combination with isotopic labeling, TR-MIMS became an essential and powerful research tool for the study of the mechanism of photosynthetic water-oxidation to molecular oxygen catalyzed by the water-oxidizing complex of photosystem II. Moreover, recently, the TR-MIMS and $^{\textrm{18}}$O-labeling approach was successfully applied for testing newly developed catalysts for artificial water-splitting and provided important insight about the mechanism and pathways of O$_{\textrm{2}}$ formation. In this mini-review we summarize these results and provide a brief introduction into key aspects of the TR-MIMS technique and its perspectives for future studies of the enigmatic water-splitting chemistry.\textless/p\textgreater

@article{messinger_institutional_2012,
	title = {An {Institutional} {Approach} to {Solar} {Fuels} {Research}},
	volume = {65},
	issn = {1445-0038},
	url = {https://www.publish.csiro.au/ch/CH12020},
	doi = {10.1071/CH12020},
	abstract = {This account gives a brief overview of various directions in current solar fuels research. On that basis, the necessity for an interdisciplinary approach is argued, and an institutional way for promoting this development is presented using the example of the Chemistry Biology Centre (KBC) at Umeå University in Sweden.},
	language = {en},
	number = {6},
	urldate = {2024-12-10},
	journal = {Australian Journal of Chemistry},
	publisher = {CSIRO PUBLISHING},
	author = {Messinger, Johannes},
	month = may,
	year = {2012},
	pages = {573--576},
}

This account gives a brief overview of various directions in current solar fuels research. On that basis, the necessity for an interdisciplinary approach is argued, and an institutional way for promoting this development is presented using the example of the Chemistry Biology Centre (KBC) at Umeå University in Sweden.

@article{rapatskiy_detection_2012,
	title = {Detection of the {Water}-{Binding} {Sites} of the {Oxygen}-{Evolving} {Complex} of {Photosystem} {II} {Using} {W}-{Band} 17 {O} {Electron}–{Electron} {Double} {Resonance}-{Detected} {NMR} {Spectroscopy}},
	volume = {134},
	issn = {0002-7863, 1520-5126},
	url = {https://pubs.acs.org/doi/10.1021/ja3053267},
	doi = {10/f2z5c6},
	language = {en},
	number = {40},
	urldate = {2021-06-08},
	journal = {Journal of the American Chemical Society},
	author = {Rapatskiy, Leonid and Cox, Nicholas and Savitsky, Anton and Ames, William M. and Sander, Julia and Nowaczyk, Marc. M. and Rögner, Matthias and Boussac, Alain and Neese, Frank and Messinger, Johannes and Lubitz, Wolfgang},
	month = oct,
	year = {2012},
	pages = {16619--16634},
}

@article{alonso-mori_energy-dispersive_2012,
	title = {Energy-dispersive {X}-ray emission spectroscopy using an {X}-ray free-electron laser in a shot-by-shot mode},
	volume = {109},
	url = {https://www.pnas.org/doi/10.1073/pnas.1211384109},
	doi = {10.1073/pnas.1211384109},
	abstract = {The ultrabright femtosecond X-ray pulses provided by X-ray free-electron lasers open capabilities for studying the structure and dynamics of a wide variety of systems beyond what is possible with synchrotron sources. Recently, this “probe-before-destroy” approach has been demonstrated for atomic structure determination by serial X-ray diffraction of microcrystals. There has been the question whether a similar approach can be extended to probe the local electronic structure by X-ray spectroscopy. To address this, we have carried out femtosecond X-ray emission spectroscopy (XES) at the Linac Coherent Light Source using redox-active Mn complexes. XES probes the charge and spin states as well as the ligand environment, critical for understanding the functional role of redox-active metal sites. Kβ1,3 XES spectra of MnII and Mn2III,IV complexes at room temperature were collected using a wavelength dispersive spectrometer and femtosecond X-ray pulses with an individual dose of up to {\textgreater}100 MGy. The spectra were found in agreement with undamaged spectra collected at low dose using synchrotron radiation. Our results demonstrate that the intact electronic structure of redox active transition metal compounds in different oxidation states can be characterized with this shot-by-shot method. This opens the door for studying the chemical dynamics of metal catalytic sites by following reactions under functional conditions. The technique can be combined with X-ray diffraction to simultaneously obtain the geometric structure of the overall protein and the local chemistry of active metal sites and is expected to prove valuable for understanding the mechanism of important metalloproteins, such as photosystem II.},
	number = {47},
	urldate = {2024-12-10},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Alonso-Mori, Roberto and Kern, Jan and Gildea, Richard J. and Sokaras, Dimosthenis and Weng, Tsu-Chien and Lassalle-Kaiser, Benedikt and Tran, Rosalie and Hattne, Johan and Laksmono, Hartawan and Hellmich, Julia and Glöckner, Carina and Echols, Nathaniel and Sierra, Raymond G. and Schafer, Donald W. and Sellberg, Jonas and Kenney, Christopher and Herbst, Ryan and Pines, Jack and Hart, Philip and Herrmann, Sven and Grosse-Kunstleve, Ralf W. and Latimer, Matthew J. and Fry, Alan R. and Messerschmidt, Marc M. and Miahnahri, Alan and Seibert, M. Marvin and Zwart, Petrus H. and White, William E. and Adams, Paul D. and Bogan, Michael J. and Boutet, Sébastien and Williams, Garth J. and Zouni, Athina and Messinger, Johannes and Glatzel, Pieter and Sauter, Nicholas K. and Yachandra, Vittal K. and Yano, Junko and Bergmann, Uwe},
	month = nov,
	year = {2012},
	pages = {19103--19107},
}

The ultrabright femtosecond X-ray pulses provided by X-ray free-electron lasers open capabilities for studying the structure and dynamics of a wide variety of systems beyond what is possible with synchrotron sources. Recently, this “probe-before-destroy” approach has been demonstrated for atomic structure determination by serial X-ray diffraction of microcrystals. There has been the question whether a similar approach can be extended to probe the local electronic structure by X-ray spectroscopy. To address this, we have carried out femtosecond X-ray emission spectroscopy (XES) at the Linac Coherent Light Source using redox-active Mn complexes. XES probes the charge and spin states as well as the ligand environment, critical for understanding the functional role of redox-active metal sites. Kβ1,3 XES spectra of MnII and Mn2III,IV complexes at room temperature were collected using a wavelength dispersive spectrometer and femtosecond X-ray pulses with an individual dose of up to \textgreater100 MGy. The spectra were found in agreement with undamaged spectra collected at low dose using synchrotron radiation. Our results demonstrate that the intact electronic structure of redox active transition metal compounds in different oxidation states can be characterized with this shot-by-shot method. This opens the door for studying the chemical dynamics of metal catalytic sites by following reactions under functional conditions. The technique can be combined with X-ray diffraction to simultaneously obtain the geometric structure of the overall protein and the local chemistry of active metal sites and is expected to prove valuable for understanding the mechanism of important metalloproteins, such as photosystem II.

@article{sierra_nanoflow_2012,
	title = {Nanoflow electrospinning serial femtosecond crystallography},
	volume = {68},
	issn = {0907-4449},
	url = {https://journals.iucr.org/d/issues/2012/11/00/lv5021/},
	doi = {10.1107/S0907444912038152},
	abstract = {An electrospun liquid microjet has been developed that delivers protein microcrystal suspensions at flow rates of 0.14–3.1 µl min−1 to perform serial femtosecond crystallography (SFX) studies with X-ray lasers. Thermolysin microcrystals flowed at 0.17 µl min−1 and diffracted to beyond 4 Å resolution, producing 14 000 indexable diffraction patterns, or four per second, from 140 µg of protein. Nanoflow electrospinning extends SFX to biological samples that necessitate minimal sample consumption.},
	language = {en},
	number = {11},
	urldate = {2024-12-10},
	journal = {Acta Crystallographica Section D: Biological Crystallography},
	publisher = {International Union of Crystallography},
	author = {Sierra, R. G. and Laksmono, H. and Kern, J. and Tran, R. and Hattne, J. and Alonso-Mori, R. and Lassalle-Kaiser, B. and Glöckner, C. and Hellmich, J. and Schafer, D. W. and Echols, N. and Gildea, R. J. and Grosse-Kunstleve, R. W. and Sellberg, J. and McQueen, T. A. and Fry, A. R. and Messerschmidt, M. M. and Miahnahri, A. and Seibert, M. M. and Hampton, C. Y. and Starodub, D. and Loh, N. D. and Sokaras, D. and Weng, T.-C. and Zwart, P. H. and Glatzel, P. and Milathianaki, D. and White, W. E. and Adams, P. D. and Williams, G. J. and Boutet, S. and Zouni, A. and Messinger, J. and Sauter, N. K. and Bergmann, U. and Yano, J. and Yachandra, V. K. and Bogan, M. J.},
	month = nov,
	year = {2012},
	pages = {1584--1587},
}

An electrospun liquid microjet has been developed that delivers protein microcrystal suspensions at flow rates of 0.14–3.1 µl min−1 to perform serial femtosecond crystallography (SFX) studies with X-ray lasers. Thermolysin microcrystals flowed at 0.17 µl min−1 and diffracted to beyond 4 Å resolution, producing 14 000 indexable diffraction patterns, or four per second, from 140 µg of protein. Nanoflow electrospinning extends SFX to biological samples that necessitate minimal sample consumption.

@article{shevela_probing_2012,
	series = {Photosynthesis {Research} for {Sustainability}: {From} {Natural} to {Artificial}},
	title = {Probing the turnover efficiency of photosystem {II} membrane fragments with different electron acceptors},
	volume = {1817},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272812001235},
	doi = {10.1016/j.bbabio.2012.03.038},
	abstract = {In this study we employ isotope ratio membrane-inlet mass spectrometry to probe the turnover efficiency of photosystem II (PSII) membrane fragments isolated from spinach at flash frequencies between 1Hz and 50Hz in the presence of the commonly used exogenous electron acceptors potassium ferricyanide(III) (FeCy), 2,5-dichloro-p-benzoquinone (DCBQ), and 2-phenyl-p-benzoquinone (PPBQ). The data obtained clearly indicate that among the tested acceptors PPBQ is the best at high flash frequencies. If present at high enough concentration, the PSII turnover efficiency is unaffected by flash frequency of up to 30Hz, and at 40Hz and 50Hz only a slight decrease by about 5–7\% is observed. In contrast, drastic reductions of the O2 yields by about 40\% and 65\% were found at 50Hz for DCBQ and FeCy, respectively. Comparison with literature data reveals that PPBQ accepts electrons from QA− in PSII membrane fragments with similar efficiency as plastoquinone in intact cells. Our data also confirm that at high flashing rates O2 evolution is limited by the reactions on the electron-acceptor side of PSII. The relevance of these data to the evolutionary development of the water-splitting complex in PSII and with regard to the potential of artificial water-splitting catalysts is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.},
	number = {8},
	urldate = {2024-12-10},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Shevela, Dmitriy and Messinger, Johannes},
	month = aug,
	year = {2012},
	keywords = {Artificial photosynthesis, Electron acceptor, Membrane-inlet mass spectrometry, Oxygen evolution, Photosystem II, Turnover frequency},
	pages = {1208--1212},
}

In this study we employ isotope ratio membrane-inlet mass spectrometry to probe the turnover efficiency of photosystem II (PSII) membrane fragments isolated from spinach at flash frequencies between 1Hz and 50Hz in the presence of the commonly used exogenous electron acceptors potassium ferricyanide(III) (FeCy), 2,5-dichloro-p-benzoquinone (DCBQ), and 2-phenyl-p-benzoquinone (PPBQ). The data obtained clearly indicate that among the tested acceptors PPBQ is the best at high flash frequencies. If present at high enough concentration, the PSII turnover efficiency is unaffected by flash frequency of up to 30Hz, and at 40Hz and 50Hz only a slight decrease by about 5–7% is observed. In contrast, drastic reductions of the O2 yields by about 40% and 65% were found at 50Hz for DCBQ and FeCy, respectively. Comparison with literature data reveals that PPBQ accepts electrons from QA− in PSII membrane fragments with similar efficiency as plastoquinone in intact cells. Our data also confirm that at high flashing rates O2 evolution is limited by the reactions on the electron-acceptor side of PSII. The relevance of these data to the evolutionary development of the water-splitting complex in PSII and with regard to the potential of artificial water-splitting catalysts is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

@article{kern_room_2012,
	title = {Room temperature femtosecond {X}-ray diffraction of photosystem {II} microcrystals},
	volume = {109},
	url = {https://www.pnas.org/doi/10.1073/pnas.1204598109},
	doi = {10.1073/pnas.1204598109},
	abstract = {Most of the dioxygen on earth is generated by the oxidation of water by photosystem II (PS II) using light from the sun. This light-driven, four-photon reaction is catalyzed by the Mn4CaO5 cluster located at the lumenal side of PS II. Various X-ray studies have been carried out at cryogenic temperatures to understand the intermediate steps involved in the water oxidation mechanism. However, the necessity for collecting data at room temperature, especially for studying the transient steps during the O–O bond formation, requires the development of new methodologies. In this paper we report room temperature X-ray diffraction data of PS II microcrystals obtained using ultrashort ({\textless} 50 fs) 9 keV X-ray pulses from a hard X-ray free electron laser, namely the Linac Coherent Light Source. The results presented here demonstrate that the ”probe before destroy” approach using an X-ray free electron laser works even for the highly-sensitive Mn4CaO5 cluster in PS II at room temperature. We show that these data are comparable to those obtained in synchrotron radiation studies as seen by the similarities in the overall structure of the helices, the protein subunits and the location of the various cofactors. This work is, therefore, an important step toward future studies for resolving the structure of the Mn4CaO5 cluster without any damage at room temperature, and of the reaction intermediates of PS II during O–O bond formation.},
	number = {25},
	urldate = {2024-12-10},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Kern, Jan and Alonso-Mori, Roberto and Hellmich, Julia and Tran, Rosalie and Hattne, Johan and Laksmono, Hartawan and Glöckner, Carina and Echols, Nathaniel and Sierra, Raymond G. and Sellberg, Jonas and Lassalle-Kaiser, Benedikt and Gildea, Richard J. and Glatzel, Pieter and Grosse-Kunstleve, Ralf W. and Latimer, Matthew J. and McQueen, Trevor A. and DiFiore, Dörte and Fry, Alan R. and Messerschmidt, Marc and Miahnahri, Alan and Schafer, Donald W. and Seibert, M. Marvin and Sokaras, Dimosthenis and Weng, Tsu-Chien and Zwart, Petrus H. and White, William E. and Adams, Paul D. and Bogan, Michael J. and Boutet, Sébastien and Williams, Garth J. and Messinger, Johannes and Sauter, Nicholas K. and Zouni, Athina and Bergmann, Uwe and Yano, Junko and Yachandra, Vittal K.},
	month = jun,
	year = {2012},
	pages = {9721--9726},
}

Most of the dioxygen on earth is generated by the oxidation of water by photosystem II (PS II) using light from the sun. This light-driven, four-photon reaction is catalyzed by the Mn4CaO5 cluster located at the lumenal side of PS II. Various X-ray studies have been carried out at cryogenic temperatures to understand the intermediate steps involved in the water oxidation mechanism. However, the necessity for collecting data at room temperature, especially for studying the transient steps during the O–O bond formation, requires the development of new methodologies. In this paper we report room temperature X-ray diffraction data of PS II microcrystals obtained using ultrashort (\textless 50 fs) 9 keV X-ray pulses from a hard X-ray free electron laser, namely the Linac Coherent Light Source. The results presented here demonstrate that the ”probe before destroy” approach using an X-ray free electron laser works even for the highly-sensitive Mn4CaO5 cluster in PS II at room temperature. We show that these data are comparable to those obtained in synchrotron radiation studies as seen by the similarities in the overall structure of the helices, the protein subunits and the location of the various cofactors. This work is, therefore, an important step toward future studies for resolving the structure of the Mn4CaO5 cluster without any damage at room temperature, and of the reaction intermediates of PS II during O–O bond formation.

@article{lohmiller_basic_2012,
	title = {The {Basic} {Properties} of the {Electronic} {Structure} of the {Oxygen}-evolving {Complex} of {Photosystem} {II} {Are} {Not} {Perturbed} by {Ca2}+ {Removal}*},
	volume = {287},
	issn = {0021-9258},
	url = {https://www.sciencedirect.com/science/article/pii/S002192582043323X},
	doi = {10.1074/jbc.M112.365288},
	abstract = {Ca2+ is an integral component of the Mn4O5Ca cluster of the oxygen-evolving complex in photosystem II (PS II). Its removal leads to the loss of the water oxidizing functionality. The S2′ state of the Ca2+-depleted cluster from spinach is examined by X- and Q-band EPR and 55Mn electron nuclear double resonance (ENDOR) spectroscopy. Spectral simulations demonstrate that upon Ca2+ removal, its electronic structure remains essentially unaltered, i.e. that of a manganese tetramer. No redistribution of the manganese valence states and only minor perturbation of the exchange interactions between the manganese ions were found. Interestingly, the S2′ state in spinach PS II is very similar to the native S2 state of Thermosynechococcus elongatus in terms of spin state energies and insensitivity to methanol addition. These results assign the Ca2+ a functional as opposed to a structural role in water splitting catalysis, such as (i) being essential for efficient proton-coupled electron transfer between YZ and the manganese cluster and/or (ii) providing an initial binding site for substrate water. Additionally, a novel 55Mn2+ signal, detected by Q-band pulse EPR and ENDOR, was observed in Ca2+-depleted PS II. Mn2+ titration, monitored by 55Mn ENDOR, revealed a specific Mn2+ binding site with a submicromolar KD. Ca2+ titration of Mn2+-loaded, Ca2+-depleted PS II demonstrated that the site is reversibly made accessible to Mn2+ by Ca2+ depletion and reconstitution. Mn2+ is proposed to bind at one of the extrinsic subunits. This process is possibly relevant for the formation of the Mn4O5Ca cluster during photoassembly and/or D1 repair. Background: EPR/55Mn ENDOR spectroscopy of the oxygen-evolving complex (OEC) and Mn2+ in Ca2+-depleted photosystem II. Results: Electronic model of the Ca2+-depleted OEC; characterization of Mn2+ binding. Conclusion: Ca2+ is not critical for maintaining the electronic and spatial structure of the OEC. Its removal exposes a Mn2+ binding site supposedly in an extrinsic subunit. Significance: Mechanistic implications for water oxidation; Mn2+ in photoassembly/D1 protein repair.},
	number = {29},
	urldate = {2024-12-10},
	journal = {Journal of Biological Chemistry},
	author = {Lohmiller, Thomas and Cox, Nicholas and Su, Ji-Hu and Messinger, Johannes and Lubitz, Wolfgang},
	month = jul,
	year = {2012},
	keywords = {Calcium, ENDOR Spectroscopy, Electron Paramagnetic Resonance (EPR), Manganese, Metalloproteins, Oxygen-evolving Complex (OEC), Photoassembly/Photoactivation, Photosystem II, Water-oxidizing Complex (WOC), Zero-field Splitting},
	pages = {24721--24733},
}

Ca2+ is an integral component of the Mn4O5Ca cluster of the oxygen-evolving complex in photosystem II (PS II). Its removal leads to the loss of the water oxidizing functionality. The S2′ state of the Ca2+-depleted cluster from spinach is examined by X- and Q-band EPR and 55Mn electron nuclear double resonance (ENDOR) spectroscopy. Spectral simulations demonstrate that upon Ca2+ removal, its electronic structure remains essentially unaltered, i.e. that of a manganese tetramer. No redistribution of the manganese valence states and only minor perturbation of the exchange interactions between the manganese ions were found. Interestingly, the S2′ state in spinach PS II is very similar to the native S2 state of Thermosynechococcus elongatus in terms of spin state energies and insensitivity to methanol addition. These results assign the Ca2+ a functional as opposed to a structural role in water splitting catalysis, such as (i) being essential for efficient proton-coupled electron transfer between YZ and the manganese cluster and/or (ii) providing an initial binding site for substrate water. Additionally, a novel 55Mn2+ signal, detected by Q-band pulse EPR and ENDOR, was observed in Ca2+-depleted PS II. Mn2+ titration, monitored by 55Mn ENDOR, revealed a specific Mn2+ binding site with a submicromolar KD. Ca2+ titration of Mn2+-loaded, Ca2+-depleted PS II demonstrated that the site is reversibly made accessible to Mn2+ by Ca2+ depletion and reconstitution. Mn2+ is proposed to bind at one of the extrinsic subunits. This process is possibly relevant for the formation of the Mn4O5Ca cluster during photoassembly and/or D1 repair. Background: EPR/55Mn ENDOR spectroscopy of the oxygen-evolving complex (OEC) and Mn2+ in Ca2+-depleted photosystem II. Results: Electronic model of the Ca2+-depleted OEC; characterization of Mn2+ binding. Conclusion: Ca2+ is not critical for maintaining the electronic and spatial structure of the OEC. Its removal exposes a Mn2+ binding site supposedly in an extrinsic subunit. Significance: Mechanistic implications for water oxidation; Mn2+ in photoassembly/D1 protein repair.

@article{shevela_calcium_2011,
	title = {Calcium {Manganese} {Oxides} as {Oxygen} {Evolution} {Catalysts}: {O2} {Formation} {Pathways} {Indicated} by {18O}-{Labelling} {Studies}},
	volume = {17},
	copyright = {Copyright © 2011 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
	issn = {1521-3765},
	shorttitle = {Calcium {Manganese} {Oxides} as {Oxygen} {Evolution} {Catalysts}},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/chem.201002548},
	doi = {10.1002/chem.201002548},
	abstract = {Oxygen evolution catalysed by calcium manganese and manganese-only oxides was studied in 18O-enriched water. Using membrane-inlet mass spectrometry, we monitored the formation of the different O2 isotopologues 16O2, 16O18O and 18O2 in such reactions simultaneously with good time resolution. From the analysis of the data, we conclude that entirely different pathways of dioxygen formation catalysis exist for reactions involving hydrogen peroxide (H2O2), hydrogen persulfate (HSO5−) or single-electron oxidants such as CeIV and [RuIII(bipy)3]3+. Like the studied oxide catalysts, the active sites of manganese catalase and the oxygen-evolving complex (OEC) of photosystem II (PSII) consist of μ-oxido manganese or μ-oxido calcium manganese sites. The studied processes show very similar 18O-labelling behaviour to the natural enzymes and are therefore interesting model systems for in vivo oxygen formation by manganese metalloenzymes such as PSII.},
	language = {en},
	number = {19},
	urldate = {2024-12-10},
	journal = {Chemistry – A European Journal},
	author = {Shevela, Dmitriy and Koroidov, Sergey and Najafpour, M. Mahdi and Messinger, Johannes and Kurz, Philipp},
	year = {2011},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201002548},
	keywords = {heterogeneous catalysis, isotopic labelling, manganese, mass spectrometry, water splitting},
	pages = {5415--5423},
}

Oxygen evolution catalysed by calcium manganese and manganese-only oxides was studied in 18O-enriched water. Using membrane-inlet mass spectrometry, we monitored the formation of the different O2 isotopologues 16O2, 16O18O and 18O2 in such reactions simultaneously with good time resolution. From the analysis of the data, we conclude that entirely different pathways of dioxygen formation catalysis exist for reactions involving hydrogen peroxide (H2O2), hydrogen persulfate (HSO5−) or single-electron oxidants such as CeIV and [RuIII(bipy)3]3+. Like the studied oxide catalysts, the active sites of manganese catalase and the oxygen-evolving complex (OEC) of photosystem II (PSII) consist of μ-oxido manganese or μ-oxido calcium manganese sites. The studied processes show very similar 18O-labelling behaviour to the natural enzymes and are therefore interesting model systems for in vivo oxygen formation by manganese metalloenzymes such as PSII.

@article{cox_effect_2011,
	title = {Effect of {Ca2}+/{Sr2}+ {Substitution} on the {Electronic} {Structure} of the {Oxygen}-{Evolving} {Complex} of {Photosystem} {II}: {A} {Combined} {Multifrequency} {EPR}, {55Mn}-{ENDOR}, and {DFT} {Study} of the {S2} {State}},
	volume = {133},
	issn = {0002-7863},
	shorttitle = {Effect of {Ca2}+/{Sr2}+ {Substitution} on the {Electronic} {Structure} of the {Oxygen}-{Evolving} {Complex} of {Photosystem} {II}},
	url = {https://doi.org/10.1021/ja110145v},
	doi = {10.1021/ja110145v},
	abstract = {The electronic structures of the native Mn4OxCa cluster and the biosynthetically substituted Mn4OxSr cluster of the oxygen evolving complex (OEC) of photosystem II (PSII) core complexes isolated from Thermosynechococcus elongatus, poised in the S2 state, were studied by X- and Q-band CW-EPR and by pulsed Q-band 55Mn-ENDOR spectroscopy. Both wild type and tyrosine D less mutants grown photoautotrophically in either CaCl2 or SrCl2 containing media were measured. The obtained CW-EPR spectra of the S2 state displayed the characteristic, clearly noticeable differences in the hyperfine pattern of the multiline EPR signal [Boussac et al. J. Biol. Chem.2004, 279, 22809−22819]. In sharp contrast, the manganese (55Mn) ENDOR spectra of the Ca and Sr forms of the OEC were remarkably similar. Multifrequency simulations of the X- and Q-band CW-EPR and 55Mn-pulsed ENDOR spectra using the Spin Hamiltonian formalism were performed to investigate this surprising result. It is shown that (i) all four manganese ions contribute to the 55Mn-ENDOR spectra; (ii) only small changes are seen in the fitted isotropic hyperfine values for the Ca2+ and Sr2+ containing OEC, suggesting that there is no change in the overall spin distribution (electronic coupling scheme) upon Ca2+/Sr2+ substitution; (iii) the changes in the CW-EPR hyperfine pattern can be explained by a small decrease in the anisotropy of at least two hyperfine tensors. It is proposed that modifications at the Ca2+ site may modulate the fine structure tensor of the MnIII ion. DFT calculations support the above conclusions. Our data analysis also provides strong support for the notion that in the S2 state the coordination of the MnIII ion is square-pyramidal (5-coordinate) or octahedral (6-coordinate) with tetragonal elongation. In addition, it is shown that only one of the currently published OEC models, the Siegbahn structure [Siegbahn, P. E. M. Acc. Chem. Res.2009, 42, 1871−1880, Pantazis, D. A. et al. Phys. Chem. Chem. Phys.2009, 11, 6788−6798], is consistent with all data presented here. These results provide important information for the structure of the OEC and the water-splitting mechanism. In particular, the 5-coordinate MnIII is a potential site for substrate ‘water’ (H2O, OH−) binding. Its location within the cuboidal structural unit, as opposed to the external ‘dangler’ position, may have important consequences for the mechanism of O−O bond formation.},
	number = {10},
	urldate = {2024-12-10},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Cox, Nicholas and Rapatskiy, Leonid and Su, Ji-Hu and Pantazis, Dimitrios A. and Sugiura, Miwa and Kulik, Leonid and Dorlet, Pierre and Rutherford, A. William and Neese, Frank and Boussac, Alain and Lubitz, Wolfgang and Messinger, Johannes},
	month = mar,
	year = {2011},
	pages = {3635--3648},
}

The electronic structures of the native Mn4OxCa cluster and the biosynthetically substituted Mn4OxSr cluster of the oxygen evolving complex (OEC) of photosystem II (PSII) core complexes isolated from Thermosynechococcus elongatus, poised in the S2 state, were studied by X- and Q-band CW-EPR and by pulsed Q-band 55Mn-ENDOR spectroscopy. Both wild type and tyrosine D less mutants grown photoautotrophically in either CaCl2 or SrCl2 containing media were measured. The obtained CW-EPR spectra of the S2 state displayed the characteristic, clearly noticeable differences in the hyperfine pattern of the multiline EPR signal [Boussac et al. J. Biol. Chem.2004, 279, 22809−22819]. In sharp contrast, the manganese (55Mn) ENDOR spectra of the Ca and Sr forms of the OEC were remarkably similar. Multifrequency simulations of the X- and Q-band CW-EPR and 55Mn-pulsed ENDOR spectra using the Spin Hamiltonian formalism were performed to investigate this surprising result. It is shown that (i) all four manganese ions contribute to the 55Mn-ENDOR spectra; (ii) only small changes are seen in the fitted isotropic hyperfine values for the Ca2+ and Sr2+ containing OEC, suggesting that there is no change in the overall spin distribution (electronic coupling scheme) upon Ca2+/Sr2+ substitution; (iii) the changes in the CW-EPR hyperfine pattern can be explained by a small decrease in the anisotropy of at least two hyperfine tensors. It is proposed that modifications at the Ca2+ site may modulate the fine structure tensor of the MnIII ion. DFT calculations support the above conclusions. Our data analysis also provides strong support for the notion that in the S2 state the coordination of the MnIII ion is square-pyramidal (5-coordinate) or octahedral (6-coordinate) with tetragonal elongation. In addition, it is shown that only one of the currently published OEC models, the Siegbahn structure [Siegbahn, P. E. M. Acc. Chem. Res.2009, 42, 1871−1880, Pantazis, D. A. et al. Phys. Chem. Chem. Phys.2009, 11, 6788−6798], is consistent with all data presented here. These results provide important information for the structure of the OEC and the water-splitting mechanism. In particular, the 5-coordinate MnIII is a potential site for substrate ‘water’ (H2O, OH−) binding. Its location within the cuboidal structural unit, as opposed to the external ‘dangler’ position, may have important consequences for the mechanism of O−O bond formation.

@article{cox_electronic_2011,
	title = {Electronic {Structure} of a {Weakly} {Antiferromagnetically} {Coupled} {MnIIMnIII} {Model} {Relevant} to {Manganese} {Proteins}: {A} {Combined} {EPR}, {55Mn}-{ENDOR}, and {DFT} {Study}},
	volume = {50},
	issn = {0020-1669},
	shorttitle = {Electronic {Structure} of a {Weakly} {Antiferromagnetically} {Coupled} {MnIIMnIII} {Model} {Relevant} to {Manganese} {Proteins}},
	url = {https://doi.org/10.1021/ic200767e},
	doi = {10.1021/ic200767e},
	abstract = {An analysis of the electronic structure of the [MnIIMnIII(μ-OH)-(μ-piv)2(Me3tacn)2](ClO4)2 (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-μ-hydroxo bridge, bis-μ-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the MnIII. All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the 55Mn-ENDOR constrains the 55Mn hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the MnII and MnIII ions. The large effective hyperfine tensor anisotropy of the MnII, a d5 ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the MnIII ion strongly perturbs the zero-field energy levels of the MnIIMnIII complex. An estimate of the fine structure parameter (d) for the MnIII of −4 cm–1 was made, by assuming the intrinsic anisotropy of the MnII ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the MnIII is consistent with the known coordination environment of the MnIII ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5\%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.},
	number = {17},
	urldate = {2024-12-10},
	journal = {Inorganic Chemistry},
	publisher = {American Chemical Society},
	author = {Cox, Nicholas and Ames, William and Epel, Boris and Kulik, Leonid V. and Rapatskiy, Leonid and Neese, Frank and Messinger, Johannes and Wieghardt, Karl and Lubitz, Wolfgang},
	month = sep,
	year = {2011},
	pages = {8238--8251},
}

An analysis of the electronic structure of the [MnIIMnIII(μ-OH)-(μ-piv)2(Me3tacn)2](ClO4)2 (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-μ-hydroxo bridge, bis-μ-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the MnIII. All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the 55Mn-ENDOR constrains the 55Mn hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the MnII and MnIII ions. The large effective hyperfine tensor anisotropy of the MnII, a d5 ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the MnIII ion strongly perturbs the zero-field energy levels of the MnIIMnIII complex. An estimate of the fine structure parameter (d) for the MnIII of −4 cm–1 was made, by assuming the intrinsic anisotropy of the MnII ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the MnIII is consistent with the known coordination environment of the MnIII ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.

@article{buren_importance_2011,
	title = {Importance of {Post}-{Translational} {Modifications} for {Functionality} of a {Chloroplast}-{Localized} {Carbonic} {Anhydrase} ({CAH1}) in {Arabidopsis} thaliana},
	volume = {6},
	issn = {1932-6203},
	url = {https://dx.plos.org/10.1371/journal.pone.0021021},
	doi = {10/bdgdgk},
	language = {en},
	number = {6},
	urldate = {2021-06-08},
	journal = {PLoS ONE},
	author = {Burén, Stefan and Ortega-Villasante, Cristina and Blanco-Rivero, Amaya and Martínez-Bernardini, Andrea and Shutova, Tatiana and Shevela, Dmitriy and Messinger, Johannes and Bakó, Laszlo and Villarejo, Arsenio and Samuelsson, Göran},
	editor = {Bassham, Diane},
	month = jun,
	year = {2011},
	pages = {e21021},
}

@article{shevela_membrane-inlet_2011,
	title = {Membrane-inlet mass spectrometry reveals a high driving force for oxygen production by photosystem {II}},
	volume = {108},
	issn = {0027-8424, 1091-6490},
	url = {http://www.pnas.org/lookup/doi/10.1073/pnas.1014249108},
	doi = {10/dtfj9n},
	language = {en},
	number = {9},
	urldate = {2021-06-08},
	journal = {Proceedings of the National Academy of Sciences},
	author = {Shevela, Dmitriy and Beckmann, Katrin and Clausen, Jürgen and Junge, Wolfgang and Messinger, Johannes},
	month = mar,
	year = {2011},
	pages = {3602--3607},
}

@article{messinger_photosynthetic_2011,
	title = {Photosynthetic {O2} {Evolution}},
	url = {https://books.rsc.org/books/edited-volume/1811/chapter/2135927/Photosynthetic-O2-Evolution},
	doi = {10.1039/9781849733038-00163},
	abstract = {Oxygen evolution by photosynthetic water oxidation has shaped life on planet Earth. This unique biological reaction may provide important clues for develop},
	language = {en},
	urldate = {2024-12-10},
	journal = {Molecular Solar Fuels RSC},
	author = {Messinger, Johannes and Noguchi, Takumi and Yano, Junko},
	month = dec,
	year = {2011},
	pages = {302--314},
}

Oxygen evolution by photosynthetic water oxidation has shaped life on planet Earth. This unique biological reaction may provide important clues for develop

@incollection{cahen_principles_2011,
	address = {Cambridge},
	title = {Principles of photosynthesis},
	isbn = {978-1-107-00023-0},
	url = {https://www.cambridge.org/core/books/fundamentals-of-materials-for-energy-and-environmental-sustainability/principles-of-photosynthesis/E498F19B0D9D77EEC9B92A3B12DF5C9B},
	doi = {10.1017/CBO9780511718786.028},
	abstract = {FocusPhotosynthesis is the biological process that converts sunlight into chemical energy. It provides the basis for life on Earth and is the ultimate source of all fossil fuels and of the oxygen we breathe. The primary light reactions occur with high quantum yield and drive free-energy-demanding chemical reactions with unsurpassed efficiency. Coupling of photosynthesis to hydrogenases allows some organisms to evolve H2. Research into understanding and applying the molecular details and reaction mechanisms of the involved catalysts is well under way.SynopsisLife needs free energy. On our planet this free energy is mostly provided by the Sun. The sunlight is captured and converted into chemical energy by a process known as photosynthesis (from Greek, photo, “light,” and synthesis, “putting together”). This process occurs in plants and many bacteria. The “big bang” of evolution was the development of oxygenic photosynthesis. In this process sunlight is employed to split the abundant water into the molecular oxygen we breathe. The protons and electrons gained are employed by the organism within complex reaction sequences to reduce CO2 to carbohydrates. The widespread availability of the electron source water allowed oxygenic organisms to spread and diversify rapidly. The O2 produced was initially toxic for most species, but those which learned to cope with the emerging oxygen-rich atmosphere were able to gain additional energy by “burning” organic matter.},
	urldate = {2024-12-10},
	booktitle = {Fundamentals of {Materials} for {Energy} and {Environmental} {Sustainability}},
	publisher = {Cambridge University Press},
	author = {Messinger, Johannes and Shevela, Dmitriy},
	editor = {Cahen, David and Ginley, David S.},
	year = {2011},
	pages = {302--314},
}

FocusPhotosynthesis is the biological process that converts sunlight into chemical energy. It provides the basis for life on Earth and is the ultimate source of all fossil fuels and of the oxygen we breathe. The primary light reactions occur with high quantum yield and drive free-energy-demanding chemical reactions with unsurpassed efficiency. Coupling of photosynthesis to hydrogenases allows some organisms to evolve H2. Research into understanding and applying the molecular details and reaction mechanisms of the involved catalysts is well under way.SynopsisLife needs free energy. On our planet this free energy is mostly provided by the Sun. The sunlight is captured and converted into chemical energy by a process known as photosynthesis (from Greek, photo, “light,” and synthesis, “putting together”). This process occurs in plants and many bacteria. The “big bang” of evolution was the development of oxygenic photosynthesis. In this process sunlight is employed to split the abundant water into the molecular oxygen we breathe. The protons and electrons gained are employed by the organism within complex reaction sequences to reduce CO2 to carbohydrates. The widespread availability of the electron source water allowed oxygenic organisms to spread and diversify rapidly. The O2 produced was initially toxic for most species, but those which learned to cope with the emerging oxygen-rich atmosphere were able to gain additional energy by “burning” organic matter.

@article{su_probing_2011,
	title = {Probing {Mode} and {Site} of {Substrate} {Water} {Binding} to the {Oxygen}-{Evolving} {Complex} in the {S2} {State} of {Photosystem} {II} by {17O}-{HYSCORE} {Spectroscopy}},
	volume = {133},
	issn = {0002-7863},
	url = {https://doi.org/10.1021/ja205377n},
	doi = {10.1021/ja205377n},
	number = {31},
	urldate = {2024-11-29},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Su, Ji-Hu and Lubitz, Wolfgang and Messinger, Johannes},
	month = aug,
	year = {2011},
	pages = {12317--12317},
}

@article{su_electronic_2011,
	title = {The electronic structures of the \textit{{S}}2 states of the oxygen-evolving complexes of photosystem {II} in plants and cyanobacteria in the presence and absence of methanol},
	volume = {1807},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272811000533},
	doi = {10.1016/j.bbabio.2011.03.002},
	abstract = {The electronic properties of the Mn4OxCa cluster in the S2 state of the oxygen-evolving complex (OEC) were studied using X- and Q-band EPR and Q-band 55Mn-ENDOR using photosystem II preparations isolated from the thermophilic cyanobacterium T. elongatus and higher plants (spinach). The data presented here show that there is very little difference between the two species. Specifically it is shown that: (i) only small changes are seen in the fitted isotropic hyperfine values, suggesting that there is no significant difference in the overall spin distribution (electronic coupling scheme) between the two species; (ii) the inferred fine-structure tensor of the only MnIII ion in the cluster is of the same magnitude and geometry for both species types, suggesting that the MnIII ion has the same coordination sphere in both sample preparations; and (iii) the data from both species are consistent with only one structural model available in the literature, namely the Siegbahn structure [Siegbahn, P. E. M. Accounts Chem. Res. 2009, 42, 1871–1880, Pantazis, D. A. et al., Phys. Chem. Chem. Phys. 2009, 11, 6788–6798]. These measurements were made in the presence of methanol because it confers favorable magnetic relaxation properties to the cluster that facilitate pulse-EPR techniques. In the absence of methanol the separation of the ground state and the first excited state of the spin system is smaller. For cyanobacteria this effect is minor but in plant PS II it leads to a break-down of the ST=½ spin model of the S2 state. This suggests that the methanol–OEC interaction is species dependent. It is proposed that the effect of small organic solvents on the electronic structure of the cluster is to change the coupling between the outer Mn (MnA) and the other three Mn ions that form the trimeric part of the cluster (MnB, MnC, MnD), by perturbing the linking bis-μ-oxo bridge. The flexibility of this bridging unit is discussed with regard to the mechanism of O-O bond formation.},
	number = {7},
	urldate = {2024-12-10},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Su, Ji-Hu and Cox, Nicholas and Ames, William and Pantazis, Dimitrios A. and Rapatskiy, Leonid and Lohmiller, Thomas and Kulik, Leonid V. and Dorlet, Pierre and Rutherford, A. William and Neese, Frank and Boussac, Alain and Lubitz, Wolfgang and Messinger, Johannes},
	month = jul,
	year = {2011},
	keywords = {EPR, Methanol, Mn-ENDOR, MnOCa cluster, OEC, Orbach process, Photosystem II, Raman process, Spin Hamiltonian},
	pages = {829--840},
}

The electronic properties of the Mn4OxCa cluster in the S2 state of the oxygen-evolving complex (OEC) were studied using X- and Q-band EPR and Q-band 55Mn-ENDOR using photosystem II preparations isolated from the thermophilic cyanobacterium T. elongatus and higher plants (spinach). The data presented here show that there is very little difference between the two species. Specifically it is shown that: (i) only small changes are seen in the fitted isotropic hyperfine values, suggesting that there is no significant difference in the overall spin distribution (electronic coupling scheme) between the two species; (ii) the inferred fine-structure tensor of the only MnIII ion in the cluster is of the same magnitude and geometry for both species types, suggesting that the MnIII ion has the same coordination sphere in both sample preparations; and (iii) the data from both species are consistent with only one structural model available in the literature, namely the Siegbahn structure [Siegbahn, P. E. M. Accounts Chem. Res. 2009, 42, 1871–1880, Pantazis, D. A. et al., Phys. Chem. Chem. Phys. 2009, 11, 6788–6798]. These measurements were made in the presence of methanol because it confers favorable magnetic relaxation properties to the cluster that facilitate pulse-EPR techniques. In the absence of methanol the separation of the ground state and the first excited state of the spin system is smaller. For cyanobacteria this effect is minor but in plant PS II it leads to a break-down of the ST=½ spin model of the S2 state. This suggests that the methanol–OEC interaction is species dependent. It is proposed that the effect of small organic solvents on the electronic structure of the cluster is to change the coupling between the outer Mn (MnA) and the other three Mn ions that form the trimeric part of the cluster (MnB, MnC, MnD), by perturbing the linking bis-μ-oxo bridge. The flexibility of this bridging unit is discussed with regard to the mechanism of O-O bond formation.

@article{ames_theoretical_2011,
	title = {Theoretical {Evaluation} of {Structural} {Models} of the {S2} {State} in the {Oxygen} {Evolving} {Complex} of {Photosystem} {II}: {Protonation} {States} and {Magnetic} {Interactions}},
	volume = {133},
	issn = {0002-7863},
	shorttitle = {Theoretical {Evaluation} of {Structural} {Models} of the {S2} {State} in the {Oxygen} {Evolving} {Complex} of {Photosystem} {II}},
	url = {https://doi.org/10.1021/ja2041805},
	doi = {10.1021/ja2041805},
	abstract = {Protonation states of water ligands and oxo bridges are intimately involved in tuning the electronic structures and oxidation potentials of the oxygen evolving complex (OEC) in Photosystem II, steering the mechanistic pathway, which involves at least five redox state intermediates Sn (n = 0–4) resulting in the oxidation of water to molecular oxygen. Although protons are practically invisible in protein crystallography, their effects on the electronic structure and magnetic properties of metal active sites can be probed using spectroscopy. With the twin purpose of aiding the interpretation of the complex electron paramagnetic resonance (EPR) spectroscopic data of the OEC and of improving the view of the cluster at the atomic level, a complete set of protonation configurations for the S2 state of the OEC were investigated, and their distinctive effects on magnetic properties of the cluster were evaluated. The most recent X-ray structure of Photosystem II at 1.9 Å resolution was used and refined to obtain the optimum structure for the Mn4O5Ca core within the protein pocket. Employing this model, a set of 26 structures was constructed that tested various protonation scenarios of the water ligands and oxo bridges. Our results suggest that one of the two water molecules that are proposed to coordinate the outer Mn ion (MnA) of the cluster is deprotonated in the S2 state, as this leads to optimal experimental agreement, reproducing the correct ground state spin multiplicity (S = 1/2), spin expectation values, and EXAFS-derived metal–metal distances. Deprotonation of Ca2+-bound water molecules is strongly disfavored in the S2 state, but dissociation of one of the two water ligands appears to be facile. The computed isotropic hyperfine couplings presented here allow distinctions between models to be made and call into question the assumption that the largest coupling is always attributable to MnIII. The present results impose limits for the total charge and the proton configuration of the OEC in the S2 state, with implications for the cascade of events in the Kok cycle and for the water splitting mechanism.},
	number = {49},
	urldate = {2024-12-10},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Ames, William and Pantazis, Dimitrios A. and Krewald, Vera and Cox, Nicholas and Messinger, Johannes and Lubitz, Wolfgang and Neese, Frank},
	month = dec,
	year = {2011},
	pages = {19743--19757},
}

Protonation states of water ligands and oxo bridges are intimately involved in tuning the electronic structures and oxidation potentials of the oxygen evolving complex (OEC) in Photosystem II, steering the mechanistic pathway, which involves at least five redox state intermediates Sn (n = 0–4) resulting in the oxidation of water to molecular oxygen. Although protons are practically invisible in protein crystallography, their effects on the electronic structure and magnetic properties of metal active sites can be probed using spectroscopy. With the twin purpose of aiding the interpretation of the complex electron paramagnetic resonance (EPR) spectroscopic data of the OEC and of improving the view of the cluster at the atomic level, a complete set of protonation configurations for the S2 state of the OEC were investigated, and their distinctive effects on magnetic properties of the cluster were evaluated. The most recent X-ray structure of Photosystem II at 1.9 Å resolution was used and refined to obtain the optimum structure for the Mn4O5Ca core within the protein pocket. Employing this model, a set of 26 structures was constructed that tested various protonation scenarios of the water ligands and oxo bridges. Our results suggest that one of the two water molecules that are proposed to coordinate the outer Mn ion (MnA) of the cluster is deprotonated in the S2 state, as this leads to optimal experimental agreement, reproducing the correct ground state spin multiplicity (S = 1/2), spin expectation values, and EXAFS-derived metal–metal distances. Deprotonation of Ca2+-bound water molecules is strongly disfavored in the S2 state, but dissociation of one of the two water ligands appears to be facile. The computed isotropic hyperfine couplings presented here allow distinctions between models to be made and call into question the assumption that the largest coupling is always attributable to MnIII. The present results impose limits for the total charge and the proton configuration of the OEC in the S2 state, with implications for the cascade of events in the Kok cycle and for the water splitting mechanism.

@article{su_is_2010,
	title = {Is {Mn}-{Bound} {Substrate} {Water} {Protonated} in the {S2} {State} of {Photosystem} {II}?},
	volume = {37},
	issn = {1613-7507},
	url = {https://doi.org/10.1007/s00723-009-0051-1},
	doi = {10.1007/s00723-009-0051-1},
	abstract = {In spite of great progress in resolving the geometric structure of the water-splitting Mn4OxCa cluster in photosystem II, the binding sites and modes of the two substrate water molecules are still insufficiently characterized. While time-resolved membrane-inlet mass spectrometry measurements indicate that both substrate water molecules are bound to the oxygen-evolving complex (OEC) in the S2 and S3 states (Hendry and Wydrzynski in Biochemistry 41:13328–13334, 2002), it is not known (1) if they are both Mn-bound, (2) if they are terminal or bridging ligands, and (3) in what protonation state they are bound in the different oxidation states Si(i = 0, 1, 2, 3, 4) of the OEC. By employing 17O hyperfine sublevel correlation (HYSCORE) spectroscopy we recently demonstrated that in the S2 state there is only one (type of) Mn-bound oxygen that is water exchangeable. We therefore tentatively identified this oxygen as one substrate ‘water’ molecule, and on the basis of the finding that it has a hyperfine interaction of about 10 MHz with the electron spin of the Mn4OxCa cluster, we suggest that it is bound as a Mn–O–Mn bridge within a bis-μ2 oxo-bridged unit (Su et al. in J Am Chem Soc 130:786–787, 2008). Employing pulse electron paramagnetic resonance, 1H/2H Mims electron-nuclear double resonance and 2H-HYSCORE spectroscopies together with 1H/2H-exchange here, we test this hypothesis by probing the protonation state of this exchangeable oxygen. We conclude that this oxygen is fully deprotonated. This result is discussed in the light of earlier reports in the literature.},
	language = {en},
	number = {1},
	urldate = {2024-12-10},
	journal = {Applied Magnetic Resonance},
	author = {Su, Ji-Hu and Messinger, Johannes},
	month = jan,
	year = {2010},
	keywords = {Electron Paramagnetic Resonance, Electron Spin Echo, Electron Spin Echo Envelope Modulation, Pulse Electron Paramagnetic Resonance, Substrate Water},
	pages = {123--136},
}

In spite of great progress in resolving the geometric structure of the water-splitting Mn4OxCa cluster in photosystem II, the binding sites and modes of the two substrate water molecules are still insufficiently characterized. While time-resolved membrane-inlet mass spectrometry measurements indicate that both substrate water molecules are bound to the oxygen-evolving complex (OEC) in the S2 and S3 states (Hendry and Wydrzynski in Biochemistry 41:13328–13334, 2002), it is not known (1) if they are both Mn-bound, (2) if they are terminal or bridging ligands, and (3) in what protonation state they are bound in the different oxidation states Si(i = 0, 1, 2, 3, 4) of the OEC. By employing 17O hyperfine sublevel correlation (HYSCORE) spectroscopy we recently demonstrated that in the S2 state there is only one (type of) Mn-bound oxygen that is water exchangeable. We therefore tentatively identified this oxygen as one substrate ‘water’ molecule, and on the basis of the finding that it has a hyperfine interaction of about 10 MHz with the electron spin of the Mn4OxCa cluster, we suggest that it is bound as a Mn–O–Mn bridge within a bis-μ2 oxo-bridged unit (Su et al. in J Am Chem Soc 130:786–787, 2008). Employing pulse electron paramagnetic resonance, 1H/2H Mims electron-nuclear double resonance and 2H-HYSCORE spectroscopies together with 1H/2H-exchange here, we test this hypothesis by probing the protonation state of this exchangeable oxygen. We conclude that this oxygen is fully deprotonated. This result is discussed in the light of earlier reports in the literature.

@incollection{govindjee_photosystem_2010,
	address = {Chichester, UK},
	title = {Photosystem {II}},
	copyright = {Copyright © 2010 John Wiley \& Sons, Ltd. All rights reserved.},
	isbn = {978-0-470-01590-2},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470015902.a0000669.pub2},
	doi = {10.1002/9780470015902.a0000669.pub2},
	abstract = {Photosystem II (PSII) is a specialized protein complex that uses light energy to drive the transfer of electrons from water to plastoquinone, resulting in the production of oxygen and the release of reduced plastoquinone into the photosynthetic membrane. The key components of the PSII complex include a peripheral antenna system that employs chlorophyll and other pigment molecules to absorb light, a reaction centre at the core of the complex that is the site of the initial electron transfer reactions, an Mn4OxCa cluster that catalyses water oxidation and a binding pocket for the reduction of plastoquinone. PSII is the sole source of oxygen production in all oxygenic photosynthetic organisms, which include plants, algae and cyanobacteria. In these organisms, PSII operates in series with other protein complexes, including the PSI reaction centre, to produce the reduced form of nicotenamide–adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), which is used in the Calvin–Benson cycle to produce carbohydrates from carbon dioxide. Key concepts Photosystem II (PSII) is a membrane-embedded protein–pigment complex, containing more than 20 subunits and approximately 100 cofactors. Antenna and reaction centre regions in PSII are in separate protein complexes. Light is absorbed by chlorophyll, carotenoid and phycobilin pigments in the antenna regions and the excitation energy is rapidly transferred to the reaction centre domain. PSII can switch among different modes to either utilize up to 90\% of the incident light for charge separation (under low light conditions) or convert a large portion of the excess light into heat and light (fluorescence) (under high light conditions). The initial light-induced charge separation results in the formation of a chlorophyll cation and a pheophytin anion which are approximately 10 Å apart; this charge separation is rapidly stabilized by transfer of the charges to other more distant cofactors. The oxidation of water occurs at an Mn4OxCa cluster embedded in the protein environment of subunits D1 and CP43. To oxidize two molecules of water four oxidizing equivalents must be accumulated in the Mn4OxCa cluster by four consecutive light-induced charge separation(s). There are several conflicting proposals on the mechanism of water oxidation at the Mn4OxCa cluster in PSII. The electrons and protons extracted from water by PSII are finally used to drive the reduction of NADP+ and the production of ATP, respectively.},
	language = {en},
	urldate = {2024-12-11},
	booktitle = {Encyclopedia of {Life} {Sciences} ({eLS})},
	publisher = {John Wiley \& Sons, Ltd},
	author = {{Govindjee} and Kern, Jan F and Messinger, Johannes and Whitmarsh, John},
	year = {2010},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/9780470015902.a0000669.pub2},
	keywords = {chlorophyll, electron transport, oxygen evolution, photosynthesis, primary photochemistry, reaction centre},
}

Photosystem II (PSII) is a specialized protein complex that uses light energy to drive the transfer of electrons from water to plastoquinone, resulting in the production of oxygen and the release of reduced plastoquinone into the photosynthetic membrane. The key components of the PSII complex include a peripheral antenna system that employs chlorophyll and other pigment molecules to absorb light, a reaction centre at the core of the complex that is the site of the initial electron transfer reactions, an Mn4OxCa cluster that catalyses water oxidation and a binding pocket for the reduction of plastoquinone. PSII is the sole source of oxygen production in all oxygenic photosynthetic organisms, which include plants, algae and cyanobacteria. In these organisms, PSII operates in series with other protein complexes, including the PSI reaction centre, to produce the reduced form of nicotenamide–adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), which is used in the Calvin–Benson cycle to produce carbohydrates from carbon dioxide. Key concepts Photosystem II (PSII) is a membrane-embedded protein–pigment complex, containing more than 20 subunits and approximately 100 cofactors. Antenna and reaction centre regions in PSII are in separate protein complexes. Light is absorbed by chlorophyll, carotenoid and phycobilin pigments in the antenna regions and the excitation energy is rapidly transferred to the reaction centre domain. PSII can switch among different modes to either utilize up to 90% of the incident light for charge separation (under low light conditions) or convert a large portion of the excess light into heat and light (fluorescence) (under high light conditions). The initial light-induced charge separation results in the formation of a chlorophyll cation and a pheophytin anion which are approximately 10 Å apart; this charge separation is rapidly stabilized by transfer of the charges to other more distant cofactors. The oxidation of water occurs at an Mn4OxCa cluster embedded in the protein environment of subunits D1 and CP43. To oxidize two molecules of water four oxidizing equivalents must be accumulated in the Mn4OxCa cluster by four consecutive light-induced charge separation(s). There are several conflicting proposals on the mechanism of water oxidation at the Mn4OxCa cluster in PSII. The electrons and protons extracted from water by PSII are finally used to drive the reduction of NADP+ and the production of ATP, respectively.

@article{pantazis_new_2009,
	title = {A {New} {Quantum} {Chemical} {Approach} to the {Magnetic} {Properties} of {Oligonuclear} {Transition}-{Metal} {Complexes}: {Application} to a {Model} for the {Tetranuclear} {Manganese} {Cluster} of {Photosystem} {II}},
	volume = {15},
	copyright = {Copyright © 2009 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
	issn = {1521-3765},
	shorttitle = {A {New} {Quantum} {Chemical} {Approach} to the {Magnetic} {Properties} of {Oligonuclear} {Transition}-{Metal} {Complexes}},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/chem.200802456},
	doi = {10.1002/chem.200802456},
	abstract = {Broken-symmetry DFT calculations on transition-metal clusters with more than two centers allow the hyperfine coupling constants to be extracted. Application of the proposed theoretical scheme to a tetranuclear manganese complex that models the S2 state of the oxygen-evolving complex of photosystem II yields hyperfine parameters that can be directly compared with experimental data. The picture shows the metal–oxo core of the model and the following parameters; exchange coupling constant Jij, the expectation value of the site-spin operator \{\vphantom{\}}łeftłangle {\textbackslash}rm S\_{\textbackslash}rm z{\textasciicircum}({\textbackslash}rm{\textbackslash}rm K) {\textbackslash}right{\textbackslash}rangle {\textbackslash}, and the isotropic hyperfine coupling \{\vphantom{\}}{\textbackslash}rm A\_{\textbackslash}rm{\textbackslash}rm iso{\textasciicircum}{\textbackslash}rm({\textbackslash}rm K) {\textbackslash} parameters. The reliable correlation of structural features and magnetic or spectroscopic properties of oligonuclear transition-metal complexes is a critical requirement both for research into innovative magnetic materials and for elucidating the structure and function of many metalloenzymes. We have developed a novel method that for the first time enables the extraction of hyperfine coupling constants (HFCs) from broken-symmetry density functional theory (BS-DFT) calculations on clusters. Using the geometry-optimized tetranuclear manganese complex [Mn4O6(bpy)6]4+/3+ as a model, we first examine in detail the calculation of exchange coupling constants J through the BS-DFT approach. Complications arising from the indeterminacy of experimentally fitted J constants are identified and analyzed. It is found that only the energy levels derived from Hamiltonian diagonalization are a physically meaningful basis for comparing theory and experiment. Subsequently, the proposed theoretical scheme is applied to the calculation of 55Mn HFCs of the MnIII,IV,IV,IV state of the complex, which is similar to the S2 state of the oxygen-evolving complex (OEC) in photosystem II of oxygenic photosynthesis. The new approach performs reliably and accurately, and yields calculated HFCs that can be directly compared with experimental data. Finally, we carefully examine the dependence of HFC on the J value and draw attention to the sensitivity of the calculated values to the exchange coupling parameters. The proposed strategy extends naturally to hetero-oligonuclear clusters of arbitrary shape and nuclearity, and hence is of general validity and usefulness in the study of magnetic metal clusters. The successful application of the new approach presented here is a first step in the effort to establish correlations between the available spectroscopic information and the structural features of complex metalloenzymes like OEC.},
	language = {en},
	number = {20},
	urldate = {2024-12-10},
	journal = {Chemistry – A European Journal},
	author = {Pantazis, Dimitrios A. and Orio, Maylis and Petrenko, Taras and Zein, Samir and Bill, Eckhard and Lubitz, Wolfgang and Messinger, Johannes and Neese, Frank},
	year = {2009},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/chem.200802456},
	keywords = {cluster compounds, density functional calculations, magnetic properties, manganese},
	pages = {5108--5123},
}

Broken-symmetry DFT calculations on transition-metal clusters with more than two centers allow the hyperfine coupling constants to be extracted. Application of the proposed theoretical scheme to a tetranuclear manganese complex that models the S2 state of the oxygen-evolving complex of photosystem II yields hyperfine parameters that can be directly compared with experimental data. The picture shows the metal–oxo core of the model and the following parameters; exchange coupling constant Jij, the expectation value of the site-spin operator \p̌hantom\łeftłangle \rm S_\rm z\textasciicircum(\rm\rm K) \right\rangle \, and the isotropic hyperfine coupling \p̌hantom\\rm A_\rm\rm iso\textasciicircum\rm(\rm K) \ parameters. The reliable correlation of structural features and magnetic or spectroscopic properties of oligonuclear transition-metal complexes is a critical requirement both for research into innovative magnetic materials and for elucidating the structure and function of many metalloenzymes. We have developed a novel method that for the first time enables the extraction of hyperfine coupling constants (HFCs) from broken-symmetry density functional theory (BS-DFT) calculations on clusters. Using the geometry-optimized tetranuclear manganese complex [Mn4O6(bpy)6]4+/3+ as a model, we first examine in detail the calculation of exchange coupling constants J through the BS-DFT approach. Complications arising from the indeterminacy of experimentally fitted J constants are identified and analyzed. It is found that only the energy levels derived from Hamiltonian diagonalization are a physically meaningful basis for comparing theory and experiment. Subsequently, the proposed theoretical scheme is applied to the calculation of 55Mn HFCs of the MnIII,IV,IV,IV state of the complex, which is similar to the S2 state of the oxygen-evolving complex (OEC) in photosystem II of oxygenic photosynthesis. The new approach performs reliably and accurately, and yields calculated HFCs that can be directly compared with experimental data. Finally, we carefully examine the dependence of HFC on the J value and draw attention to the sensitivity of the calculated values to the exchange coupling parameters. The proposed strategy extends naturally to hetero-oligonuclear clusters of arbitrary shape and nuclearity, and hence is of general validity and usefulness in the study of magnetic metal clusters. The successful application of the new approach presented here is a first step in the effort to establish correlations between the available spectroscopic information and the structural features of complex metalloenzymes like OEC.

@article{messinger_catalysts_2009,
	title = {Catalysts for {Solar} {Water} {Splitting}},
	volume = {2},
	copyright = {Copyright © 2009 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
	issn = {1864-564X},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/cssc.200800226},
	doi = {10.1002/cssc.200800226},
	abstract = {Making H2 while the sun shines: Recently, an inorganic catalyst based on Co2+ and phosphate ions was developed that operates in neutral water under ambient conditions to produce O2 from H2O at low overpotentials. Coupling the setup to a counter electrode at which H2 formation takes place as well as to a solar cell could lead to solar water splitting into H2 and O2.},
	number = {1},
	urldate = {2024-12-10},
	journal = {ChemSusChem},
	author = {Messinger, Johannes},
	year = {2009},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.200800226},
	keywords = {cobalt, hydrogen, oxygen, water splitting},
	pages = {47--48},
}

Making H2 while the sun shines: Recently, an inorganic catalyst based on Co2+ and phosphate ions was developed that operates in neutral water under ambient conditions to produce O2 from H2O at low overpotentials. Coupling the setup to a counter electrode at which H2 formation takes place as well as to a solar cell could lead to solar water splitting into H2 and O2.

@incollection{messinger_mastering_2009,
	series = {Bd. 1},
	title = {Mastering sustainable energy. {Light}-induced water-splitting in nature: {Electron} paramagnetic resonance},
	booktitle = {K. {M}. {Salichov} ({Ed}.) -{The} {Treasures} of {EUREKA}},
	publisher = {Springer},
	author = {Messinger, Johannes and Kulik, Leonid V and Lubitz, Wolfgang},
	year = {2009},
	pages = {164--165},
}

@article{beckmann_-line_2009,
	title = {On-line mass spectrometry: membrane inlet sampling},
	volume = {102},
	issn = {1573-5079},
	shorttitle = {On-line mass spectrometry},
	url = {https://doi.org/10.1007/s11120-009-9474-7},
	doi = {10.1007/s11120-009-9474-7},
	abstract = {Significant insights into plant photosynthesis and respiration have been achieved using membrane inlet mass spectrometry (MIMS) for the analysis of stable isotope distribution of gases. The MIMS approach is based on using a gas permeable membrane to enable the entry of gas molecules into the mass spectrometer source. This is a simple yet durable approach for the analysis of volatile gases, particularly atmospheric gases. The MIMS technique strongly lends itself to the study of reaction flux where isotopic labeling is employed to differentiate two competing processes; i.e., O2 evolution versus O2 uptake reactions from PSII or terminal oxidase/rubisco reactions. Such investigations have been used for in vitro studies of whole leaves and isolated cells. The MIMS approach is also able to follow rates of isotopic exchange, which is useful for obtaining chemical exchange rates. These types of measurements have been employed for oxygen ligand exchange in PSII and to discern reaction rates of the carbonic anhydrase reactions. Recent developments have also engaged MIMS for online isotopic fractionation and for the study of reactions in inorganic systems that are capable of water splitting or H2 generation. The simplicity of the sampling approach coupled to the high sensitivity of modern instrumentation is a reason for the growing applicability of this technique for a range of problems in plant photosynthesis and respiration. This review offers some insights into the sampling approaches and the experiments that have been conducted with MIMS.},
	language = {en},
	number = {2},
	urldate = {2024-12-10},
	journal = {Photosynthesis Research},
	author = {Beckmann, Katrin and Messinger, Johannes and Badger, Murray Ronald and Wydrzynski, Tom and Hillier, Warwick},
	month = dec,
	year = {2009},
	keywords = {Artificial photosynthesis, Carbonic anhydrase, Membrane-inlet mass spectrometry, Oxygenic photosynthesis, Water binding, Water-splitting},
	pages = {511--522},
}

Significant insights into plant photosynthesis and respiration have been achieved using membrane inlet mass spectrometry (MIMS) for the analysis of stable isotope distribution of gases. The MIMS approach is based on using a gas permeable membrane to enable the entry of gas molecules into the mass spectrometer source. This is a simple yet durable approach for the analysis of volatile gases, particularly atmospheric gases. The MIMS technique strongly lends itself to the study of reaction flux where isotopic labeling is employed to differentiate two competing processes; i.e., O2 evolution versus O2 uptake reactions from PSII or terminal oxidase/rubisco reactions. Such investigations have been used for in vitro studies of whole leaves and isolated cells. The MIMS approach is also able to follow rates of isotopic exchange, which is useful for obtaining chemical exchange rates. These types of measurements have been employed for oxygen ligand exchange in PSII and to discern reaction rates of the carbonic anhydrase reactions. Recent developments have also engaged MIMS for online isotopic fractionation and for the study of reactions in inorganic systems that are capable of water splitting or H2 generation. The simplicity of the sampling approach coupled to the high sensitivity of modern instrumentation is a reason for the growing applicability of this technique for a range of problems in plant photosynthesis and respiration. This review offers some insights into the sampling approaches and the experiments that have been conducted with MIMS.

@article{conlan_photo-catalytic_2009,
	title = {Photo-catalytic oxidation of a di-nuclear manganese centre in an engineered bacterioferritin ‘reaction centre’},
	volume = {1787},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272809001406},
	doi = {10.1016/j.bbabio.2009.04.011},
	abstract = {Photosynthesis involves the conversion of light into chemical energy through a series of electron transfer reactions within membrane-bound pigment/protein complexes. The Photosystem II (PSII) complex in plants, algae and cyanobacteria catalyse the oxidation of water to molecular O2. The complexity of PSII has thus far limited attempts to chemically replicate its function. Here we introduce a reverse engineering approach to build a simple, light-driven photo-catalyst based on the organization and function of the donor side of the PSII reaction centre. We have used bacterioferritin (BFR) (cytochrome b1) from Escherichia coli as the protein scaffold since it has several, inherently useful design features for engineering light-driven electron transport. Among these are: (i.) a di-iron binding site; (ii.) a potentially redox-active tyrosine residue; and (iii.) the ability to dimerise and form an inter-protein heme binding pocket within electron tunnelling distance of the di-iron binding site. Upon replacing the heme with the photoactive zinc–chlorin e6 (ZnCe6) molecule and the di-iron binding site with two manganese ions, we show that the two Mn ions bind as a weakly coupled di-nuclear Mn2II,II centre, and that ZnCe6 binds in stoichiometric amounts of 1:2 with respect to the dimeric form of BFR. Upon illumination the bound ZnCe6 initiates electron transfer, followed by oxidation of the di-nuclear Mn centre possibly via one of the inherent tyrosine residues in the vicinity of the Mn cluster. The light dependent loss of the MnII EPR signals and the formation of low field parallel mode Mn EPR signals are attributed to the formation of MnIII species. The formation of the MnIII is concomitant with consumption of oxygen. Our model is the first artificial reaction centre developed for the photo-catalytic oxidation of a di-metal site within a protein matrix which potentially mimics water oxidation centre (WOC) photo-assembly.},
	number = {9},
	urldate = {2024-12-10},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Conlan, Brendon and Cox, Nicholas and Su, Ji-Hu and Hillier, Warwick and Messinger, Johannes and Lubitz, Wolfgang and Dutton, P. Leslie and Wydrzynski, Tom},
	month = sep,
	year = {2009},
	keywords = {Artificial photosynthesis, Bacterioferritin, EPR, Electron transfer, Manganese, Protein engineering, Zinc chlorin e},
	pages = {1112--1121},
}

Photosynthesis involves the conversion of light into chemical energy through a series of electron transfer reactions within membrane-bound pigment/protein complexes. The Photosystem II (PSII) complex in plants, algae and cyanobacteria catalyse the oxidation of water to molecular O2. The complexity of PSII has thus far limited attempts to chemically replicate its function. Here we introduce a reverse engineering approach to build a simple, light-driven photo-catalyst based on the organization and function of the donor side of the PSII reaction centre. We have used bacterioferritin (BFR) (cytochrome b1) from Escherichia coli as the protein scaffold since it has several, inherently useful design features for engineering light-driven electron transport. Among these are: (i.) a di-iron binding site; (ii.) a potentially redox-active tyrosine residue; and (iii.) the ability to dimerise and form an inter-protein heme binding pocket within electron tunnelling distance of the di-iron binding site. Upon replacing the heme with the photoactive zinc–chlorin e6 (ZnCe6) molecule and the di-iron binding site with two manganese ions, we show that the two Mn ions bind as a weakly coupled di-nuclear Mn2II,II centre, and that ZnCe6 binds in stoichiometric amounts of 1:2 with respect to the dimeric form of BFR. Upon illumination the bound ZnCe6 initiates electron transfer, followed by oxidation of the di-nuclear Mn centre possibly via one of the inherent tyrosine residues in the vicinity of the Mn cluster. The light dependent loss of the MnII EPR signals and the formation of low field parallel mode Mn EPR signals are attributed to the formation of MnIII species. The formation of the MnIII is concomitant with consumption of oxygen. Our model is the first artificial reaction centre developed for the photo-catalytic oxidation of a di-metal site within a protein matrix which potentially mimics water oxidation centre (WOC) photo-assembly.

@article{messinger_special_2009,
	title = {Special educational issue on ‘{Basics} and application of biophysical techniques in photosynthesis and related processes’},
	volume = {101},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-009-9471-x},
	doi = {10.1007/s11120-009-9471-x},
	language = {en},
	number = {2},
	urldate = {2024-12-11},
	journal = {Photosynthesis Research},
	author = {Messinger, Johannes and Alia, A. and {Govindjee}},
	month = sep,
	year = {2009},
	pages = {89--92},
}

@article{messinger_special_2009,
	title = {Special educational issue on ‘{Basics} and application of biophysical techniques in photosynthesis and related processes’—{Part} {B}},
	volume = {102},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-009-9494-3},
	doi = {10.1007/s11120-009-9494-3},
	language = {en},
	number = {2},
	urldate = {2024-12-10},
	journal = {Photosynthesis Research},
	author = {Messinger, Johannes and Alia, A. and {Govindjee}},
	month = dec,
	year = {2009},
	pages = {103--106},
}

@article{pantazis_structure_2009,
	title = {Structure of the oxygen-evolving complex of photosystem {II}: information on the {S2} state through quantum chemical calculation of its magnetic properties},
	volume = {11},
	issn = {1463-9084},
	shorttitle = {Structure of the oxygen-evolving complex of photosystem {II}},
	url = {https://pubs.rsc.org/en/content/articlelanding/2009/cp/b907038a},
	doi = {10.1039/B907038A},
	abstract = {Twelve structural models for the S2 state of the oxygen-evolving complex (OEC) of photosystem II are evaluated in terms of their magnetic properties. The set includes ten models based on the ‘fused twist’ core topology derived by polarized EXAFS spectra and two related models proposed in recent mechanistic investigations. Optimized geometries and spin population analyses suggest that Mn(III), which is most often identified with the manganese ion at site D, is always associated with a penta-coordinate environment, unless a chloride is directly ligated to the metal. Exchange coupling constants were determined by broken-symmetry density functional theory calculations and the complete spectrum of magnetic sublevels was obtained by direct diagonalization of the Heisenberg Hamiltonian. Seven models display a doublet ground state and are considered spectroscopic models for the ground state corresponding to the multiline signal (MLS) of the S2 state of the OEC, whereas the remaining five models display a sextet ground state and could be related to the g = 4.1 signal of the S2 state. It is found that the sign of the exchange coupling constant between the Mn centres at positions A and B of the cluster is directly related to the ground state multiplicity, implying that interconversion between the doublet and sextet can be induced by only small structural perturbations. The recently proposed quantum chemical method for the calculation of 55Mn hyperfine coupling constants is subsequently applied to the S2 MLS state models and the quantities that enter into the individual steps of the procedure (site-spin expectation values, intrinsic site isotropic hyperfine parameters and projected 55Mn isotropic hyperfine constants) are analyzed and discussed in detail with respect to the structural and electronic features of each model. The current approach performs promisingly. It reacts sensitively to structural distortions and hence may be able to distinguish between different structural proposals. Thus it emerges as a useful contributor to the ongoing efforts that aim at establishing correlations between the body of spectroscopic data available for the various Si states of the OEC and their actual geometric features.},
	language = {en},
	number = {31},
	urldate = {2024-12-10},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {The Royal Society of Chemistry},
	author = {Pantazis, Dimitrios A. and Orio, Maylis and Petrenko, Taras and Zein, Samir and Lubitz, Wolfgang and Messinger, Johannes and Neese, Frank},
	month = jul,
	year = {2009},
	pages = {6788--6798},
}

Twelve structural models for the S2 state of the oxygen-evolving complex (OEC) of photosystem II are evaluated in terms of their magnetic properties. The set includes ten models based on the ‘fused twist’ core topology derived by polarized EXAFS spectra and two related models proposed in recent mechanistic investigations. Optimized geometries and spin population analyses suggest that Mn(III), which is most often identified with the manganese ion at site D, is always associated with a penta-coordinate environment, unless a chloride is directly ligated to the metal. Exchange coupling constants were determined by broken-symmetry density functional theory calculations and the complete spectrum of magnetic sublevels was obtained by direct diagonalization of the Heisenberg Hamiltonian. Seven models display a doublet ground state and are considered spectroscopic models for the ground state corresponding to the multiline signal (MLS) of the S2 state of the OEC, whereas the remaining five models display a sextet ground state and could be related to the g = 4.1 signal of the S2 state. It is found that the sign of the exchange coupling constant between the Mn centres at positions A and B of the cluster is directly related to the ground state multiplicity, implying that interconversion between the doublet and sextet can be induced by only small structural perturbations. The recently proposed quantum chemical method for the calculation of 55Mn hyperfine coupling constants is subsequently applied to the S2 MLS state models and the quantities that enter into the individual steps of the procedure (site-spin expectation values, intrinsic site isotropic hyperfine parameters and projected 55Mn isotropic hyperfine constants) are analyzed and discussed in detail with respect to the structural and electronic features of each model. The current approach performs promisingly. It reacts sensitively to structural distortions and hence may be able to distinguish between different structural proposals. Thus it emerges as a useful contributor to the ongoing efforts that aim at establishing correlations between the body of spectroscopic data available for the various Si states of the OEC and their actual geometric features.

@article{smolentsev_x-ray_2009,
	title = {X-ray {Emission} {Spectroscopy} {To} {Study} {Ligand} {Valence} {Orbitals} in {Mn} {Coordination} {Complexes}},
	volume = {131},
	issn = {0002-7863},
	url = {https://doi.org/10.1021/ja808526m},
	doi = {10.1021/ja808526m},
	abstract = {We discuss a spectroscopic method to determine the character of chemical bonding and for the identification of metal ligands in coordination and bioinorganic chemistry. It is based on the analysis of satellite lines in X-ray emission spectra that arise from transitions between valence orbitals and the metal ion 1s level (valence-to-core XES). The spectra, in connection with calculations based on density functional theory (DFT), provide information that is complementary to other spectroscopic techniques, in particular X-ray absorption (XANES and EXAFS). The spectral shape is sensitive to protonation of ligands and allows ligands, which differ only slightly in atomic number (e.g., C, N, O...), to be distinguished. A theoretical discussion of the main spectral features is presented in terms of molecular orbitals for a series of Mn model systems: [Mn(H2O)6]2+, [Mn(H2O)5OH]+, and [Mn(H2O)5NH3]2+. An application of the method, with comparison between theory and experiment, is presented for the solvated Mn2+ ion in water and three Mn coordination complexes, namely [LMn(acac)N3]BPh4, [LMn(B2O3Ph2)(ClO4)], and [LMn(acac)N]BPh4, where L represents 1,4,7-trimethyl-1,4,7-triazacyclononane, acac stands for the 2,4-pentanedionate anion, and B2O3Ph2 represents the 1,3-diphenyl-1,3-dibora-2-oxapropane-1,3-diolato dianion.},
	number = {36},
	urldate = {2024-12-10},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Smolentsev, Grigory and Soldatov, Alexander V. and Messinger, Johannes and Merz, Kathrin and Weyhermüller, Thomas and Bergmann, Uwe and Pushkar, Yulia and Yano, Junko and Yachandra, Vittal K. and Glatzel, Pieter},
	month = sep,
	year = {2009},
	pages = {13161--13167},
}

We discuss a spectroscopic method to determine the character of chemical bonding and for the identification of metal ligands in coordination and bioinorganic chemistry. It is based on the analysis of satellite lines in X-ray emission spectra that arise from transitions between valence orbitals and the metal ion 1s level (valence-to-core XES). The spectra, in connection with calculations based on density functional theory (DFT), provide information that is complementary to other spectroscopic techniques, in particular X-ray absorption (XANES and EXAFS). The spectral shape is sensitive to protonation of ligands and allows ligands, which differ only slightly in atomic number (e.g., C, N, O...), to be distinguished. A theoretical discussion of the main spectral features is presented in terms of molecular orbitals for a series of Mn model systems: [Mn(H2O)6]2+, [Mn(H2O)5OH]+, and [Mn(H2O)5NH3]2+. An application of the method, with comparison between theory and experiment, is presented for the solvated Mn2+ ion in water and three Mn coordination complexes, namely [LMn(acac)N3]BPh4, [LMn(B2O3Ph2)(ClO4)], and [LMn(acac)N]BPh4, where L represents 1,4,7-trimethyl-1,4,7-triazacyclononane, acac stands for the 2,4-pentanedionate anion, and B2O3Ph2 represents the 1,3-diphenyl-1,3-dibora-2-oxapropane-1,3-diolato dianion.

@inproceedings{beckmann_effects_2008,
	address = {Dordrecht},
	title = {Effects of {Chloride}/{Bromide} {Substitution} on {Substrate} {Water} {Exchange} {Rates} in {Photosystem} {II}},
	isbn = {978-1-4020-6709-9},
	doi = {10.1007/978-1-4020-6709-9_83},
	abstract = {The role of chloride in photosynthetic water oxidation is still under discussion since both its precise function and possible binding site are unresolved. In the present study the effects of Cl− on substrate water binding was investigated by monitoring the substrate water exchange kinetics in the S3-state. This was measured with time-resolved membrane-inlet mass spectrometry in thylakoids isolated from Thermosynechococcus elongatus grown in either CaCl2- or CaBr2-containing media. The results show that the exchange rate of the slowly exchangeable substrate water molecule is accelerated by replacement of Cl− by Br−.},
	language = {en},
	booktitle = {Photosynthesis. {Energy} from the {Sun}},
	publisher = {Springer Netherlands},
	author = {Beckmann, Katrin and Ishida, Naoko and Boussac, Alain and Messinger, Johannes},
	editor = {Allen, John F. and Gantt, Elisabeth and Golbeck, John H. and Osmond, Barry},
	year = {2008},
	keywords = {Bromide, chloride, isotope exchange, membrane-inlet mass spectrometry, oxygen evolving complex, photosystem II, substrate water},
	pages = {369--371},
}

The role of chloride in photosynthetic water oxidation is still under discussion since both its precise function and possible binding site are unresolved. In the present study the effects of Cl− on substrate water binding was investigated by monitoring the substrate water exchange kinetics in the S3-state. This was measured with time-resolved membrane-inlet mass spectrometry in thylakoids isolated from Thermosynechococcus elongatus grown in either CaCl2- or CaBr2-containing media. The results show that the exchange rate of the slowly exchangeable substrate water molecule is accelerated by replacement of Cl− by Br−.

@article{noring_effects_2008,
	title = {Effects of methanol on the {Si}-state transitions in photosynthetic water-splitting},
	volume = {98},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-008-9364-4},
	doi = {10.1007/s11120-008-9364-4},
	abstract = {From a chemical point of view methanol is one of the closest analogues of water. Consistent with this idea EPR spectroscopy studies have shown that methanol binds at—or at least very close to—the Mn4OxCa cluster of photosystem II (PSII). In contrast, Clark-type oxygen rate measurements demonstrate that the O2 evolving activity of PSII is surprisingly unaffected by methanol concentrations of up to 10\%. Here we study for the first time in detail the effect of methanol on photosynthetic water-splitting by employing a Joliot-type bare platinum electrode. We demonstrate a linear dependence of the miss parameter for Sistate advancement on the methanol concentrations in the range of 0–10\% (v/v). This finding is consistent with the idea that methanol binds in PSII with similar affinity as water to one or both substrate binding sites at the Mn4OxCa cluster. The possibility is discussed that the two substrate water molecules bind at different stages of the cycle, one during the S4 → S0 and the other during the S2 → S3 transition.},
	language = {en},
	number = {1},
	urldate = {2024-11-29},
	journal = {Photosynthesis Research},
	author = {Nöring, Birgit and Shevela, Dmitriy and Renger, Gernot and Messinger, Johannes},
	month = oct,
	year = {2008},
	keywords = {Manganese cluster, Methanol, Oxygen evolution, Photosystem II, Water-splitting},
	pages = {251--260},
}

From a chemical point of view methanol is one of the closest analogues of water. Consistent with this idea EPR spectroscopy studies have shown that methanol binds at—or at least very close to—the Mn4OxCa cluster of photosystem II (PSII). In contrast, Clark-type oxygen rate measurements demonstrate that the O2 evolving activity of PSII is surprisingly unaffected by methanol concentrations of up to 10%. Here we study for the first time in detail the effect of methanol on photosynthetic water-splitting by employing a Joliot-type bare platinum electrode. We demonstrate a linear dependence of the miss parameter for Sistate advancement on the methanol concentrations in the range of 0–10% (v/v). This finding is consistent with the idea that methanol binds in PSII with similar affinity as water to one or both substrate binding sites at the Mn4OxCa cluster. The possibility is discussed that the two substrate water molecules bind at different stages of the cycle, one during the S4 → S0 and the other during the S2 → S3 transition.

@inproceedings{yano_electronic_2008,
	address = {Dordrecht},
	title = {Electronic {Structure} and {Oxidation} {State} {Changes} in the {Mn4Ca} {Cluster} of {Photosystem} {II}},
	isbn = {978-1-4020-6709-9},
	doi = {10.1007/978-1-4020-6709-9_120},
	abstract = {Oxygen-evolving complex (Mn4Ca cluster) of Photosystem II cycles through five intermediate states (Si-states, i = 0–4) before a molecule of dioxygen is released. During the Sstate transitions, electrons are extracted from the OEC, either from Mn or alternatively from an oxo ligand. The oxidation state of Mn is widely accepted as Mn4(III2,IV2) and Mn4(III,IV3) for S1 and S2 states, while it is still controversial for the S0 and S3 states. We used resonant inelastic X-ray scattering (RIXS) to study the electronic structure of Mn4Ca complex in the OEC. The RIXS data yield two- dimensional plots that provide a significant advantage by obtaining both K-edge pre-edge (charge density sensitive) and L-edge-like spectra (metal spin state sensitive) simultaneously. The spectral changes in the Mn 1s2p3/2 RIXS spectra between the S-states were compared to those of the Mn oxides and coordination complexes. The results indicate strong covalency for the electronic configuration in the OEC, and we conclude that the electron is transferred from a strongly delocalized orbital, compared to those in Mn oxides or coordination complexes. The magnitude for the S0 to S1, and S1 to S2 transitions is twice as large as that during the S2 to S3 transition, indicating that the electron for this transition is extracted from a highly delocalized orbital with little change in charge density at the Mn atoms.},
	language = {en},
	booktitle = {Photosynthesis. {Energy} from the {Sun}},
	publisher = {Springer Netherlands},
	author = {Yano, Junko and Pushkar, Yulia and Messinger, Johannes and Bergmann, Uwe and Glatzel, Pieter and Yachandra, Vittal K.},
	editor = {Allen, John F. and Gantt, Elisabeth and Golbeck, John H. and Osmond, Barry},
	year = {2008},
	keywords = {Electronic structure, Mn4Ca cluster, RIXS, X-ray spectroscopy, oxygen-evolving complex, photosystem II},
	pages = {529--532},
}

Oxygen-evolving complex (Mn4Ca cluster) of Photosystem II cycles through five intermediate states (Si-states, i = 0–4) before a molecule of dioxygen is released. During the Sstate transitions, electrons are extracted from the OEC, either from Mn or alternatively from an oxo ligand. The oxidation state of Mn is widely accepted as Mn4(III2,IV2) and Mn4(III,IV3) for S1 and S2 states, while it is still controversial for the S0 and S3 states. We used resonant inelastic X-ray scattering (RIXS) to study the electronic structure of Mn4Ca complex in the OEC. The RIXS data yield two- dimensional plots that provide a significant advantage by obtaining both K-edge pre-edge (charge density sensitive) and L-edge-like spectra (metal spin state sensitive) simultaneously. The spectral changes in the Mn 1s2p3/2 RIXS spectra between the S-states were compared to those of the Mn oxides and coordination complexes. The results indicate strong covalency for the electronic configuration in the OEC, and we conclude that the electron is transferred from a strongly delocalized orbital, compared to those in Mn oxides or coordination complexes. The magnitude for the S0 to S1, and S1 to S2 transitions is twice as large as that during the S2 to S3 transition, indicating that the electron for this transition is extracted from a highly delocalized orbital with little change in charge density at the Mn atoms.

@article{dau_ftir_2008,
	title = {{FTIR} detection of water reactions in the oxygen-evolving centre of photosystem {II} - {Discussion}},
	volume = {363},
	issn = {0962-8436},
	url = {https://www.webofscience.com/wos/woscc/full-record/WOS:000253117000015},
	language = {English},
	number = {1494},
	urldate = {2024-11-29},
	journal = {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES},
	publisher = {Royal Soc},
	author = {Dau, H. and Noguchi, T. and Messinger, J. and Moran, K. and Hillier, W.},
	month = mar,
	year = {2008},
	note = {Num Pages: 2
Place: London
Web of Science ID: WOS:000253117000015},
	pages = {1194--1195},
}

@article{pace_focusing_2008,
	title = {Focusing the view on nature's water-splitting catalyst - {Discussion}},
	volume = {363},
	issn = {0962-8436},
	language = {English},
	number = {1494},
	journal = {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES},
	publisher = {Royal Soc},
	author = {Pace, R. and Messinger, J. and Boussac, A. and Britt, R. D. and Dismukes, C.},
	month = mar,
	year = {2008},
	note = {Num Pages: 1
Place: London
Web of Science ID: WOS:000253117000011},
	keywords = {COMPLEX, PHOTOSYSTEM-II, S-0 STATE, SIGNAL},
	pages = {1177--1177},
}

@inproceedings{shevela_formate-induced_2008,
	address = {Dordrecht},
	title = {Formate-{Induced} {Release} of {Carbon} {Dioxide}/ {Hydrogencarbonate} from {Photosystem} {II}},
	isbn = {978-1-4020-6709-9},
	doi = {10.1007/978-1-4020-6709-9_112},
	abstract = {Using Membrane-Inlet Mass Spectrometry (Mims) We Confirm That Injections Of High Concentrations (100 Mm Final Concentration) Of Formate Into The Psii Samples Induce A Slow Release Of Carbon Dioxide/Hydrogencarbonate (Co2/ Hco3 -) (Govindjee Et Al. 1997). The Amount Of The Released Inorganic Carbon (CI) Is Proportional Tothe Concentration Of The Bby Samples. Hydrazinepre- Treated Bby, Leading To An ̃90\% Inhibition Of The Overall Oxygen Evolution Activity, Release After Formate Injection Practically The Same Amount Of CI As Non-Treated Control Samples. These Results Indicate That The Released CI Originates From The Acceptor Side Of Psii And Evidently None From The Mn4OXCa Cluster. Thus, No Evidence Was Found In This Study For Hydrogencarbonate Being Tight Ligand Of The Mn4OXCa Cluster, Which Can Be Released By Formate Injection. The Possibility Of Hco3 - Release From Psii During Degassation In The Mass Spec Cell Is Discussed.},
	language = {en},
	booktitle = {Photosynthesis. {Energy} from the {Sun}},
	publisher = {Springer Netherlands},
	author = {Shevela, Dmitriy and Klimov, Vyacheslav and Messinger, Johannes},
	editor = {Allen, John F. and Gantt, Elisabeth and Golbeck, John H. and Osmond, Barry},
	year = {2008},
	keywords = {Membrane-inlet mass spectrometry (MIMS), bicarbonate, formate, hydrogencarbonate, photosystem II (PSII)},
	pages = {495--498},
}

Using Membrane-Inlet Mass Spectrometry (Mims) We Confirm That Injections Of High Concentrations (100 Mm Final Concentration) Of Formate Into The Psii Samples Induce A Slow Release Of Carbon Dioxide/Hydrogencarbonate (Co2/ Hco3 -) (Govindjee Et Al. 1997). The Amount Of The Released Inorganic Carbon (CI) Is Proportional Tothe Concentration Of The Bby Samples. Hydrazinepre- Treated Bby, Leading To An ̃90% Inhibition Of The Overall Oxygen Evolution Activity, Release After Formate Injection Practically The Same Amount Of CI As Non-Treated Control Samples. These Results Indicate That The Released CI Originates From The Acceptor Side Of Psii And Evidently None From The Mn4OXCa Cluster. Thus, No Evidence Was Found In This Study For Hydrogencarbonate Being Tight Ligand Of The Mn4OXCa Cluster, Which Can Be Released By Formate Injection. The Possibility Of Hco3 - Release From Psii During Degassation In The Mass Spec Cell Is Discussed.

@article{beckmann_formation_2008,
	title = {Formation of stoichiometrically {18O}-labelled oxygen from the oxidation of {18O}-enriched water mediated by a dinuclear manganese complex—a mass spectrometry and {EPR} study},
	volume = {1},
	issn = {1754-5706},
	url = {https://pubs.rsc.org/en/content/articlelanding/2008/ee/b811806j},
	doi = {10.1039/B811806J},
	abstract = {Oxygen formation was detected for the oxidations of various multinuclear manganese complexes by oxone (HSO5−) in aqueous solution. To determine to what extent water was the source of the evolved O2, H218O isotope-labelling experiments coupled with membrane inlet mass spectrometry (MIMS) were carried out. We discovered that during the reaction of oxone with [Mn2(OAc)2(bpmp)]+ (1), stoichiometrically labelled oxygen (18O2) was formed. This is the first example of a homogeneous reaction mediated by a synthetic manganese complex where the addition of a strong chemical oxidant yields 18O2 with labelling percentages matching the theoretically expected values for the case of both O-atoms originating from water. Experiments using lead acetate as an alternative oxidant supported this finding. A detailed investigation of the reaction by EPR spectroscopy, MIMS and Clark-type oxygen detection enabled us to propose potential reaction pathways.},
	language = {en},
	number = {6},
	urldate = {2024-11-29},
	journal = {Energy \& Environmental Science},
	publisher = {The Royal Society of Chemistry},
	author = {Beckmann, Katrin and Uchtenhagen, Hannes and Berggren, Gustav and Anderlund, Magnus F. and Thapper, Anders and Messinger, Johannes and Styring, Stenbjörn and Kurz, Philipp},
	month = dec,
	year = {2008},
	pages = {668--676},
}

Oxygen formation was detected for the oxidations of various multinuclear manganese complexes by oxone (HSO5−) in aqueous solution. To determine to what extent water was the source of the evolved O2, H218O isotope-labelling experiments coupled with membrane inlet mass spectrometry (MIMS) were carried out. We discovered that during the reaction of oxone with [Mn2(OAc)2(bpmp)]+ (1), stoichiometrically labelled oxygen (18O2) was formed. This is the first example of a homogeneous reaction mediated by a synthetic manganese complex where the addition of a strong chemical oxidant yields 18O2 with labelling percentages matching the theoretically expected values for the case of both O-atoms originating from water. Experiments using lead acetate as an alternative oxidant supported this finding. A detailed investigation of the reaction by EPR spectroscopy, MIMS and Clark-type oxygen detection enabled us to propose potential reaction pathways.

@article{shevela_hydrogencarbonate_2008,
	title = {Hydrogencarbonate is not a tightly bound constituent of the water-oxidizing complex in photosystem {II}},
	volume = {1777},
	issn = {0005-2728},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272808000650},
	doi = {10.1016/j.bbabio.2008.03.031},
	abstract = {Since the end of the 1950s hydrogencarbonate (‘bicarbonate’) is discussed as a possible cofactor of photosynthetic water-splitting, and in a recent X-ray crystallography model of photosystem II (PSII) it was displayed as a ligand of the Mn4OxCa cluster. Employing membrane-inlet mass spectrometry (MIMS) and isotope labelling we confirm the release of less than one (≈0.3) HCO3− per PSII upon addition of formate. The same amount of HCO3− release is observed upon formate addition to Mn-depleted PSII samples. This suggests that formate does not replace HCO3− from the donor side, but only from the non-heme iron at the acceptor side of PSII. The absence of a firmly bound HCO3− is corroborated by showing that a reductive destruction of the Mn4OxCa cluster inside the MIMS cell by NH2OH addition does not lead to any CO2/HCO3− release. We note that even after an essentially complete HCO3−/CO2 removal from the sample medium by extensive degassing in the MIMS cell the PSII samples retain ≥75\% of their initial flash-induced O2-evolving capacity. We therefore conclude that HCO3− has only ‘indirect’ effects on water-splitting in PSII, possibly by being part of a proton relay network and/or by participating in assembly and stabilization of the water-oxidizing complex.},
	number = {6},
	urldate = {2024-11-29},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Shevela, Dmitriy and Su, Ji-Hu and Klimov, Vyacheslav and Messinger, Johannes},
	month = jun,
	year = {2008},
	keywords = {Bicarbonate, Hydrogencarbonate, Membrane-inlet mass spectrometry (MIMS), Photosystem II, Water oxidation, Water-splitting},
	pages = {532--539},
}

Since the end of the 1950s hydrogencarbonate (‘bicarbonate’) is discussed as a possible cofactor of photosynthetic water-splitting, and in a recent X-ray crystallography model of photosystem II (PSII) it was displayed as a ligand of the Mn4OxCa cluster. Employing membrane-inlet mass spectrometry (MIMS) and isotope labelling we confirm the release of less than one (≈0.3) HCO3− per PSII upon addition of formate. The same amount of HCO3− release is observed upon formate addition to Mn-depleted PSII samples. This suggests that formate does not replace HCO3− from the donor side, but only from the non-heme iron at the acceptor side of PSII. The absence of a firmly bound HCO3− is corroborated by showing that a reductive destruction of the Mn4OxCa cluster inside the MIMS cell by NH2OH addition does not lead to any CO2/HCO3− release. We note that even after an essentially complete HCO3−/CO2 removal from the sample medium by extensive degassing in the MIMS cell the PSII samples retain ≥75% of their initial flash-induced O2-evolving capacity. We therefore conclude that HCO3− has only ‘indirect’ effects on water-splitting in PSII, possibly by being part of a proton relay network and/or by participating in assembly and stabilization of the water-oxidizing complex.

@incollection{konermann_mass_2008,
	address = {Dordrecht},
	title = {Mass {Spectrometry}-{Based} {Methods} for {Studying} {Kinetics} and {Dynamics} in {Biological} {Systems}},
	isbn = {978-1-4020-8250-4},
	url = {https://doi.org/10.1007/978-1-4020-8250-4_9},
	doi = {10.1007/978-1-4020-8250-4_9},
	abstract = {In recent years, mass spectrometry (MS) has become one of the most widely used analytical techniques. MS allows studies on compounds ranging in size from single atoms to mega-Dalton biomolecular assemblies. This chapter provides an overview of recent MS applications in biophysical chemistry. The focus of our discussion is on ‘time-resolved’ techniques for tracking changes in complex biological reaction mixtures on time scales of milliseconds to days, thereby providing important structural and mechanistic insights. After a general introduction to biological MS, we discuss practical aspects of time-resolved membrane inlet mass spectrometry (MIMS), such as membrane properties and the use of different sample chambers. The MIMS technique allows online detection of dissolved gases and volatile compounds. It is particularly useful for resolving competing biochemical reactions involving common reactants, because isotopic labeling of substrates can be performed. As examples we present mechanistic studies on Photosystem II, carbonic anhydrase and hydrogenase. In the third part of this chapter we discuss the kinetics and mechanisms of protein folding and unfolding in solution, which can be explored via electrospray ionization mass spectrometry (ESI-MS). On-line coupling of ESI-MS with continuous-flow rapid mixing devices allows monitoring conformational changes of polypeptide chains with millisecond time resolution, as well as the detection and characterization of (un)folding intermediates. Due to its ‘softness’ the ESI process retains even weakly bound noncovalent complexes during the transition into the gas phase, such that protein-protein and protein-ligand interactions can be monitored directly. Additional insights into the conformational dynamics of proteins can be obtained by using time-resolved ESI-MS in conjunction with hydrogen/deuterium exchange methods. It is hoped that this chapter will stimulate the application of time-resolved MS techniques to a wide range of hitherto unexplored research areas.},
	language = {en},
	urldate = {2024-11-29},
	booktitle = {Biophysical {Techniques} in {Photosynthesis}},
	publisher = {Springer Netherlands},
	author = {Konermann, Lars and Messinger, Johannes and Hillier, Warwick},
	editor = {Aartsma, Thijs J. and Matysik, Jörg},
	year = {2008},
	keywords = {Electrospray Ionization Mass Spectrometry, Electrospray Mass Spectrometry, Oxygen Evolve Complex, Phys Chem Chem Phys, Sample Chamber},
	pages = {167--190},
}

In recent years, mass spectrometry (MS) has become one of the most widely used analytical techniques. MS allows studies on compounds ranging in size from single atoms to mega-Dalton biomolecular assemblies. This chapter provides an overview of recent MS applications in biophysical chemistry. The focus of our discussion is on ‘time-resolved’ techniques for tracking changes in complex biological reaction mixtures on time scales of milliseconds to days, thereby providing important structural and mechanistic insights. After a general introduction to biological MS, we discuss practical aspects of time-resolved membrane inlet mass spectrometry (MIMS), such as membrane properties and the use of different sample chambers. The MIMS technique allows online detection of dissolved gases and volatile compounds. It is particularly useful for resolving competing biochemical reactions involving common reactants, because isotopic labeling of substrates can be performed. As examples we present mechanistic studies on Photosystem II, carbonic anhydrase and hydrogenase. In the third part of this chapter we discuss the kinetics and mechanisms of protein folding and unfolding in solution, which can be explored via electrospray ionization mass spectrometry (ESI-MS). On-line coupling of ESI-MS with continuous-flow rapid mixing devices allows monitoring conformational changes of polypeptide chains with millisecond time resolution, as well as the detection and characterization of (un)folding intermediates. Due to its ‘softness’ the ESI process retains even weakly bound noncovalent complexes during the transition into the gas phase, such that protein-protein and protein-ligand interactions can be monitored directly. Additional insights into the conformational dynamics of proteins can be obtained by using time-resolved ESI-MS in conjunction with hydrogen/deuterium exchange methods. It is hoped that this chapter will stimulate the application of time-resolved MS techniques to a wide range of hitherto unexplored research areas.

@article{su_probing_2008,
	title = {Probing {Mode} and {Site} of {Substrate} {Water} {Binding} to the {Oxygen}-{Evolving} {Complex} in the {S2} {State} of {Photosystem} {II} by {17O}-{HYSCORE} {Spectroscopy}},
	volume = {130},
	issn = {0002-7863},
	url = {https://doi.org/10.1021/ja076620i},
	doi = {10.1021/ja076620i},
	abstract = {In the oxygen-evolving complex (OEC) of photosystem II (PSII) molecular oxygen is formed from two substrate water molecules that are ligated to a μ-oxo bridged cluster containing four Mn ions and one Ca ion (Mn4OxCa cluster; Ox symbolizes the unknown number of μ-oxo bridges; x ≥ 5). There is a long-standing enigma as to when, where, and how the two substrate water molecules bind to the Mn4OxCa cluster during the cyclic water-splitting reaction, which involves five distinct redox intermediates (Si-states; i = 0,...,4). To address this question we employed hyperfine sublevel correlation (HYSCORE) spectroscopy on H217O-enriched PSII samples poised in the paramagnetic S2 state. This approach allowed us to resolve the magnetic interaction between one solvent exchangeable 17O that is directly ligated to one or more Mn ions of the Mn4OxCa cluster in the S2 state of PSII. Direct coordination of 17O to Mn is supported by the strong (A ≈ 10 MHz) hyperfine coupling. Because these are properties expected from a substrate water molecule, this spectroscopic signature holds the potential for gaining long-sought information about the binding mode and site of one of the two substrate water molecules in the S2 state of PSII.},
	number = {3},
	urldate = {2024-12-12},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Su, Ji-Hu and Lubitz, Wolfgang and Messinger, Johannes},
	month = jan,
	year = {2008},
	pages = {786--787},
}

In the oxygen-evolving complex (OEC) of photosystem II (PSII) molecular oxygen is formed from two substrate water molecules that are ligated to a μ-oxo bridged cluster containing four Mn ions and one Ca ion (Mn4OxCa cluster; Ox symbolizes the unknown number of μ-oxo bridges; x ≥ 5). There is a long-standing enigma as to when, where, and how the two substrate water molecules bind to the Mn4OxCa cluster during the cyclic water-splitting reaction, which involves five distinct redox intermediates (Si-states; i = 0,...,4). To address this question we employed hyperfine sublevel correlation (HYSCORE) spectroscopy on H217O-enriched PSII samples poised in the paramagnetic S2 state. This approach allowed us to resolve the magnetic interaction between one solvent exchangeable 17O that is directly ligated to one or more Mn ions of the Mn4OxCa cluster in the S2 state of PSII. Direct coordination of 17O to Mn is supported by the strong (A ≈ 10 MHz) hyperfine coupling. Because these are properties expected from a substrate water molecule, this spectroscopic signature holds the potential for gaining long-sought information about the binding mode and site of one of the two substrate water molecules in the S2 state of PSII.

@article{lubitz_solar_2008,
	title = {Solar water-splitting into {H2} and {O2}: design principles of photosystem {II} and hydrogenases},
	volume = {1},
	issn = {1754-5706},
	shorttitle = {Solar water-splitting into {H2} and {O2}},
	url = {https://pubs.rsc.org/en/content/articlelanding/2008/ee/b808792j},
	doi = {10.1039/B808792J},
	abstract = {This review aims at presenting the principles of water-oxidation in photosystem II and of hydrogen production by the two major classes of hydrogenases in order to facilitate application for the design of artificial catalysts for solar fuel production.},
	language = {en},
	number = {1},
	urldate = {2024-11-29},
	journal = {Energy \& Environmental Science},
	publisher = {The Royal Society of Chemistry},
	author = {Lubitz, Wolfgang and Reijerse, Edward J. and Messinger, Johannes},
	month = jul,
	year = {2008},
	pages = {15--31},
}

This review aims at presenting the principles of water-oxidation in photosystem II and of hydrogen production by the two major classes of hydrogenases in order to facilitate application for the design of artificial catalysts for solar fuel production.

@inproceedings{yano_structure_2008,
	address = {Dordrecht},
	title = {Structure of the {Photosynthetic} {Mn4Ca} {Cluster} {Using} {X}-ray {Spectroscopy}},
	isbn = {978-1-4020-6709-9},
	doi = {10.1007/978-1-4020-6709-9_121},
	abstract = {Single crystals of Photosystem II (PSII) isolated from thermophilic cyanobacteria have been studied by X-ray diffraction (XRD) with resolutions between 3 and 3.8 Å (Ferreira et al. 2004; Loll et al. 2005). These studies have localized electron density associated with the wateroxidizing Mn4Ca cluster within the large complex of PSII peptides, but the limited resolution is short of what is needed to place individual metal atoms precisely in the cluster. Examination of the orientation dependence of the EXAFS of single crystals of PSII can provide structural information about the Mn sites at a resolution higher than that is presently available from single-crystal X-ray diffraction. We have successfully collected single crystal XANES and EXAFS data from the native S1 state with the X-ray e-vector parallel to the a, b, and c axes of the crystal, under non-damaging conditions by monitoring the Mn K-edge for any X-ray induced Mn reduction. The EXAFS spectra show that the Fourier peaks are clearly dichroic, demonstrating an asymmetric Mn cluster. We have used the EXAFS dichroism to evaluate the Mn cluster geometry. Three Mn4Ca models which satisfy the trend of EXAFS dichroism were further fit into the ligand environment obtained from XRD, in order to discriminate between the several symmetry-related orientations which arise from the crystal symmetry. Furthermore, single crystals in the S1 state were illuminated either by continuous illumination or by laser flashes to create intermediate S-states (S2 and S3). Polarized XANES and EXAFS spectra from these crystals show unique orientational dependence. Additionally, a review of how the resolution of traditional EXAFS techniques can be improved, using methods such as rangeextended EXAFS, is presented.},
	language = {en},
	booktitle = {Photosynthesis. {Energy} from the {Sun}},
	publisher = {Springer Netherlands},
	author = {Yano, Junko and Kern, Jan and Sauer, Kenneth and Pushkar, Yulia and Bergmann, Uwe and Glatzel, Pieter and Messinger, Johannes and Zouni, Athina and Yachandra, Vittal K.},
	editor = {Allen, John F. and Gantt, Elisabeth and Golbeck, John H. and Osmond, Barry},
	year = {2008},
	keywords = {Mn4Ca cluster, X-ray spectroscopy, oxygen-evolving complex, photosystem II},
	pages = {533--538},
}

Single crystals of Photosystem II (PSII) isolated from thermophilic cyanobacteria have been studied by X-ray diffraction (XRD) with resolutions between 3 and 3.8 Å (Ferreira et al. 2004; Loll et al. 2005). These studies have localized electron density associated with the wateroxidizing Mn4Ca cluster within the large complex of PSII peptides, but the limited resolution is short of what is needed to place individual metal atoms precisely in the cluster. Examination of the orientation dependence of the EXAFS of single crystals of PSII can provide structural information about the Mn sites at a resolution higher than that is presently available from single-crystal X-ray diffraction. We have successfully collected single crystal XANES and EXAFS data from the native S1 state with the X-ray e-vector parallel to the a, b, and c axes of the crystal, under non-damaging conditions by monitoring the Mn K-edge for any X-ray induced Mn reduction. The EXAFS spectra show that the Fourier peaks are clearly dichroic, demonstrating an asymmetric Mn cluster. We have used the EXAFS dichroism to evaluate the Mn cluster geometry. Three Mn4Ca models which satisfy the trend of EXAFS dichroism were further fit into the ligand environment obtained from XRD, in order to discriminate between the several symmetry-related orientations which arise from the crystal symmetry. Furthermore, single crystals in the S1 state were illuminated either by continuous illumination or by laser flashes to create intermediate S-states (S2 and S3). Polarized XANES and EXAFS spectra from these crystals show unique orientational dependence. Additionally, a review of how the resolution of traditional EXAFS techniques can be improved, using methods such as rangeextended EXAFS, is presented.

@inproceedings{su_substrate_2008,
	address = {Dordrecht},
	title = {Substrate {Water} {Bound} to the {S2}-{State} of the {Mn4OxCa} {Cluster} in {Photosystem} {II} {Studied} by {Advanced} {Pulse} {EPR} {Spectroscopy}},
	isbn = {978-1-4020-6709-9},
	doi = {10.1007/978-1-4020-6709-9_114},
	abstract = {In The Present Study, We Employed Several Advanced Pulse Epr Techniques To Detect Weak Hyperfine Couplings To Study Substrate Water Binding To The Mn4OXCa Cluster In The S2, State Of Photosystem Ii. Our Data Show That The Hyperfine Coupling Between The 2H And The Mn4OXCa Cluster Is Too Weak To Be Resolved. We Propose That The Information Does Not Allow Make Precise Conclusions About The Mode Of Substrate Water Binding To The Mn4OXCa Cluster. Our Preliminary Data Suggest That More Detailed Information Can Be Obtained From The H216O/ H217O Exchange Experiments.},
	language = {en},
	booktitle = {Photosynthesis. {Energy} from the {Sun}},
	publisher = {Springer Netherlands},
	author = {Su, Ji-Hu and Lubitz, Wolfgang and Messinger, Johannes},
	editor = {Allen, John F. and Gantt, Elisabeth and Golbeck, John H. and Osmond, Barry},
	year = {2008},
	keywords = {ESEEM, HYSCORE, Mims ENDOR, PSII, Water binding, pulse EPR},
	pages = {503--507},
}

In The Present Study, We Employed Several Advanced Pulse Epr Techniques To Detect Weak Hyperfine Couplings To Study Substrate Water Binding To The Mn4OXCa Cluster In The S2, State Of Photosystem Ii. Our Data Show That The Hyperfine Coupling Between The 2H And The Mn4OXCa Cluster Is Too Weak To Be Resolved. We Propose That The Information Does Not Allow Make Precise Conclusions About The Mode Of Substrate Water Binding To The Mn4OXCa Cluster. Our Preliminary Data Suggest That More Detailed Information Can Be Obtained From The H216O/ H217O Exchange Experiments.

@article{pecoraro_structure_2008,
	title = {The structure of the {Mn}$_{\textrm{4}}${Ca}{\textless}{SUP}{\textgreater}2+{\textless}/{SUP}{\textgreater} cluster of photosystem {II} and its protein environment as revealed by {X}-ray crystallography -: {Discussion}},
	volume = {363},
	issn = {0962-8436},
	language = {English},
	number = {1494},
	journal = {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES},
	publisher = {Royal Soc},
	author = {Pecoraro, V. L. and Barber, J. and Dau, H. and Brudvig, G. and Siegbahn, P. and Messinger, J.},
	month = mar,
	year = {2008},
	note = {Num Pages: 2
Place: London
Web of Science ID: WOS:000253117000003},
	pages = {1137--1138},
}

@article{barber_using_2008,
	title = {Using small molecule complexes to elucidate features of photosynthetic water oxidation - {Discussion}},
	volume = {363},
	issn = {0962-8436, 1471-2970},
	language = {English},
	number = {1494},
	journal = {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES},
	publisher = {Royal Soc},
	author = {Barber, J. and Pecoraro, V. and Brudvig, G. and Aukauloo, A. and Messinger, J.},
	month = mar,
	year = {2008},
	note = {Num Pages: 3
Place: London
Web of Science ID: WOS:000253117000034},
	keywords = {S-3 STATE},
	pages = {1279--1281},
}

@article{pecoraro_water_2008,
	title = {Water oxidation chemistry of photosystem {II} - {Discussion}},
	volume = {363},
	issn = {0962-8436},
	language = {English},
	number = {1494},
	journal = {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES},
	publisher = {Royal Soc},
	author = {Pecoraro, V. and Brudvig, G. and Dau, H. and Aukauloo, A. and Nocera, D. and Siegbalm, P. and Messinger, J.},
	month = mar,
	year = {2008},
	note = {Num Pages: 2
Place: London
Web of Science ID: WOS:000253117000021},
	pages = {1218--1219},
}

@article{yano_electronic_2007,
	title = {Electronic {Structure} of the {Mn4Ca} {Cluster} in the {Oxygen}‐{Evolving} {Complex} of {Photosystem} {II} {Studied} by {Resonant} {Inelastic} {X}‐{Ray} {Scattering}},
	volume = {882},
	issn = {0094-243X},
	url = {https://doi.org/10.1063/1.2644510},
	doi = {10.1063/1.2644510},
	abstract = {Oxygen‐evolving complex (Mn4Ca cluster) of Photosystem II cycles through five intermediate states (Si‐states, i =0–4) before a molecule of dioxygen is released. During the S‐state transitions, electrons are extracted from the OEC, either from Mn or alternatively from a Mn ligand. The oxidation state of Mn is widely accepted as Mn4(III2,IV2) and Mn4(III,IV3) for S1 and S2 states, while it is still controversial for the S0 and S3 states. We used resonant inelastic X‐ray scattering (RIXS) to study the electronic structure of Mn4Ca complex in the OEC. The RIXS data yield two‐dimensional plots that provide a significant advantage by obtaining both K‐edge pre‐edge and L‐edge‐like spectra simultaneously. The second energy dimension separates the pre‐edge (1s to 3d) transitions from the main K‐edge (1s to 4p), and thus more precise analysis is possible. The 1s2p RIXS final state electron configuration along the energy transfer axis is identical to conventional L‐edge absorption spectroscopy and the RIXS spectra are therefore sensitive to the metal spin state. We have collected data from PS II samples in the each of the S‐states and compared them with data from various inorganic Mn complexes. The spectral changes in the Mn 1s2p3/2 RIXS spectra between the S‐states are small compared to those of the oxides of Mn and coordination complexes. The results indicate strong covalency for the electronic configuration in the OEC, and we conclude that the electron is transferred from a strongly delocalized orbital, compared to those in Mn oxides or coordination complexes. The magnitude for the S0 to S1, and S1 to S2 transitions is twice as large as that during the S2 to S3 transition, indicating that the electron for this transition is extracted from a highly delocalized orbital with little change in charge density at the Mn atoms. The RIXS spectra of S0 and S3 states also showed characteristic features which were not clear from the K‐edge spectroscopy.},
	number = {1},
	urldate = {2024-11-29},
	journal = {AIP Conference Proceedings},
	author = {Yano, Junko and Pushkar, Yulia and Messinger, Johannes and Bergmann, Uwe and Glatzel, Pieter and Yachandra, Vittal K.},
	month = feb,
	year = {2007},
	pages = {316--318},
}

Oxygen‐evolving complex (Mn4Ca cluster) of Photosystem II cycles through five intermediate states (Si‐states, i =0–4) before a molecule of dioxygen is released. During the S‐state transitions, electrons are extracted from the OEC, either from Mn or alternatively from a Mn ligand. The oxidation state of Mn is widely accepted as Mn4(III2,IV2) and Mn4(III,IV3) for S1 and S2 states, while it is still controversial for the S0 and S3 states. We used resonant inelastic X‐ray scattering (RIXS) to study the electronic structure of Mn4Ca complex in the OEC. The RIXS data yield two‐dimensional plots that provide a significant advantage by obtaining both K‐edge pre‐edge and L‐edge‐like spectra simultaneously. The second energy dimension separates the pre‐edge (1s to 3d) transitions from the main K‐edge (1s to 4p), and thus more precise analysis is possible. The 1s2p RIXS final state electron configuration along the energy transfer axis is identical to conventional L‐edge absorption spectroscopy and the RIXS spectra are therefore sensitive to the metal spin state. We have collected data from PS II samples in the each of the S‐states and compared them with data from various inorganic Mn complexes. The spectral changes in the Mn 1s2p3/2 RIXS spectra between the S‐states are small compared to those of the oxides of Mn and coordination complexes. The results indicate strong covalency for the electronic configuration in the OEC, and we conclude that the electron is transferred from a strongly delocalized orbital, compared to those in Mn oxides or coordination complexes. The magnitude for the S0 to S1, and S1 to S2 transitions is twice as large as that during the S2 to S3 transition, indicating that the electron for this transition is extracted from a highly delocalized orbital with little change in charge density at the Mn atoms. The RIXS spectra of S0 and S3 states also showed characteristic features which were not clear from the K‐edge spectroscopy.

@article{kulik_electronic_2007,
	title = {Electronic {Structure} of the {Mn4OxCa} {Cluster} in the {S0} and {S2} {States} of the {Oxygen}-{Evolving} {Complex} of {Photosystem} {II} {Based} on {Pulse} {55Mn}-{ENDOR} and {EPR} {Spectroscopy}},
	volume = {129},
	issn = {0002-7863},
	url = {https://doi.org/10.1021/ja071487f},
	doi = {10.1021/ja071487f},
	abstract = {The heart of the oxygen-evolving complex (OEC) of photosystem II is a Mn4OxCa cluster that cycles through five different oxidation states (S0 to S4) during the light-driven water-splitting reaction cycle. In this study we interpret the recently obtained 55Mn hyperfine coupling constants of the S0 and S2 states of the OEC [Kulik et al. J. Am. Chem. Soc. 2005, 127, 2392−2393] on the basis of Y-shaped spin-coupling schemes with up to four nonzero exchange coupling constants, J. This analysis rules out the presence of one or more Mn(II) ions in S0 in methanol (3\%) containing samples and thereby establishes that the oxidation states of the manganese ions in S0 and S2 are, at 4 K, Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. By applying a “structure filter” that is based on the recently reported single-crystal EXAFS data on the Mn4OxCa cluster [Yano et al. Science 2006, 314, 821−825] we (i) show that this new structural model is fully consistent with EPR and 55Mn-ENDOR data, (ii) assign the Mn oxidation states to the individual Mn ions, and (iii) propose that the known shortening of one 2.85 Å Mn−Mn distance in S0 to 2.75 Å in S1 [Robblee et al. J. Am. Chem. Soc. 2002, 124, 7459−7471] corresponds to a deprotonation of a μ-hydroxo bridge between MnA and MnB, i.e., between the outer Mn and its neighboring Mn of the μ3-oxo bridged moiety of the cluster. We summarize our results in a molecular model for the S0 → S1 and S1 → S2 transitions.},
	number = {44},
	urldate = {2024-12-12},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Kulik, Leonid V. and Epel, Boris and Lubitz, Wolfgang and Messinger, Johannes},
	month = nov,
	year = {2007},
	pages = {13421--13435},
}

The heart of the oxygen-evolving complex (OEC) of photosystem II is a Mn4OxCa cluster that cycles through five different oxidation states (S0 to S4) during the light-driven water-splitting reaction cycle. In this study we interpret the recently obtained 55Mn hyperfine coupling constants of the S0 and S2 states of the OEC [Kulik et al. J. Am. Chem. Soc. 2005, 127, 2392−2393] on the basis of Y-shaped spin-coupling schemes with up to four nonzero exchange coupling constants, J. This analysis rules out the presence of one or more Mn(II) ions in S0 in methanol (3%) containing samples and thereby establishes that the oxidation states of the manganese ions in S0 and S2 are, at 4 K, Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. By applying a “structure filter” that is based on the recently reported single-crystal EXAFS data on the Mn4OxCa cluster [Yano et al. Science 2006, 314, 821−825] we (i) show that this new structural model is fully consistent with EPR and 55Mn-ENDOR data, (ii) assign the Mn oxidation states to the individual Mn ions, and (iii) propose that the known shortening of one 2.85 Å Mn−Mn distance in S0 to 2.75 Å in S1 [Robblee et al. J. Am. Chem. Soc. 2002, 124, 7459−7471] corresponds to a deprotonation of a μ-hydroxo bridge between MnA and MnB, i.e., between the outer Mn and its neighboring Mn of the μ3-oxo bridged moiety of the cluster. We summarize our results in a molecular model for the S0 → S1 and S1 → S2 transitions.

@article{zein_focusing_2007,
	title = {Focusing the view on nature's water-splitting catalyst},
	volume = {363},
	url = {https://royalsocietypublishing.org/doi/10.1098/rstb.2007.2212},
	doi = {10.1098/rstb.2007.2212},
	abstract = {Nature invented a catalyst about 3 Gyr ago, which splits water with high efficiency into molecular oxygen and hydrogen equivalents (protons and electrons). This reaction is energetically driven by sunlight and the active centre contains relatively cheap and abundant metals: manganese and calcium. This biological system therefore forms the paradigm for all man-made attempts for direct solar fuel production, and several studies are underway to determine the electronic and geometric structures of this catalyst. In this report we briefly summarize the problems and the current status of these efforts and propose a density functional theory-based strategy for obtaining a reliable high-resolution structure of this unique catalyst that includes both the inorganic core and the first ligand sphere.},
	number = {1494},
	urldate = {2024-11-29},
	journal = {Philosophical Transactions of the Royal Society B: Biological Sciences},
	publisher = {Royal Society},
	author = {Zein, Samir and Kulik, Leonid V and Yano, Junko and Kern, Jan and Pushkar, Yulia and Zouni, Athina and Yachandra, Vittal K and Lubitz, Wolfgang and Neese, Frank and Messinger, Johannes},
	month = oct,
	year = {2007},
	keywords = {EPR/ENDOR, EXAFS, density functional theory, manganese cluster, oxygen evolution, photosystem II},
	pages = {1167--1177},
}

Nature invented a catalyst about 3 Gyr ago, which splits water with high efficiency into molecular oxygen and hydrogen equivalents (protons and electrons). This reaction is energetically driven by sunlight and the active centre contains relatively cheap and abundant metals: manganese and calcium. This biological system therefore forms the paradigm for all man-made attempts for direct solar fuel production, and several studies are underway to determine the electronic and geometric structures of this catalyst. In this report we briefly summarize the problems and the current status of these efforts and propose a density functional theory-based strategy for obtaining a reliable high-resolution structure of this unique catalyst that includes both the inorganic core and the first ligand sphere.

@incollection{schmidt_grundlage_2007,
	edition = {R. Hoer, T. Lehmann},
	title = {Grundlage irdischen {Lebens}},
	booktitle = {{HighChem} hautnah. {Aktuelles} aus der {Elektrochemie} und zum {Thema} {Energie}},
	publisher = {Gesellschaft Deutscher Chemiker, Fachgruppe Angewandte Elektrochemie, Germany},
	author = {Schmidt, J. and Messinger, Johannes and Holzwarth, Alfred R. and Lubitz, Wolfgang},
	year = {2007},
	pages = {84--85},
}

@article{yano_high-resolution_2007,
	title = {High-resolution structure of the photosynthetic {Mn4Ca} catalyst from {X}-ray spectroscopy},
	volume = {363},
	url = {https://royalsocietypublishing.org/doi/10.1098/rstb.2007.2209},
	doi = {10.1098/rstb.2007.2209},
	abstract = {The application of high-resolution X-ray spectroscopy methods to study the photosynthetic water oxidizing complex, which contains a unique hetero-nuclear catalytic Mn4Ca cluster, is described. Issues of X-ray damage, especially at the metal sites in the Mn4Ca cluster, are discussed. The structure of the Mn4Ca catalyst at high resolution, which has so far eluded attempts of determination by X-ray diffraction, X-ray absorption fine structure (EXAFS) and other spectroscopic techniques, has been addressed using polarized EXAFS techniques applied to oriented photosystem II (PSII) membrane preparations and PSII single crystals. A review of how the resolution of traditional EXAFS techniques can be improved, using methods such as range-extended EXAFS, is presented, and the changes that occur in the structure of the cluster as it advances through the catalytic cycle are described. X-ray absorption and emission techniques (XANES and Kβ emission) have been used earlier to determine the oxidation states of the Mn4Ca cluster, and in this report we review the use of X-ray resonant Raman spectroscopy to understand the electronic structure of the Mn4Ca cluster as it cycles through the intermediate S-states.},
	number = {1494},
	urldate = {2024-11-29},
	journal = {Philosophical Transactions of the Royal Society B: Biological Sciences},
	publisher = {Royal Society},
	author = {Yano, Junko and Kern, Jan and Pushkar, Yulia and Sauer, Kenneth and Glatzel, Pieter and Bergmann, Uwe and Messinger, Johannes and Zouni, Athina and Yachandra, Vittal K},
	month = oct,
	year = {2007},
	keywords = {X-ray absorption fine structure, manganese, oxygen-evolving complex, photosystem II, photosystem II single crystals, polarized X-ray absorption fine structure},
	pages = {1139--1147},
}

The application of high-resolution X-ray spectroscopy methods to study the photosynthetic water oxidizing complex, which contains a unique hetero-nuclear catalytic Mn4Ca cluster, is described. Issues of X-ray damage, especially at the metal sites in the Mn4Ca cluster, are discussed. The structure of the Mn4Ca catalyst at high resolution, which has so far eluded attempts of determination by X-ray diffraction, X-ray absorption fine structure (EXAFS) and other spectroscopic techniques, has been addressed using polarized EXAFS techniques applied to oriented photosystem II (PSII) membrane preparations and PSII single crystals. A review of how the resolution of traditional EXAFS techniques can be improved, using methods such as range-extended EXAFS, is presented, and the changes that occur in the structure of the cluster as it advances through the catalytic cycle are described. X-ray absorption and emission techniques (XANES and Kβ emission) have been used earlier to determine the oxidation states of the Mn4Ca cluster, and in this report we review the use of X-ray resonant Raman spectroscopy to understand the electronic structure of the Mn4Ca cluster as it cycles through the intermediate S-states.

@article{shevela_interactions_2007,
	title = {Interactions of photosystem {II} with bicarbonate, formate and acetate},
	volume = {94},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-007-9200-2},
	doi = {10.1007/s11120-007-9200-2},
	abstract = {In this study, we probe the effects of bicarbonate (hydrogencarbonate), BC, removal from photosystem II in spinach thylakoids by measuring flash-induced oxygen evolution patterns (FIOPs) with a Joliot-type electrode. For this we compared three commonly employed methods: (1) washing in BC-free medium, (2) formate addition, and (3) acetate addition. Washing of the samples with buffers depleted of BC and CO2 by bubbling with argon (Method 1) under our conditions leads to an increase in the double hit parameter of the first flash (β1), while the miss parameter and the overall activity remain unchanged. In contrast, addition of 40–50 mM formate or acetate results in a significant increase in the miss parameter and to an ∼50\% (formate) and ∼10\% (acetate) inhibition of the overall oxygen evolution activity, but not to an increased β1 parameter. All described effects could be reversed by washing with formate/acetate free buffer and/or addition of 2–10 mM bicarbonate. The redox potential of the water-oxidizing complex (WOC) in samples treated by Method 1 is compared to samples containing 2 mM bicarbonate in two ways: (1) The lifetimes of the S0, S2, and S3 states were measured, and no differences were found between the two sample types. (2) The S1, S0, S−1, and S−2 states were probed by incubation with small concentrations of NH2OH. These experiments displayed a subtle, yet highly reproducible difference in the apparent Si/S−i state distribution which is shown to arise from the interaction of BC with PSII in the already reduced states of the WOC. These data are discussed in detail by also taking into account the CO2 concentrations present in the buffers after argon bubbling and during the measurements. These values were measured by membrane-inlet mass spectrometry (MIMS).},
	language = {en},
	number = {2},
	urldate = {2024-11-29},
	journal = {Photosynthesis Research},
	author = {Shevela, Dmitriy and Klimov, Vyacheslav and Messinger, Johannes},
	month = nov,
	year = {2007},
	keywords = {Acetate, Bicarbonate, Flash-induced oxygen evolution pattern (FIOPs), Formate, Hydrogencarbonate, Membrane-inlet mass spectrometry (MIMS), Oxygen evolution, S states, Water splitting},
	pages = {247--264},
}

In this study, we probe the effects of bicarbonate (hydrogencarbonate), BC, removal from photosystem II in spinach thylakoids by measuring flash-induced oxygen evolution patterns (FIOPs) with a Joliot-type electrode. For this we compared three commonly employed methods: (1) washing in BC-free medium, (2) formate addition, and (3) acetate addition. Washing of the samples with buffers depleted of BC and CO2 by bubbling with argon (Method 1) under our conditions leads to an increase in the double hit parameter of the first flash (β1), while the miss parameter and the overall activity remain unchanged. In contrast, addition of 40–50 mM formate or acetate results in a significant increase in the miss parameter and to an ∼50% (formate) and ∼10% (acetate) inhibition of the overall oxygen evolution activity, but not to an increased β1 parameter. All described effects could be reversed by washing with formate/acetate free buffer and/or addition of 2–10 mM bicarbonate. The redox potential of the water-oxidizing complex (WOC) in samples treated by Method 1 is compared to samples containing 2 mM bicarbonate in two ways: (1) The lifetimes of the S0, S2, and S3 states were measured, and no differences were found between the two sample types. (2) The S1, S0, S−1, and S−2 states were probed by incubation with small concentrations of NH2OH. These experiments displayed a subtle, yet highly reproducible difference in the apparent Si/S−i state distribution which is shown to arise from the interaction of BC with PSII in the already reduced states of the WOC. These data are discussed in detail by also taking into account the CO2 concentrations present in the buffers after argon bubbling and during the measurements. These values were measured by membrane-inlet mass spectrometry (MIMS).

@article{messinger_photosynthetic_2007,
	title = {Photosynthetic {Water} {Splitting}},
	url = {https://books.rsc.org/books/edited-volume/1817/chapter/2047788/Photosynthetic-Water-Splitting},
	doi = {10.1039/9781847558169-00291},
	abstract = {This chapter reviews our current state of knowledge on the structure and functional pattern of the water oxidizing complex (WOC) in photosynthesis. The rea},
	language = {en},
	urldate = {2024-12-10},
	author = {Messinger, Johannes and Renger, Gernot},
	month = nov,
	year = {2007},
}

This chapter reviews our current state of knowledge on the structure and functional pattern of the water oxidizing complex (WOC) in photosynthesis. The rea

@article{pushkar_polarized_2007,
	title = {Polarized {Range}‐{Extended} {X}‐{Ray} {Absorption} {Spectroscopy} of {Oriented} {Photosystem} {II} {Membranes} in the {S1} {State}},
	volume = {882},
	issn = {0094-243X},
	url = {https://doi.org/10.1063/1.2644521},
	doi = {10.1063/1.2644521},
	abstract = {Detailed information about the orientation of particular Mn‐Mn and Mn‐Ca vectors in the oxygen evolving complex (OEC) of the Photosystem II in the S1 state provide a critical starting point for the analysis of the structural changes in the OEC along the catalytic Si‐state cycle. The method of polarized range‐extended EXAFS is an important technical development, that allows: i) resolution of the 2.7 Å and 2.8 Å Mn‐Mn interactions; ii) resolution of 3.2 Å Mn‐Mn and 3.4 Å Mn‐Ca; iii) determination of 2.7 Å, 2.8 Å, 3.2 Å Mn‐Mn and 3.4 Å Mn‐Ca vectors orientation relative to the membrane normal.},
	number = {1},
	urldate = {2024-11-29},
	journal = {AIP Conference Proceedings},
	author = {Pushkar, Yulia and Yano, Junko and Glatzel, Pieter and Messinger, Johannes and Lewis, Azul and Sauer, Kenneth and Bergmann, Uwe and Yachandra, Vittal K.},
	month = feb,
	year = {2007},
	pages = {346--348},
}

Detailed information about the orientation of particular Mn‐Mn and Mn‐Ca vectors in the oxygen evolving complex (OEC) of the Photosystem II in the S1 state provide a critical starting point for the analysis of the structural changes in the OEC along the catalytic Si‐state cycle. The method of polarized range‐extended EXAFS is an important technical development, that allows: i) resolution of the 2.7 Å and 2.8 Å Mn‐Mn interactions; ii) resolution of 3.2 Å Mn‐Mn and 3.4 Å Mn‐Ca; iii) determination of 2.7 Å, 2.8 Å, 3.2 Å Mn‐Mn and 3.4 Å Mn‐Ca vectors orientation relative to the membrane normal.

@article{pushkar_structure_2007,
	title = {Structure and {Orientation} of the {Mn4Ca} {Cluster} in {Plant} {Photosystem} {II} {Membranes} {Studied} by {Polarized} {Range}-extended {X}-ray {Absorption} {Spectroscopy}*♦},
	volume = {282},
	issn = {0021-9258},
	url = {https://www.sciencedirect.com/science/article/pii/S0021925820635621},
	doi = {10.1074/jbc.M610505200},
	abstract = {X-ray absorption spectroscopy has provided important insights into the structure and function of the Mn4Ca cluster in the oxygen-evolving complex of Photosystem II (PS II). The range of manganese extended x-ray absorption fine structure data collected from PS II until now has been, however, limited by the presence of iron in PS II. Using a crystal spectrometer with high energy resolution to detect solely the manganese Kα fluorescence, we are able to extend the extended x-ray absorption fine structure range beyond the onset of the iron absorption edge. This results in improvement in resolution of the manganese-backscatterer distances in PS II from 0.14 to 0.09Ä. The high resolution data obtained from oriented spinach PS II membranes in the S1 state show that there are three di-μ-oxo-bridged manganese-manganese distances of ∼2.7 and ∼2.8Ä in a 2:1 ratio and that these three manganese-manganese vectors are aligned at an average orientation of ∼60° relative to the membrane normal. Furthermore, we are able to observe the separation of the Fourier peaks corresponding to the ∼3.2Ä manganese-manganese and the ∼3.4Ä manganese-calcium interactions in oriented PS II samples and determine their orientation relative to the membrane normal. The average of the manganese-calcium vectors at ∼3.4Ä is aligned along the membrane normal, while the ∼3.2Ä manganese-manganese vector is oriented near the membrane plane. A comparison of this structural information with the proposed Mn4Ca cluster models based on spectroscopic and diffraction data provides input for refining and selecting among these models.},
	number = {10},
	urldate = {2024-11-29},
	journal = {Journal of Biological Chemistry},
	author = {Pushkar, Yulia and Yano, Junko and Glatzel, Pieter and Messinger, Johannes and Lewis, Azul and Sauer, Kenneth and Bergmann, Uwe and Yachandra, Vittal},
	month = mar,
	year = {2007},
	pages = {7198--7208},
}

X-ray absorption spectroscopy has provided important insights into the structure and function of the Mn4Ca cluster in the oxygen-evolving complex of Photosystem II (PS II). The range of manganese extended x-ray absorption fine structure data collected from PS II until now has been, however, limited by the presence of iron in PS II. Using a crystal spectrometer with high energy resolution to detect solely the manganese Kα fluorescence, we are able to extend the extended x-ray absorption fine structure range beyond the onset of the iron absorption edge. This results in improvement in resolution of the manganese-backscatterer distances in PS II from 0.14 to 0.09Ä. The high resolution data obtained from oriented spinach PS II membranes in the S1 state show that there are three di-μ-oxo-bridged manganese-manganese distances of ∼2.7 and ∼2.8Ä in a 2:1 ratio and that these three manganese-manganese vectors are aligned at an average orientation of ∼60° relative to the membrane normal. Furthermore, we are able to observe the separation of the Fourier peaks corresponding to the ∼3.2Ä manganese-manganese and the ∼3.4Ä manganese-calcium interactions in oriented PS II samples and determine their orientation relative to the membrane normal. The average of the manganese-calcium vectors at ∼3.4Ä is aligned along the membrane normal, while the ∼3.2Ä manganese-manganese vector is oriented near the membrane plane. A comparison of this structural information with the proposed Mn4Ca cluster models based on spectroscopic and diffraction data provides input for refining and selecting among these models.

@article{smolentsev_valencecore_2007,
	title = {Valence‐to‐{Core} {X}‐{Ray} {Emission} {Spectroscopy} as a {Tool} for {Investigation} of {Organometallic} {Systems}},
	volume = {882},
	issn = {0094-243X},
	url = {https://doi.org/10.1063/1.2644522},
	doi = {10.1063/1.2644522},
	abstract = {The fine structure of X‐ray emission satellite lines just below the Fermi level (Kβ satellite lines), which arise from transitions from valence bands to the core 1s level in 3d transition metals, is a developing technique and the full potential is not well known or understood. On the basis of DFT calculations for some theoretical model complexes that are relevant in bioinorganic chemistry we show that the method can be used to distinguish bonds of the metal atom with different light atoms O/N/C even if the coordination geometries are exactly the same, which is not possible using EXAFS or XANES spectroscopy. Moreover the method is sensitive to the bonding of H to the ligands, which allows to discriminate, for example, between OH− and water near the metal. Both these aspects clearly demonstrate that the Kβ satellite lines yield information that is complementary to traditional X‐ray absorption spectroscopy. Good agreement between theoretical and experimental spectra for a complex Mn system was also achieved.},
	number = {1},
	urldate = {2024-12-10},
	journal = {AIP Conference Proceedings},
	author = {Smolentsev, Grigory and Soldatov, Alexander V. and Messinger, Johannes and Merz, Katrin and Weyhermuller, Thomas and Pushkar, Yulia and Yano, Junko and Yachandra, Vittal K. and Glatzel, Pieter},
	month = feb,
	year = {2007},
	pages = {349--351},
}

The fine structure of X‐ray emission satellite lines just below the Fermi level (Kβ satellite lines), which arise from transitions from valence bands to the core 1s level in 3d transition metals, is a developing technique and the full potential is not well known or understood. On the basis of DFT calculations for some theoretical model complexes that are relevant in bioinorganic chemistry we show that the method can be used to distinguish bonds of the metal atom with different light atoms O/N/C even if the coordination geometries are exactly the same, which is not possible using EXAFS or XANES spectroscopy. Moreover the method is sensitive to the bonding of H to the ligands, which allows to discriminate, for example, between OH− and water near the metal. Both these aspects clearly demonstrate that the Kβ satellite lines yield information that is complementary to traditional X‐ray absorption spectroscopy. Good agreement between theoretical and experimental spectra for a complex Mn system was also achieved.

@article{shevela_characterization_2006,
	title = {Characterization of the water oxidizing complex of photosystem {II} of the {Chl} d-containing cyanobacterium {Acaryochloris} marina via its reactivity towards endogenous electron donors and acceptors},
	volume = {8},
	issn = {1463-9084},
	url = {https://pubs.rsc.org/en/content/articlelanding/2006/cp/b604389e},
	doi = {10.1039/B604389E},
	abstract = {Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the Si state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different Si states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of YZ are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S2 and S3 states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S2 and S3 decay between A. marina cells and spinach thylakoids reflect a shift of the Em of YD/YoxD to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S0 oxidation to S1 by YoxD. A slightly increased double hit probability in A. marina cells is indicative of a faster QA− to QB electron transfer in these cells compared to spinach thylakoids.},
	language = {en},
	number = {29},
	urldate = {2024-11-29},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {The Royal Society of Chemistry},
	author = {Shevela, Dmitriy and Nöring, Birgit and Eckert, Hann-Jörg and Messinger, Johannes and Renger, Gernot},
	month = jul,
	year = {2006},
	pages = {3460--3466},
}

Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the Si state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different Si states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of YZ are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S2 and S3 states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S2 and S3 decay between A. marina cells and spinach thylakoids reflect a shift of the Em of YD/YoxD to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S0 oxidation to S1 by YoxD. A slightly increased double hit probability in A. marina cells is indicative of a faster QA− to QB electron transfer in these cells compared to spinach thylakoids.

@article{yano_where_2006,
	title = {Where {Water} {Is} {Oxidized} to {Dioxygen}: {Structure} of the {Photosynthetic} {Mn4Ca} {Cluster}},
	volume = {314},
	shorttitle = {Where {Water} {Is} {Oxidized} to {Dioxygen}},
	url = {https://www.science.org/doi/10.1126/science.1128186},
	doi = {10.1126/science.1128186},
	abstract = {The oxidation of water to dioxygen is catalyzed within photosystem II (PSII) by a Mn4Ca cluster, the structure of which remains elusive. Polarized extended x-ray absorption fine structure (EXAFS) measurements on PSII single crystals constrain the Mn4Ca cluster geometry to a set of three similar high-resolution structures. Combining polarized EXAFS and x-ray diffraction data, the cluster was placed within PSII, taking into account the overall trend of the electron density of the metal site and the putative ligands. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 angstrom–resolution x-ray structures or other previously proposed models.},
	number = {5800},
	urldate = {2024-11-29},
	journal = {Science},
	publisher = {American Association for the Advancement of Science},
	author = {Yano, Junko and Kern, Jan and Sauer, Kenneth and Latimer, Matthew J. and Pushkar, Yulia and Biesiadka, Jacek and Loll, Bernhard and Saenger, Wolfram and Messinger, Johannes and Zouni, Athina and Yachandra, Vittal K.},
	month = nov,
	year = {2006},
	pages = {821--825},
}

The oxidation of water to dioxygen is catalyzed within photosystem II (PSII) by a Mn4Ca cluster, the structure of which remains elusive. Polarized extended x-ray absorption fine structure (EXAFS) measurements on PSII single crystals constrain the Mn4Ca cluster geometry to a set of three similar high-resolution structures. Combining polarized EXAFS and x-ray diffraction data, the cluster was placed within PSII, taking into account the overall trend of the electron density of the metal site and the putative ligands. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 angstrom–resolution x-ray structures or other previously proposed models.

@article{kulik_55mn_2005,
	title = {{55Mn} {Pulse} {ENDOR} at 34 {GHz} of the {S0} and {S2} {States} of the {Oxygen}-{Evolving} {Complex} in {Photosystem} {II}},
	volume = {127},
	issn = {0002-7863},
	url = {https://doi.org/10.1021/ja043012j},
	doi = {10.1021/ja043012j},
	abstract = {55Mn pulse ENDOR experiments at 34 GHz (Q-band) are reported for the S0 and S2 states of the oxygen-evolving complex of photosystem II. Their numerical analysis (i) shows that in both states all four Mn ions are magnetically coupled, (ii) allows a refinement of the hyperfine interaction (HFI) parameters obtained earlier for the S2 state at X-band (Peloquin, J. M.; Campbell, K. A.; Randall, D. W.; Evanchik, M. A.; Pecoraro, V. L.; Armstrong, W. H.; Britt, R. D. J. Am. Chem. Soc. 2000, 122, 10926−10942), (iii) provides the first reliable 55Mn HFI tensors for the S0 state, and (iv) leads to the suggestion that the Mn oxidation states in S0 and S2 are Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. In addition, a Q-band EPR spectrum is reported for the S0 state, and inversion−recovery experiments at 4.5 K directly show that the electron spin−lattice relaxation for the S0 state is about 2 orders of magnitude faster than that for the S2 state.},
	number = {8},
	urldate = {2025-01-13},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Kulik, Leonid V. and Epel, Boris and Lubitz, Wolfgang and Messinger, Johannes},
	month = mar,
	year = {2005},
	pages = {2392--2393},
}

55Mn pulse ENDOR experiments at 34 GHz (Q-band) are reported for the S0 and S2 states of the oxygen-evolving complex of photosystem II. Their numerical analysis (i) shows that in both states all four Mn ions are magnetically coupled, (ii) allows a refinement of the hyperfine interaction (HFI) parameters obtained earlier for the S2 state at X-band (Peloquin, J. M.; Campbell, K. A.; Randall, D. W.; Evanchik, M. A.; Pecoraro, V. L.; Armstrong, W. H.; Britt, R. D. J. Am. Chem. Soc. 2000, 122, 10926−10942), (iii) provides the first reliable 55Mn HFI tensors for the S0 state, and (iv) leads to the suggestion that the Mn oxidation states in S0 and S2 are Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. In addition, a Q-band EPR spectrum is reported for the S0 state, and inversion−recovery experiments at 4.5 K directly show that the electron spin−lattice relaxation for the S0 state is about 2 orders of magnitude faster than that for the S2 state.

@article{kulik_electron_2005,
	title = {Electron {Spin}−{Lattice} {Relaxation} of the {S0} {State} of the {Oxygen}-{Evolving} {Complex} in {Photosystem} {II} and of {Dinuclear} {Manganese} {Model} {Complexes}},
	volume = {44},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi050411y},
	doi = {10.1021/bi050411y},
	abstract = {The temperature dependence of the electron spin−lattice relaxation time T1 was measured for the S0 state of the oxygen-evolving complex (OEC) in photosystem II and for two dinuclear manganese model complexes by pulse EPR using the inversion−recovery method. For [Mn(III)Mn(IV)(μ-O)2bipy4]ClO4, the Raman relaxation process dominates at temperatures below 50 K. In contrast, Orbach type relaxation was found for [Mn(II)Mn(III)(μ-OH)(μ-piv)2(Me3tacn)2](ClO4)2 between 4.3 and 9 K. For the latter complex, an energy separation of 24.7−28.0 cm-1 between the ground and the first excited electronic state was determined. In the S0 state of photosystem II, the T1 relaxation times were measured in the range of 4.3−6.5 K. A comparison with the relaxation data (rate and pre-exponential factor) of the two model complexes and of the S2 state of photosystem II indicates that the Orbach relaxation process is dominant for the S0 state and that its first excited state lies 21.7 ± 0.4 cm-1 above its ground state. The results are discussed with respect to the structure of the OEC in photosystem II.},
	number = {26},
	urldate = {2024-11-29},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Kulik, L. V. and Lubitz, W. and Messinger, J.},
	month = jul,
	year = {2005},
	pages = {9368--9374},
}

The temperature dependence of the electron spin−lattice relaxation time T1 was measured for the S0 state of the oxygen-evolving complex (OEC) in photosystem II and for two dinuclear manganese model complexes by pulse EPR using the inversion−recovery method. For [Mn(III)Mn(IV)(μ-O)2bipy4]ClO4, the Raman relaxation process dominates at temperatures below 50 K. In contrast, Orbach type relaxation was found for [Mn(II)Mn(III)(μ-OH)(μ-piv)2(Me3tacn)2](ClO4)2 between 4.3 and 9 K. For the latter complex, an energy separation of 24.7−28.0 cm-1 between the ground and the first excited electronic state was determined. In the S0 state of photosystem II, the T1 relaxation times were measured in the range of 4.3−6.5 K. A comparison with the relaxation data (rate and pre-exponential factor) of the two model complexes and of the S2 state of photosystem II indicates that the Orbach relaxation process is dominant for the S0 state and that its first excited state lies 21.7 ± 0.4 cm-1 above its ground state. The results are discussed with respect to the structure of the OEC in photosystem II.

@article{clausen_evidence_2005,
	title = {Evidence {That} {Bicarbonate} {Is} {Not} the {Substrate} in {Photosynthetic} {Oxygen} {Evolution}},
	volume = {139},
	issn = {0032-0889},
	url = {https://doi.org/10.1104/pp.105.068437},
	doi = {10.1104/pp.105.068437},
	abstract = {It is widely accepted that the oxygen produced by photosystem II of cyanobacteria, algae, and plants is derived from water. Earlier proposals that bicarbonate may serve as substrate or catalytic intermediate are almost forgotten, though not rigorously disproved. These latter proposals imply that CO2 is an intermediate product of oxygen production in addition to O2. In this work, we investigated this possible role of exchangeable HCO3− in oxygen evolution in two independent ways. (1) We studied a possible product inhibition of the electron transfer into the catalytic Mn4Ca complex during the oxygen-evolving reaction by greatly increasing the pressure of CO2. This was monitored by absorption transients in the near UV. We found that a 3,000-fold increase of the CO2 pressure over ambient conditions did not affect the UV transient, whereas the S3 → S4 → S0 transition was half-inhibited by raising the O2 pressure only 10-fold over ambient, as previously established. (2) The flash-induced O2 and CO2 production by photosystem II was followed simultaneously with membrane inlet mass spectrometry under approximately 15\% H218O enrichment. Light flashes that revealed the known oscillatory O2 release failed to produce any oscillatory CO2 signal. Both types of results exclude that exchangeable bicarbonate is the substrate for (and CO2 an intermediate product of) oxygen evolution by photosynthesis. The possibility that a tightly bound carbonate or bicarbonate is a cofactor of photosynthetic water oxidation has remained.},
	number = {3},
	urldate = {2024-11-29},
	journal = {Plant Physiology},
	author = {Clausen, Juergen and Beckmann, Katrin and Junge, Wolfgang and Messinger, Johannes},
	month = nov,
	year = {2005},
	pages = {1444--1450},
}

It is widely accepted that the oxygen produced by photosystem II of cyanobacteria, algae, and plants is derived from water. Earlier proposals that bicarbonate may serve as substrate or catalytic intermediate are almost forgotten, though not rigorously disproved. These latter proposals imply that CO2 is an intermediate product of oxygen production in addition to O2. In this work, we investigated this possible role of exchangeable HCO3− in oxygen evolution in two independent ways. (1) We studied a possible product inhibition of the electron transfer into the catalytic Mn4Ca complex during the oxygen-evolving reaction by greatly increasing the pressure of CO2. This was monitored by absorption transients in the near UV. We found that a 3,000-fold increase of the CO2 pressure over ambient conditions did not affect the UV transient, whereas the S3 → S4 → S0 transition was half-inhibited by raising the O2 pressure only 10-fold over ambient, as previously established. (2) The flash-induced O2 and CO2 production by photosystem II was followed simultaneously with membrane inlet mass spectrometry under approximately 15% H218O enrichment. Light flashes that revealed the known oscillatory O2 release failed to produce any oscillatory CO2 signal. Both types of results exclude that exchangeable bicarbonate is the substrate for (and CO2 an intermediate product of) oxygen evolution by photosynthesis. The possibility that a tightly bound carbonate or bicarbonate is a cofactor of photosynthetic water oxidation has remained.

@article{yano_high-resolution_2005,
	title = {High-{Resolution} {Mn} {EXAFS} of the {Oxygen}-{Evolving} {Complex} in {Photosystem} {II}:  {Structural} {Implications} for the {Mn4Ca} {Cluster}},
	volume = {127},
	issn = {0002-7863},
	shorttitle = {High-{Resolution} {Mn} {EXAFS} of the {Oxygen}-{Evolving} {Complex} in {Photosystem} {II}},
	url = {https://doi.org/10.1021/ja054873a},
	doi = {10.1021/ja054873a},
	abstract = {The biological generation of oxygen by the oxygen-evolving complex in photosystem II (PS II) is one of nature's most important reactions. The recent X-ray crystal structures, while limited by resolutions of 3.2−3.5 Å, have located the electron density associated with the Mn4Ca cluster within the multiprotein PS II complex. Detailed structures critically depend on input from spectroscopic techniques, such as EXAFS and EPR/ENDOR, as the XRD resolution does not allow for accurate determination of the position of Mn/Ca or the bridging and terminal ligand atoms. The number and distances of Mn−Mn/Ca/ligand interactions determined from EXAFS provide important constraints for the structure of the Mn4Ca cluster. Here, we present data from a high-resolution EXAFS method using a novel multicrystal monochromator that show three short Mn−Mn distances between 2.7 and 2.8 Å and, hence, the presence of three di-μ-oxo-bridged units in the Mn4Ca cluster. This result imposes clear limitations on the proposed structures based on spectroscopic and diffraction data and provides input for refining such structures.},
	number = {43},
	urldate = {2024-11-29},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Yano, Junko and Pushkar, Yulia and Glatzel, Pieter and Lewis, Azul and Sauer, Kenneth and Messinger, Johannes and Bergmann, Uwe and Yachandra, Vittal},
	month = nov,
	year = {2005},
	pages = {14974--14975},
}

The biological generation of oxygen by the oxygen-evolving complex in photosystem II (PS II) is one of nature's most important reactions. The recent X-ray crystal structures, while limited by resolutions of 3.2−3.5 Å, have located the electron density associated with the Mn4Ca cluster within the multiprotein PS II complex. Detailed structures critically depend on input from spectroscopic techniques, such as EXAFS and EPR/ENDOR, as the XRD resolution does not allow for accurate determination of the position of Mn/Ca or the bridging and terminal ligand atoms. The number and distances of Mn−Mn/Ca/ligand interactions determined from EXAFS provide important constraints for the structure of the Mn4Ca cluster. Here, we present data from a high-resolution EXAFS method using a novel multicrystal monochromator that show three short Mn−Mn distances between 2.7 and 2.8 Å and, hence, the presence of three di-μ-oxo-bridged units in the Mn4Ca cluster. This result imposes clear limitations on the proposed structures based on spectroscopic and diffraction data and provides input for refining such structures.

@incollection{hillier_mechanism_2005,
	address = {Dordrecht},
	title = {Mechanism of {Photosynthetic} {Oxygen} {Production}},
	isbn = {978-1-4020-4254-6},
	url = {https://doi.org/10.1007/1-4020-4254-X_26},
	doi = {10.1007/1-4020-4254-X_26},
	abstract = {This chapter deals with the mechanism of photosynthetic water oxidation that leads to O2 formation in Photosystem II (PS II). After a brief introduction to the structure and function of the PS II complex, the S-state cycle (Kok model) is outlined and the structure and oxidation states of the catalytic Mn4OxCa complex are summarized. We then cover in detail the current information concerning substrate water binding and consider energetic and kinetic aspects of photosynthetic water oxidation. On that basis, we discuss several recent mechanistic proposals for O-O bond formation in PS II and summarize our current perceptions in a novel mechanistic proposal for photosynthetic water oxidation.},
	language = {en},
	urldate = {2024-11-29},
	booktitle = {Photosystem {II}: {The} {Light}-{Driven} {Water}:{Plastoquinone} {Oxidoreductase}},
	publisher = {Springer Netherlands},
	author = {Hillier, Warwick and Messinger, Johannes},
	editor = {Wydrzynski, Thomas J. and Satoh, Kimiyuki and Freeman, Joel A.},
	year = {2005},
	pages = {567--608},
}

This chapter deals with the mechanism of photosynthetic water oxidation that leads to O2 formation in Photosystem II (PS II). After a brief introduction to the structure and function of the PS II complex, the S-state cycle (Kok model) is outlined and the structure and oxidation states of the catalytic Mn4OxCa complex are summarized. We then cover in detail the current information concerning substrate water binding and consider energetic and kinetic aspects of photosynthetic water oxidation. On that basis, we discuss several recent mechanistic proposals for O-O bond formation in PS II and summarize our current perceptions in a novel mechanistic proposal for photosynthetic water oxidation.

@article{kulik_pulse_2005,
	title = {Pulse {EPR}, {55Mn}-{ENDOR} and {ELDOR}-detected {NMR} of the {S2}-state of the oxygen evolving complex in {Photosystem} {II}},
	volume = {84},
	issn = {1573-5079},
	url = {https://doi.org/10.1007/s11120-005-2438-7},
	doi = {10.1007/s11120-005-2438-7},
	abstract = {Pulse EPR, 55Mn-ENDOR and ELDOR-detected NMR experiments were performed on the S2-state of the oxygen-evolving complex from spinach Photosystem II. The novel technique of random acquisition in ENDOR was used to suppress heating artefacts. Our data unambiguously shows that four Mn ions have significant hyperfine coupling constants. Numerical simulation of the 55Mn-ENDOR spectrum allowed the determination of the principal values of the hyperfine interaction tensors for all four Mn ions of the oxygen-evolving complex. The results of our 55Mn-ENDOR experiments are in good agreement with previously published data [Peloquin JM et al. (2000) J Am Chem Soc 122: 10926–10942]. For the first time ELDOR-detected NMR was applied to the S2-state and revealed a broad peak that can be simulated numerically with the same parameters that were used for the simulation of the 55Mn-ENDOR spectrum. This provides strong independent support for the assigned hyperfine parameters.},
	language = {en},
	number = {1},
	urldate = {2024-11-29},
	journal = {Photosynthesis Research},
	author = {Kulik, Leonid and Epel, Boris and Messinger, Johannes and Lubitz, Wolfgang},
	month = jun,
	year = {2005},
	keywords = {ELDOR-detected NMR, ENDOR, S2-state, manganese cluster, oxygen-evolving complex, pulse EPR},
	pages = {347--353},
}

Pulse EPR, 55Mn-ENDOR and ELDOR-detected NMR experiments were performed on the S2-state of the oxygen-evolving complex from spinach Photosystem II. The novel technique of random acquisition in ENDOR was used to suppress heating artefacts. Our data unambiguously shows that four Mn ions have significant hyperfine coupling constants. Numerical simulation of the 55Mn-ENDOR spectrum allowed the determination of the principal values of the hyperfine interaction tensors for all four Mn ions of the oxygen-evolving complex. The results of our 55Mn-ENDOR experiments are in good agreement with previously published data [Peloquin JM et al. (2000) J Am Chem Soc 122: 10926–10942]. For the first time ELDOR-detected NMR was applied to the S2-state and revealed a broad peak that can be simulated numerically with the same parameters that were used for the simulation of the 55Mn-ENDOR spectrum. This provides strong independent support for the assigned hyperfine parameters.

@article{yano_x-ray_2005,
	title = {X-ray damage to the {Mn4Ca} complex in single crystals of photosystem {II}: {A} case study for metalloprotein crystallography},
	volume = {102},
	shorttitle = {X-ray damage to the {Mn4Ca} complex in single crystals of photosystem {II}},
	url = {https://www.pnas.org/doi/10.1073/pnas.0505207102},
	doi = {10.1073/pnas.0505207102},
	abstract = {X-ray absorption spectroscopy was used to measure the damage caused by exposure to x-rays to the Mn4Ca active site in single crystals of photosystem II as a function of dose and energy of x-rays, temperature, and time. These studies reveal that the conditions used for structure determination by x-ray crystallography cause serious damage specifically to the metal-site structure. The x-ray absorption spectra show that the structure changes from one that is characteristic of a high-valent Mn4(III2,IV2) oxo-bridged Mn4Ca cluster to that of Mn(II) in aqueous solution. This damage to the metal site occurs at a dose that is more than one order of magnitude lower than the dose that results in loss of diffractivity and is commonly considered safe for protein crystallography. These results establish quantitative x-ray dose parameters that are applicable to redox-active metalloproteins. This case study shows that a careful evaluation of the structural intactness of the active site(s) by spectroscopic techniques can validate structures derived from crystallography and that it can be a valuable complementary method before structure–function correlations of metalloproteins can be made on the basis of high-resolution x-ray crystal structures.},
	number = {34},
	urldate = {2024-11-29},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Yano, Junko and Kern, Jan and Irrgang, Klaus-Dieter and Latimer, Matthew J. and Bergmann, Uwe and Glatzel, Pieter and Pushkar, Yulia and Biesiadka, Jacek and Loll, Bernhard and Sauer, Kenneth and Messinger, Johannes and Zouni, Athina and Yachandra, Vittal K.},
	month = aug,
	year = {2005},
	pages = {12047--12052},
}

X-ray absorption spectroscopy was used to measure the damage caused by exposure to x-rays to the Mn4Ca active site in single crystals of photosystem II as a function of dose and energy of x-rays, temperature, and time. These studies reveal that the conditions used for structure determination by x-ray crystallography cause serious damage specifically to the metal-site structure. The x-ray absorption spectra show that the structure changes from one that is characteristic of a high-valent Mn4(III2,IV2) oxo-bridged Mn4Ca cluster to that of Mn(II) in aqueous solution. This damage to the metal site occurs at a dose that is more than one order of magnitude lower than the dose that results in loss of diffractivity and is commonly considered safe for protein crystallography. These results establish quantitative x-ray dose parameters that are applicable to redox-active metalloproteins. This case study shows that a careful evaluation of the structural intactness of the active site(s) by spectroscopic techniques can validate structures derived from crystallography and that it can be a valuable complementary method before structure–function correlations of metalloproteins can be made on the basis of high-resolution x-ray crystal structures.

@article{messinger_biophysical_2004,
	title = {Biophysical studies of {Photosystem} {II} and related model systems},
	volume = {6},
	url = {https://pubs.rsc.org/en/content/articlelanding/2004/cp/b414025g},
	doi = {10.1039/B414025G},
	language = {en},
	number = {20},
	urldate = {2024-11-29},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {Royal Society of Chemistry},
	author = {Messinger, Johannes},
	year = {2004},
	pages = {E11--E12},
}

@article{messinger_evaluation_2004,
	title = {Evaluation of different mechanistic proposals for water oxidation in photosynthesis on the basis of {Mn4OxCa} structures for the catalytic site and spectroscopic data},
	volume = {6},
	issn = {1463-9084},
	url = {https://pubs.rsc.org/en/content/articlelanding/2004/cp/b406437b},
	doi = {10.1039/B406437B},
	abstract = {Recent progress in EPR and EXAFS spectroscopy, quantum mechanical calculations and X-ray crystallography led to a tremendous improvement in our picture of the catalytic site of water oxidation in photosystem II. It is now likely that the four Mn ions are grouped in a 3 + 1 fashion with three short Mn–Mn distances of about 2.7 Å and one long Mn–Mn distance of 3.3 Å. In addition, Ca has been firmly localized close to the Mn centers, with an average distance of 3.4 Å and an average angle of the Mn–Ca vectors close to the membrane normal (≤23°). The recent crystal structure of Ferreira et al. (Science, 2004, 303, 1831–1838) suggests that the Mn3 unit and Ca form a distorted cubane like structure. The fourth Mn is ‘dangling’ from this unit either via a μ4-oxo bridge or a mono μ-oxo bridge. However, the precise Mn–Mn distances and the bridging situation still need to be worked out. On this structural basis and the available spectroscopic data two possible mechanisms for photosynthetic water oxidation are discussed in order to highlight questions that still need to be solved for a full understanding of this fascinating reaction.},
	language = {en},
	number = {20},
	urldate = {2024-11-29},
	journal = {Physical Chemistry Chemical Physics},
	publisher = {The Royal Society of Chemistry},
	author = {Messinger, Johannes},
	month = oct,
	year = {2004},
	pages = {4764--4771},
}

Recent progress in EPR and EXAFS spectroscopy, quantum mechanical calculations and X-ray crystallography led to a tremendous improvement in our picture of the catalytic site of water oxidation in photosystem II. It is now likely that the four Mn ions are grouped in a 3 + 1 fashion with three short Mn–Mn distances of about 2.7 Å and one long Mn–Mn distance of 3.3 Å. In addition, Ca has been firmly localized close to the Mn centers, with an average distance of 3.4 Å and an average angle of the Mn–Ca vectors close to the membrane normal (≤23°). The recent crystal structure of Ferreira et al. (Science, 2004, 303, 1831–1838) suggests that the Mn3 unit and Ca form a distorted cubane like structure. The fourth Mn is ‘dangling’ from this unit either via a μ4-oxo bridge or a mono μ-oxo bridge. However, the precise Mn–Mn distances and the bridging situation still need to be worked out. On this structural basis and the available spectroscopic data two possible mechanisms for photosynthetic water oxidation are discussed in order to highlight questions that still need to be solved for a full understanding of this fascinating reaction.

@article{cinco_orientation_2004,
	title = {Orientation of {Calcium} in the {Mn4Ca} {Cluster} of the {Oxygen}-{Evolving} {Complex} {Determined} {Using} {Polarized} {Strontium} {EXAFS} of {Photosystem} {II} {Membranes}},
	volume = {43},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi036308v},
	doi = {10.1021/bi036308v},
	abstract = {The oxygen-evolving complex of photosystem II (PS II) in green plants and algae contains a cluster of four Mn atoms in the active site, which catalyzes the photoinduced oxidation of water to dioxygen. Along with Mn, calcium and chloride ions are necessary cofactors for proper functioning of the complex. The current study using polarized Sr EXAFS on oriented Sr-reactivated samples shows that Fourier peak II, which fits best to Mn at 3.5 Å rather than lighter atoms (C, N, O, or Cl), is dichroic, with a larger magnitude at 10° (angle between the PS II membrane normal and the X-ray electric field vector) and a smaller magnitude at 80°. Analysis of the dichroism of the Sr EXAFS yields a lower and upper limit of 0° and 23° for the average angle between the Sr−Mn vectors and the membrane normal and an isotropic coordination number (number of Mn neighbors to Sr) of 1 or 2 for these layered PS II samples. The results confirm the contention that Ca (Sr) is proximal to the Mn cluster and lead to refined working models of the heteronuclear Mn4Ca cluster of the oxygen-evolving complex in PS II.},
	number = {42},
	urldate = {2024-11-29},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Cinco, Roehl M. and Robblee, John H. and Messinger, Johannes and Fernandez, Carmen and Holman, Karen L. McFarlane and Sauer, Kenneth and Yachandra, Vittal K.},
	month = oct,
	year = {2004},
	pages = {13271--13282},
}

The oxygen-evolving complex of photosystem II (PS II) in green plants and algae contains a cluster of four Mn atoms in the active site, which catalyzes the photoinduced oxidation of water to dioxygen. Along with Mn, calcium and chloride ions are necessary cofactors for proper functioning of the complex. The current study using polarized Sr EXAFS on oriented Sr-reactivated samples shows that Fourier peak II, which fits best to Mn at 3.5 Å rather than lighter atoms (C, N, O, or Cl), is dichroic, with a larger magnitude at 10° (angle between the PS II membrane normal and the X-ray electric field vector) and a smaller magnitude at 80°. Analysis of the dichroism of the Sr EXAFS yields a lower and upper limit of 0° and 23° for the average angle between the Sr−Mn vectors and the membrane normal and an isotropic coordination number (number of Mn neighbors to Sr) of 1 or 2 for these layered PS II samples. The results confirm the contention that Ca (Sr) is proximal to the Mn cluster and lead to refined working models of the heteronuclear Mn4Ca cluster of the oxygen-evolving complex in PS II.

@article{britt_recent_2004,
	series = {Special issue dedicated to {Jerry} {Babcock}},
	title = {Recent pulsed {EPR} studies of the {Photosystem} {II} oxygen-evolving complex: implications as to water oxidation mechanisms},
	volume = {1655},
	issn = {0005-2728},
	shorttitle = {Recent pulsed {EPR} studies of the {Photosystem} {II} oxygen-evolving complex},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272803002317},
	doi = {10.1016/j.bbabio.2003.11.009},
	abstract = {The pulsed electron paramagnetic resonance (EPR) methods of electron spin echo envelope modulation (ESEEM) and electron spin echo-electron nuclear double resonance (ESE-ENDOR) are used to investigate the structure of the Photosystem II oxygen-evolving complex (OEC), including the paramagnetic manganese cluster and its immediate surroundings. Recent unpublished results from the pulsed EPR laboratory at UC-Davis are discussed, along with aspects of recent publications, with a focus on substrate and cofactor interactions. New data on the proximity of exchangeable deuterons around the Mn cluster poised in the S0-state are presented and interpreted. These pulsed EPR results are used in an evaluation of several recently proposed mechanisms for PSII water oxidation. We strongly favor mechanistic models where the substrate waters bind within the OEC early in the S-state cycle. Models in which the OO bond is formed by a nucleophilic attack by a Ca2+-bound water on a strong S4-state electrophile provide a good match to the pulsed EPR data.},
	urldate = {2024-11-29},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Britt, R. David and Campbell, Kristy A and Peloquin, Jeffrey M and Gilchrist, M. Lane and Aznar, Constantino P and Dicus, Michelle M and Robblee, John and Messinger, Johannes},
	month = apr,
	year = {2004},
	keywords = {ENDOR, ESEEM, Multiline EPR signal, S-state, Substrate water binding},
	pages = {158--171},
}

The pulsed electron paramagnetic resonance (EPR) methods of electron spin echo envelope modulation (ESEEM) and electron spin echo-electron nuclear double resonance (ESE-ENDOR) are used to investigate the structure of the Photosystem II oxygen-evolving complex (OEC), including the paramagnetic manganese cluster and its immediate surroundings. Recent unpublished results from the pulsed EPR laboratory at UC-Davis are discussed, along with aspects of recent publications, with a focus on substrate and cofactor interactions. New data on the proximity of exchangeable deuterons around the Mn cluster poised in the S0-state are presented and interpreted. These pulsed EPR results are used in an evaluation of several recently proposed mechanisms for PSII water oxidation. We strongly favor mechanistic models where the substrate waters bind within the OEC early in the S-state cycle. Models in which the OO bond is formed by a nucleophilic attack by a Ca2+-bound water on a strong S4-state electrophile provide a good match to the pulsed EPR data.

@article{isgandarova_functional_2003,
	title = {Functional {Differences} of {Photosystem} {II} from {Synechococcus} elongatus and {Spinach} {Characterized} by {Flash} {Induced} {Oxygen} {Evolution} {Patterns}},
	volume = {42},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi034744b},
	doi = {10.1021/bi034744b},
	abstract = {Detailed comparative studies of flash induced oxygen evolution patterns in thylakoids from the thermophilic cyanobacterium Synechococcus elongatus (S. elongatus; also referred to as Thermosynechococcus elongatus) and from spinach led to the following results:  (i) the miss parameter α of S. elongatus thylakoids exhibits a pronounced temperature dependence with a minimum of 7\% at 25 °C and values of 17 and 10\% at 3 and 35 °C, respectively, while for spinach thylakoids α decreases continuously from 18\% at 35 °C down to 8\% at 3 °C; (ii) at all temperatures, the double hit probability β exceeds in S. elongatus the corresponding values of spinach by an increment Δβ of about 3\%; (iii) at 20 °C the slow relaxation of the oxidation states S2 and S3 is about 15 and 30 times, respectively, slower in S. elongatus than in spinach, while the reduction of these S states by tyrosine YD is 2−3 times faster; (iv) the reaction S0YDox → S1YD is slower by a factor of 4 in S. elongatus as compared to spinach; and (v) the activation energies of S state dark relaxations in S. elongatus are all within a factor of 1.5 as compared to the previously reported values from spinach thylakoids [Vass, I., Deak, Z., and Hideg, E. (1990) Biochim. Biophys. Acta 1017, 63−69; Messinger, J., Schröder, W. P., and Renger, G. (1993) Biochemistry 32, 7658−7668], but the difference between the activation energies of the slow S2 and S3 decays is significantly larger in S. elongatus than in spinach. These results are discussed in terms of differences between cyanobacteria and higher plants on the acceptor side of PSII and a shift of the redox potential of the couple YD/YDox. The obtained data are also suitable to address questions about effects of the redox state of YD on the miss probability and the possibility of an S state dependent miss parameter.},
	number = {30},
	urldate = {2024-11-29},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Isgandarova, Sabina and Renger, Gernot and Messinger, Johannes},
	month = aug,
	year = {2003},
	pages = {8929--8938},
}

Detailed comparative studies of flash induced oxygen evolution patterns in thylakoids from the thermophilic cyanobacterium Synechococcus elongatus (S. elongatus; also referred to as Thermosynechococcus elongatus) and from spinach led to the following results:  (i) the miss parameter α of S. elongatus thylakoids exhibits a pronounced temperature dependence with a minimum of 7% at 25 °C and values of 17 and 10% at 3 and 35 °C, respectively, while for spinach thylakoids α decreases continuously from 18% at 35 °C down to 8% at 3 °C; (ii) at all temperatures, the double hit probability β exceeds in S. elongatus the corresponding values of spinach by an increment Δβ of about 3%; (iii) at 20 °C the slow relaxation of the oxidation states S2 and S3 is about 15 and 30 times, respectively, slower in S. elongatus than in spinach, while the reduction of these S states by tyrosine YD is 2−3 times faster; (iv) the reaction S0YDox → S1YD is slower by a factor of 4 in S. elongatus as compared to spinach; and (v) the activation energies of S state dark relaxations in S. elongatus are all within a factor of 1.5 as compared to the previously reported values from spinach thylakoids [Vass, I., Deak, Z., and Hideg, E. (1990) Biochim. Biophys. Acta 1017, 63−69; Messinger, J., Schröder, W. P., and Renger, G. (1993) Biochemistry 32, 7658−7668], but the difference between the activation energies of the slow S2 and S3 decays is significantly larger in S. elongatus than in spinach. These results are discussed in terms of differences between cyanobacteria and higher plants on the acceptor side of PSII and a shift of the redox potential of the couple YD/YDox. The obtained data are also suitable to address questions about effects of the redox state of YD on the miss probability and the possibility of an S state dependent miss parameter.

@article{sarrou_nitric_2003,
	title = {Nitric {Oxide}-{Induced} {Formation} of the {S}-2 {State} in the {Oxygen}-{Evolving} {Complex} of {Photosystem} {II} from {Synechococcus} elongatus},
	volume = {42},
	url = {https://pubs.acs.org/doi/10.1021/bi026327p},
	abstract = {In spinach photosystem II (PSII) membranes, the tetranuclear manganese cluster of the oxygen-evolving complex (OEC) can be reduced by incubation with nitric oxide at −30 °C to a state which is characterized by an Mn2(II, III) EPR multiline signal [Sarrou, J., Ioannidis, N., Deligiannakis, Y., and Petrouleas, V. (1998) Biochemistry 37, 3581−3587]. This state was recently assigned to the S-2 state of the OEC [Schansker, G., Goussias, C., Petrouleas, V., and Rutherford, A. W. (2002) Biochemistry 41, 3057−3064]. On the basis of EPR spectroscopy and flash-induced oxygen evolution patterns, we show that a similar reduction process takes place in PSII samples of the thermophilic cyanobacterium Synechococcus elongatus at both −30 and 0 °C. An EPR multiline signal, very similar but not identical to that of the S-2 state in spinach, was obtained with monomeric and dimeric PSII core complexes from S. elongatus only after incubation at −30 °C. The assignment of this EPR multiline signal to the S-2 state is corroborated by measurements of flash-induced oxygen evolution patterns and detailed fits using extended Kok models. The small reproducible shifts of several low-field peak positions of the S-2 EPR multiline signal in S. elongatus compared to spinach suggest that slight differences in the coordination geometry and/or the ligands of the manganese cluster exist between thermophilic cyanobacteria and higher plants.},
	urldate = {2024-11-29},
	journal = {Biochemistry},
	author = {Sarrou, Josephine and Isgandarova, Sabina and Kern, Jan and Zouni, Athina and Renger, Gernot and Lubitz, Wolfgang and Messinger, Johannes},
	year = {2003},
	pages = {1016--1023},
}

In spinach photosystem II (PSII) membranes, the tetranuclear manganese cluster of the oxygen-evolving complex (OEC) can be reduced by incubation with nitric oxide at −30 °C to a state which is characterized by an Mn2(II, III) EPR multiline signal [Sarrou, J., Ioannidis, N., Deligiannakis, Y., and Petrouleas, V. (1998) Biochemistry 37, 3581−3587]. This state was recently assigned to the S-2 state of the OEC [Schansker, G., Goussias, C., Petrouleas, V., and Rutherford, A. W. (2002) Biochemistry 41, 3057−3064]. On the basis of EPR spectroscopy and flash-induced oxygen evolution patterns, we show that a similar reduction process takes place in PSII samples of the thermophilic cyanobacterium Synechococcus elongatus at both −30 and 0 °C. An EPR multiline signal, very similar but not identical to that of the S-2 state in spinach, was obtained with monomeric and dimeric PSII core complexes from S. elongatus only after incubation at −30 °C. The assignment of this EPR multiline signal to the S-2 state is corroborated by measurements of flash-induced oxygen evolution patterns and detailed fits using extended Kok models. The small reproducible shifts of several low-field peak positions of the S-2 EPR multiline signal in S. elongatus compared to spinach suggest that slight differences in the coordination geometry and/or the ligands of the manganese cluster exist between thermophilic cyanobacteria and higher plants.

@article{robblee_mn_2002,
	title = {The {Mn} {Cluster} in the {S0} {State} of the {Oxygen}-{Evolving} {Complex} of {Photosystem} {II} {Studied} by {EXAFS} {Spectroscopy}:  {Are} {There} {Three} {Di}-μ-oxo-bridged {Mn2} {Moieties} in the {Tetranuclear} {Mn} {Complex}?},
	volume = {124},
	issn = {0002-7863},
	shorttitle = {The {Mn} {Cluster} in the {S0} {State} of the {Oxygen}-{Evolving} {Complex} of {Photosystem} {II} {Studied} by {EXAFS} {Spectroscopy}},
	url = {https://doi.org/10.1021/ja011621a},
	doi = {10.1021/ja011621a},
	abstract = {A key component required for an understanding of the mechanism of the evolution of molecular oxygen by the photosynthetic oxygen-evolving complex (OEC) in photosystem II (PS II) is the knowledge of the structures of the Mn cluster in the OEC in each of its intermediate redox states, or S-states. In this paper, we report the first detailed structural characterization using Mn extended X-ray absorption fine structure (EXAFS) spectroscopy of the Mn cluster of the OEC in the S0 state, which exists immediately after the release of molecular oxygen. On the basis of the EXAFS spectroscopic results, the most likely interpretation is that one of the di-μ-oxo-bridged Mn−Mn moieties in the OEC has increased in distance from 2.7 Å in the dark-stable S1 state to 2.85 Å in the S0 state. Furthermore, curve fitting of the distance heterogeneity present in the EXAFS data from the S0 state leads to the intriguing possibility that three di-μ-oxo-bridged Mn−Mn moieties may exist in the OEC instead of the two di-μ-oxo-bridged Mn−Mn moieties that are widely used in proposed structural models for the OEC. This possibility is developed using novel structural models for the Mn cluster in the OEC which are consistent with the structural information available from EXAFS and the recent X-ray crystallographic structure of PS II at 3.8 Å resolution.},
	number = {25},
	urldate = {2024-11-29},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Robblee, John H. and Messinger, Johannes and Cinco, Roehl M. and McFarlane, Karen L. and Fernandez, Carmen and Pizarro, Shelly A. and Sauer, Kenneth and Yachandra, Vittal K.},
	month = jun,
	year = {2002},
	pages = {7459--7471},
}

A key component required for an understanding of the mechanism of the evolution of molecular oxygen by the photosynthetic oxygen-evolving complex (OEC) in photosystem II (PS II) is the knowledge of the structures of the Mn cluster in the OEC in each of its intermediate redox states, or S-states. In this paper, we report the first detailed structural characterization using Mn extended X-ray absorption fine structure (EXAFS) spectroscopy of the Mn cluster of the OEC in the S0 state, which exists immediately after the release of molecular oxygen. On the basis of the EXAFS spectroscopic results, the most likely interpretation is that one of the di-μ-oxo-bridged Mn−Mn moieties in the OEC has increased in distance from 2.7 Å in the dark-stable S1 state to 2.85 Å in the S0 state. Furthermore, curve fitting of the distance heterogeneity present in the EXAFS data from the S0 state leads to the intriguing possibility that three di-μ-oxo-bridged Mn−Mn moieties may exist in the OEC instead of the two di-μ-oxo-bridged Mn−Mn moieties that are widely used in proposed structural models for the OEC. This possibility is developed using novel structural models for the Mn cluster in the OEC which are consistent with the structural information available from EXAFS and the recent X-ray crystallographic structure of PS II at 3.8 Å resolution.

@article{messinger_absence_2001,
	title = {Absence of {Mn}-{Centered} {Oxidation} in the {S2} → {S3} {Transition}:  {Implications} for the {Mechanism} of {Photosynthetic} {Water} {Oxidation}},
	volume = {123},
	issn = {0002-7863},
	shorttitle = {Absence of {Mn}-{Centered} {Oxidation} in the {S2} → {S3} {Transition}},
	url = {https://doi.org/10.1021/ja004307+},
	doi = {10.1021/ja004307+},
	abstract = {A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S3 → [S4] → S0 transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn Kβ X-ray emission spectroscopy (Kβ XES) to this problem for the first time. The Kβ XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S2 → S3 transition, in contrast to the S0 → S1 and S1 → S2 transitions, does not involve a Mn-centered oxidation. On the basis of new structural data from the S3-state, manganese μ-oxo bridge radical formation is proposed for the S2 → S3 transition, and three possible mechanisms for the O−O bond formation are presented.},
	number = {32},
	urldate = {2024-11-29},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Messinger, Johannes and Robblee, John H. and Bergmann, Uwe and Fernandez, Carmen and Glatzel, Pieter and Visser, Hendrik and Cinco, Roehl M. and McFarlane, Karen L. and Bellacchio, Emanuele and Pizarro, Shelly A. and Cramer, Stephen P. and Sauer, Kenneth and Klein, Melvin P. and Yachandra, Vittal K.},
	month = aug,
	year = {2001},
	pages = {7804--7820},
}

A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S3 → [S4] → S0 transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn Kβ X-ray emission spectroscopy (Kβ XES) to this problem for the first time. The Kβ XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S2 → S3 transition, in contrast to the S0 → S1 and S1 → S2 transitions, does not involve a Mn-centered oxidation. On the basis of new structural data from the S3-state, manganese μ-oxo bridge radical formation is proposed for the S2 → S3 transition, and three possible mechanisms for the O−O bond formation are presented.

@article{bergmann_high-resolution_2001,
	title = {High-resolution {X}-ray spectroscopy of rare events: a different look at local structure and chemistry},
	volume = {8},
	issn = {0909-0495},
	shorttitle = {High-resolution {X}-ray spectroscopy of rare events},
	url = {https://journals.iucr.org/s/issues/2001/02/00/hi5024/},
	doi = {10.1107/S0909049500016484},
	abstract = {The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal Kβ fluorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weaker features of the Kβ spectrum resulting from valence-level and `interatomic' ligand to metal transitions contain detailed information on the ligand-atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (\<600 eV) X-ray absorption fine structure (XAFS) spectra using hard (∼10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated complex systems, reactive compounds and samples under extreme conditions (pressure, temperature). Recent results are discussed.},
	language = {en},
	number = {2},
	urldate = {2024-11-29},
	journal = {Journal of Synchrotron Radiation},
	publisher = {International Union of Crystallography},
	author = {Bergmann, U. and Glatzel, P. and Robblee, J. H. and Messinger, J. and Fernandez, C. and Cinco, R. and Visser, H. and McFarlane, K. and Bellacchio, E. and Pizarro, S. and Sauer, K. and Yachandra, V. K. and Klein, M. P. and Cox, B. L. and Nealson, K. H. and Cramer, S. P.},
	month = mar,
	year = {2001},
	pages = {199--203},
}

The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal Kβ fluorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weaker features of the Kβ spectrum resulting from valence-level and `interatomic' ligand to metal transitions contain detailed information on the ligand-atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (<600 eV) X-ray absorption fine structure (XAFS) spectra using hard (∼10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated complex systems, reactive compounds and samples under extreme conditions (pressure, temperature). Recent results are discussed.

@article{messinger_towards_2000,
	title = {Towards understanding the chemistry of photosynthetic oxygen evolution: dynamic structural changes, redox states and substrate water binding of the {Mn} cluster in photosystem {II}},
	volume = {1459},
	issn = {0005-2728},
	shorttitle = {Towards understanding the chemistry of photosynthetic oxygen evolution},
	url = {https://www.sciencedirect.com/science/article/pii/S0005272800001870},
	doi = {10.1016/S0005-2728(00)00187-0},
	abstract = {This mini-review summarizes my postdoctoral research in the labs of T. Wydrzynski/C.B. Osmond, J.H.A. Nugent/M.C.W. Evans and V.K. Yachandra/K. Sauer/M.P. Klein. The results are reported in the context of selected data from the literature. Special emphasis is given to the mode of substrate water binding, Mn oxidation states and the structures of the Mn cluster in the four (meta)stable redox states of the oxygen evolving complex. The paper concludes with a working model for the mechanism of photosynthetic water oxidation that combines μ-oxo bridge oxidation in the S3 state (V.K. Yachandra, K. Sauer, M.P. Klein, Chem. Rev. 96 (1996) 2927–2950) with O-O bond formation between two terminal Mn-hydroxo ligands during the S3→(S4)→S0 transition.},
	number = {2},
	urldate = {2024-11-29},
	journal = {Biochimica et Biophysica Acta (BBA) - Bioenergetics},
	author = {Messinger, Johannes},
	month = aug,
	year = {2000},
	keywords = {Manganese cluster, Oxygen evolution, Photosystem II, Water oxidation},
	pages = {481--488},
}

This mini-review summarizes my postdoctoral research in the labs of T. Wydrzynski/C.B. Osmond, J.H.A. Nugent/M.C.W. Evans and V.K. Yachandra/K. Sauer/M.P. Klein. The results are reported in the context of selected data from the literature. Special emphasis is given to the mode of substrate water binding, Mn oxidation states and the structures of the Mn cluster in the four (meta)stable redox states of the oxygen evolving complex. The paper concludes with a working model for the mechanism of photosynthetic water oxidation that combines μ-oxo bridge oxidation in the S3 state (V.K. Yachandra, K. Sauer, M.P. Klein, Chem. Rev. 96 (1996) 2927–2950) with O-O bond formation between two terminal Mn-hydroxo ligands during the S3→(S4)→S0 transition.

@incollection{fernandez_calcium_1998,
	address = {Dordrecht},
	title = {Calcium and {Chloride} {Cofactors} of the {Oxygen} {Evolving} {Complex} - {X}-{Ray} {Absorption} {Spectroscopy} {Evidence} for {A} {Mn}/{Ca}/{Cl} {Heteronuclear} {Cluster}},
	isbn = {978-94-011-3953-3},
	url = {https://doi.org/10.1007/978-94-011-3953-3_330},
	doi = {10.1007/978-94-011-3953-3_330},
	abstract = {The oxygen-evolving complex (OEC) of photosystem II (PSII) in green plants and algae contains a cluster of four Mn atoms in the active site, which catalyzes the oxidation of water to dioxygen. Along with Mn, Cl− and Ca2+ are essential cofactors for oxygen evolution (1).},
	language = {en},
	urldate = {2024-11-29},
	booktitle = {Photosynthesis: {Mechanisms} and {Effects}: {Volume} {I}–{V}: {Proceedings} of the {XIth} {International} {Congress} on {Photosynthesis}, {Budapest}, {Hungary}, {August} 17–22, 1998},
	publisher = {Springer Netherlands},
	author = {Fernandez, Carmen and Cinco, Roehl M. and Robblee, John H. and Messinger, Johannes and Pizarro, Shelly A. and Sauer, Kenneth and Klein, Melvin P. and Yachandra, Vittal K.},
	editor = {Garab, G.},
	year = {1998},
	pages = {1399--1402},
}

The oxygen-evolving complex (OEC) of photosystem II (PSII) in green plants and algae contains a cluster of four Mn atoms in the active site, which catalyzes the oxidation of water to dioxygen. Along with Mn, Cl− and Ca2+ are essential cofactors for oxygen evolution (1).

@article{hillier_kinetic_1998,
	title = {Kinetic {Determination} of the {Fast} {Exchanging} {Substrate} {Water} {Molecule} in the {S3} {State} of {Photosystem} {II},},
	volume = {37},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi980756z},
	doi = {10.1021/bi980756z},
	abstract = {In a previous communication we showed from rapid isotopic exchange measurements that the exchangeability of the substrate water at the water oxidation catalytic site in the S3 state undergoes biphasic kinetics although the fast phase could not be fully resolved at that time [Messinger, J., Badger, M., and Wydrzynski, T. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 3209−3213]. We have since improved the time resolution for these measurements by a further factor of 3 and report here the first detailed kinetics for the fast phase of exchange. First-order exchange kinetics were determined from mass spectrometric measurements of photogenerated O2 as a function of time after injection of H218O into spinach thylakoid samples preset in the S3 state at 10 °C. For measurements made at m/e = 34 (i.e., for the mixed labeled 16,18O2 product), the two kinetic components are observed:  a slow component with k1 = 2.2 ± 0.1 s-1 (t1/2 ∼ 315 ms) and a fast component with k2 = 38 ± 4 s-1 (t1/2 ∼ 18 ms). When the isotopic exchange is measured at m/e = 36 (i.e., for the double labeled 18,18O2 product), only the slow component (k1) is observed, clearly indicating that the substrate water undergoing slow isotopic exchange provides the rate-limiting step in the formation of the double labeled 18,18O2 product. When the isotopic exchange is measured as a function of temperature, the two kinetic components reveal different temperature dependencies in which k1 increases by a factor of 10 over the range 0−20 °C while k2 increases by only a factor of 3. Assuming simple Arrhenius behavior, the activation energies are estimated to be 78 ± 10 kJ mol-1 for the slow component and 39 ± 5 kJ mol-1 for the fast component. The different kinetic components in the 18O isotopic exchange provide firm evidence that the two substrate water molecules undergo separate exchange processes at two different chemical sites in the S3 state, prior to the O2 release step (t1/2 ∼ 1 ms at 20 °C). The results are discussed in terms of how the substrate water may be bound at two separate metal sites.},
	number = {48},
	urldate = {2024-12-12},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Hillier, Warwick and Messinger, Johannes and Wydrzynski, Tom},
	month = dec,
	year = {1998},
	pages = {16908--16914},
}

In a previous communication we showed from rapid isotopic exchange measurements that the exchangeability of the substrate water at the water oxidation catalytic site in the S3 state undergoes biphasic kinetics although the fast phase could not be fully resolved at that time [Messinger, J., Badger, M., and Wydrzynski, T. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 3209−3213]. We have since improved the time resolution for these measurements by a further factor of 3 and report here the first detailed kinetics for the fast phase of exchange. First-order exchange kinetics were determined from mass spectrometric measurements of photogenerated O2 as a function of time after injection of H218O into spinach thylakoid samples preset in the S3 state at 10 °C. For measurements made at m/e = 34 (i.e., for the mixed labeled 16,18O2 product), the two kinetic components are observed:  a slow component with k1 = 2.2 ± 0.1 s-1 (t1/2 ∼ 315 ms) and a fast component with k2 = 38 ± 4 s-1 (t1/2 ∼ 18 ms). When the isotopic exchange is measured at m/e = 36 (i.e., for the double labeled 18,18O2 product), only the slow component (k1) is observed, clearly indicating that the substrate water undergoing slow isotopic exchange provides the rate-limiting step in the formation of the double labeled 18,18O2 product. When the isotopic exchange is measured as a function of temperature, the two kinetic components reveal different temperature dependencies in which k1 increases by a factor of 10 over the range 0−20 °C while k2 increases by only a factor of 3. Assuming simple Arrhenius behavior, the activation energies are estimated to be 78 ± 10 kJ mol-1 for the slow component and 39 ± 5 kJ mol-1 for the fast component. The different kinetic components in the 18O isotopic exchange provide firm evidence that the two substrate water molecules undergo separate exchange processes at two different chemical sites in the S3 state, prior to the O2 release step (t1/2 ∼ 1 ms at 20 °C). The results are discussed in terms of how the substrate water may be bound at two separate metal sites.

@incollection{messinger_oxidation_1998,
	address = {Dordrecht},
	title = {Oxidation {States} and {Structure} of the {Manganese} {Cluster} in the {S0} {State} of the {Oxygen} {Evolving} {Complex}},
	isbn = {978-94-011-3953-3},
	url = {https://doi.org/10.1007/978-94-011-3953-3_301},
	doi = {10.1007/978-94-011-3953-3_301},
	abstract = {Photosystem II (PS II) catalyzes the light driven oxidation of water to molecular oxygen and the reduction of plastoquinone to plastohydroquinone. Water oxidation occurs in the oxygen evolving complex (OEC) of PS II that is known to cycle through five different redox states, referred to as the S states (S0,..,S4). A cluster of four Mn, one Ca and Cl− is thought to form the central unit of the OEC which stores most of the oxidizing equivalents and binds the substrate water.},
	language = {en},
	urldate = {2024-11-29},
	booktitle = {Photosynthesis: {Mechanisms} and {Effects}: {Volume} {I}–{V}: {Proceedings} of the {XIth} {International} {Congress} on {Photosynthesis}, {Budapest}, {Hungary}, {August} 17–22, 1998},
	publisher = {Springer Netherlands},
	author = {Messinger, J. and Robblee, J. H. and Fernandez, C. and Cinco, R. M. and Visser, H. and Bergmann, U. and Glatzel, P. and Cramer, S. P. and Campbell, K. A. and Peloquin, J. M. and Britt, R. D. and Sauer, K. and Yachandra, V. K. and Klein, M. P.},
	editor = {Garab, G.},
	year = {1998},
	pages = {1279--1282},
}

Photosystem II (PS II) catalyzes the light driven oxidation of water to molecular oxygen and the reduction of plastoquinone to plastohydroquinone. Water oxidation occurs in the oxygen evolving complex (OEC) of PS II that is known to cycle through five different redox states, referred to as the S states (S0,..,S4). A cluster of four Mn, one Ca and Cl− is thought to form the central unit of the OEC which stores most of the oxidizing equivalents and binds the substrate water.

@incollection{cinco_refined_1998,
	address = {Dordrecht},
	title = {Refined {Model} of the {Oxidation} {States} and {Structures} of the {Mn}/{Ca}/{Cl} {Cluster} of the {Oxygen} {Evolving} {Complex} of {Photosystem} {II}},
	isbn = {978-94-011-3953-3},
	url = {https://doi.org/10.1007/978-94-011-3953-3_300},
	doi = {10.1007/978-94-011-3953-3_300},
	abstract = {Central to the problem of photosynthetic oxygen evolution is the structure and function of the Mn/Ca/Cl complex that appears to be the locus of charge accumulation and water splitting. In the recent past our group has presented a topological model for the structure of the tetranuclear Mn cluster, the oxidation state assignments of the S-states of the Kok cycle, the orientation of the Mn-Mn vectors relative to the membrane normal, and evidence for the proximity of Ca to the Mn (1–3).},
	language = {en},
	urldate = {2024-11-28},
	booktitle = {Photosynthesis: {Mechanisms} and {Effects}: {Volume} {I}–{V}: {Proceedings} of the {XIth} {International} {Congress} on {Photosynthesis}, {Budapest}, {Hungary}, {August} 17–22, 1998},
	publisher = {Springer Netherlands},
	author = {Cinco, Roehl M. and Fernandez, Carmen and Messinger, Johannes and Robblee, John H. and Visser, Henk and McFarlane, Karen L. and Bergmann, Uwe and Glatzel, Pieter and Cramer, Stephen P. and Sauer, Kenneth and Klein, Melvin P. and Yachandra, Vittal K.},
	editor = {Garab, G.},
	year = {1998},
	pages = {1273--1278},
}

Central to the problem of photosynthetic oxygen evolution is the structure and function of the Mn/Ca/Cl complex that appears to be the locus of charge accumulation and water splitting. In the recent past our group has presented a topological model for the structure of the tetranuclear Mn cluster, the oxidation state assignments of the S-states of the Kok cycle, the orientation of the Mn-Mn vectors relative to the membrane normal, and evidence for the proximity of Ca to the Mn (1–3).

@incollection{hillier_substrate_1998,
	address = {Dordrecht},
	title = {Substrate {Water} {18O} {Exchange} {Kinetics} in the {S2} {State} of {Photosystem} {II}},
	isbn = {978-94-011-3953-3},
	url = {https://doi.org/10.1007/978-94-011-3953-3_308},
	doi = {10.1007/978-94-011-3953-3_308},
	abstract = {The oxidation of water into molecular oxygen during photosynthesis is catalyzed by a subdomain of photosystem II (PSII) termed the water oxidizing complex (WOC). Contained within the WOC is a redox-active Mn/tyrosine motif which forms, in part, the catalytic site. Fundamental to the water oxidation mechanism is the periodicity of four in the release of O2 upon flash illumination. This unique behavior can readily be explained by a phenomenological model in which the WOC cycles through five intermediate states called the S-states (or Sn where n=0-4). Upon attaining the metastable S4 state, O2 is released, the S0 state is regenerated, and the S-state cycle begins again [1].},
	language = {en},
	urldate = {2024-11-29},
	booktitle = {Photosynthesis: {Mechanisms} and {Effects}: {Volume} {I}–{V}: {Proceedings} of the {XIth} {International} {Congress} on {Photosynthesis}, {Budapest}, {Hungary}, {August} 17–22, 1998},
	publisher = {Springer Netherlands},
	author = {Hillier, Warwick and Messinger, Johannes and Wydrzynski, Tom},
	editor = {Garab, G.},
	year = {1998},
	pages = {1307--1310},
}

The oxidation of water into molecular oxygen during photosynthesis is catalyzed by a subdomain of photosystem II (PSII) termed the water oxidizing complex (WOC). Contained within the WOC is a redox-active Mn/tyrosine motif which forms, in part, the catalytic site. Fundamental to the water oxidation mechanism is the periodicity of four in the release of O2 upon flash illumination. This unique behavior can readily be explained by a phenomenological model in which the WOC cycles through five intermediate states called the S-states (or Sn where n=0-4). Upon attaining the metastable S4 state, O2 is released, the S0 state is regenerated, and the S-state cycle begins again [1].

@article{messinger_detection_1997,
	title = {Detection of an {EPR} {Multiline} {Signal} for the {S0}* {State} in {Photosystem} {II}},
	volume = {36},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi9711285},
	doi = {10.1021/bi9711285},
	abstract = {The S0* state was generated by incubation of dark-adapted (S1 state) photosystem II membranes either with the exogenous two electron reductant hydrazine and subsequent 273 K illumination in the presence of DCMU or by dark incubation with low amounts of the one electron reductant hydroxylamine. In agreement with earlier reports, the S1 and S-1 states were found to be electron paramagnetic resonance (EPR) silent. However, in the presence of 0.5−1.5\% methanol, a weak EPR multiline signal centered around g = 2.0 was observed at 7 K for the S0* states generated by both procedures. This signal has a similar average line splitting to the well-characterized S2 state multiline EPR signal, but can be clearly distinguished from that and other modified S2 multiline signals by differences in line position and intensities. In addition, at 4 K it can be seen that the S0* multiline has a greater spectral breadth than the S2 multilines and is composed of up to 26 peaks. The S0* signal is not seen in the absence of methanol and is not affected by 1 mM EDTA in the buffer medium. We assign the S0* multiline signal to the manganese cluster of the oxygen evolving complex in a mixed valence state of the form MnIIMnIIIMnIIIMnIII, MnIIMnIIIMnIVMnIV, or MnIIIMnIIIMnIIIMnIV. Addition of methanol may be helpful in future to find an EPR signal originating from the natural S0 state.},
	number = {37},
	urldate = {2024-11-28},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Messinger, Johannes and Nugent, Jonathan H. A. and Evans, Michael C. W.},
	month = sep,
	year = {1997},
	pages = {11055--11060},
}

The S0* state was generated by incubation of dark-adapted (S1 state) photosystem II membranes either with the exogenous two electron reductant hydrazine and subsequent 273 K illumination in the presence of DCMU or by dark incubation with low amounts of the one electron reductant hydroxylamine. In agreement with earlier reports, the S1 and S-1 states were found to be electron paramagnetic resonance (EPR) silent. However, in the presence of 0.5−1.5% methanol, a weak EPR multiline signal centered around g = 2.0 was observed at 7 K for the S0* states generated by both procedures. This signal has a similar average line splitting to the well-characterized S2 state multiline EPR signal, but can be clearly distinguished from that and other modified S2 multiline signals by differences in line position and intensities. In addition, at 4 K it can be seen that the S0* multiline has a greater spectral breadth than the S2 multilines and is composed of up to 26 peaks. The S0* signal is not seen in the absence of methanol and is not affected by 1 mM EDTA in the buffer medium. We assign the S0* multiline signal to the manganese cluster of the oxygen evolving complex in a mixed valence state of the form MnIIMnIIIMnIIIMnIII, MnIIMnIIIMnIVMnIV, or MnIIIMnIIIMnIIIMnIV. Addition of methanol may be helpful in future to find an EPR signal originating from the natural S0 state.

@article{messinger_s-3_1997,
	title = {S-3 {State} of the {Water} {Oxidase} in {Photosystem} {II}},
	volume = {36},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi962653r},
	doi = {10.1021/bi962653r},
	abstract = {The effect of the reductant hydrazine on the flash-induced oxygen oscillation patterns of spinach thylakoids was used to characterize a new super-reduced redox state of the water oxidase in photosystem II. The formation of a discrete S-3 state is evident from the shift of the first maximum of oxygen evolution from the 3rd flash through the 5th flash to the 7th flash during a 90 min incubation of dark-adapted thylakoids with 10 mM hydrazine sulfate at pH 6.8 on ice. A distinct period four oscillation with further maxima on the 11th and 15th flashes is still observed at this stage of the incubation. The data analysis within the framework of an extended Kok model reveals that a S-3 state population of almost 50\% can be achieved by this treatment. A prolonged incubation of the S-3 sample with 10 mM hydrazine (and even 100 mM) does not lead to a further shift of the first maximum toward the 9th flash that could reflect the formation of the S-5 state. Instead, a slow oxidation of S-3 to S-2 takes place by an as yet unidentified electron acceptor. A consistent simulation of all the measured oxygen oscillation patterns of this study could, however, only be achieved by including the formal redox states S-4 and S-5 in the fits (S-4 + S-5 up to 35\%). The implications of these findings for the oxidation states of the manganese in the tetranuclear cluster of the water oxidase are discussed.},
	number = {23},
	urldate = {2024-11-28},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Messinger, J. and Seaton, G. and Wydrzynski, T. and Wacker, U. and Renger, G.},
	month = jun,
	year = {1997},
	pages = {6862--6873},
}

The effect of the reductant hydrazine on the flash-induced oxygen oscillation patterns of spinach thylakoids was used to characterize a new super-reduced redox state of the water oxidase in photosystem II. The formation of a discrete S-3 state is evident from the shift of the first maximum of oxygen evolution from the 3rd flash through the 5th flash to the 7th flash during a 90 min incubation of dark-adapted thylakoids with 10 mM hydrazine sulfate at pH 6.8 on ice. A distinct period four oscillation with further maxima on the 11th and 15th flashes is still observed at this stage of the incubation. The data analysis within the framework of an extended Kok model reveals that a S-3 state population of almost 50% can be achieved by this treatment. A prolonged incubation of the S-3 sample with 10 mM hydrazine (and even 100 mM) does not lead to a further shift of the first maximum toward the 9th flash that could reflect the formation of the S-5 state. Instead, a slow oxidation of S-3 to S-2 takes place by an as yet unidentified electron acceptor. A consistent simulation of all the measured oxygen oscillation patterns of this study could, however, only be achieved by including the formal redox states S-4 and S-5 in the fits (S-4 + S-5 up to 35%). The implications of these findings for the oxidation states of the manganese in the tetranuclear cluster of the water oxidase are discussed.

@article{messinger_s0_1997,
	title = {The {S0} {State} of the {Oxygen}-{Evolving} {Complex} in {Photosystem} {II} {Is} {Paramagnetic}:  {Detection} of an {EPR} {Multiline} {Signal}},
	volume = {119},
	issn = {0002-7863},
	shorttitle = {The {S0} {State} of the {Oxygen}-{Evolving} {Complex} in {Photosystem} {II} {Is} {Paramagnetic}},
	url = {https://doi.org/10.1021/ja972696a},
	doi = {10.1021/ja972696a},
	number = {46},
	urldate = {2024-11-28},
	journal = {Journal of the American Chemical Society},
	publisher = {American Chemical Society},
	author = {Messinger, Johannes and Robblee, John H. and Yu, Wa On and Sauer, Kenneth and Yachandra, Vittal K. and Klein, Melvin P.},
	month = nov,
	year = {1997},
	pages = {11349--11350},
}

@article{wydrzynski_functional_1996,
	title = {On the functional significance of substrate accessibility in the photosynthetic water oxidation mechanism},
	volume = {96},
	issn = {1399-3054},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1399-3054.1996.tb00224.x},
	doi = {10.1111/j.1399-3054.1996.tb00224.x},
	abstract = {Recent evidence suggests that after selective perturbation of the protein structure in photosystem II, hydrogen peroxide is formed at the water oxidation catalytic site instead of molecular oxygen. In this communication, we review the interpretation of these observations and elaborate on the hypothesis that an essential factor in determining the end-product of photosynthetic water oxidation is one of substrate accessibility. It is argued that normally the access of water to the catalytic site is controlled by a hydrophobic domain in the surrounding protein matrix and that the production of O2 is optimized by an ordered binding of the two substrate water molecules. It is proposed that upon perturbation of the hydrophobic domain (for example, through the removal of various extrinsic proteins) the catalytic site becomes exposed to excess water from the external solvent phase. As a consequence, additional water binds at the catalytic site during intermediate oxidation steps and undergoes a partial oxidation reaction to form hydrogen peroxide. The importance of water accessibility to the structure/function relationships of photosystem II is discussed, particularly with respect to photoinhibitory damage through the formation of hydrogen peroxide.},
	language = {en},
	number = {2},
	urldate = {2024-11-28},
	journal = {Physiologia Plantarum},
	author = {Wydrzynski, Tom and Hillier, Warwick and Messinger, Johannes},
	year = {1996},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1399-3054.1996.tb00224.x},
	keywords = {Hydrogen peroxide formation, oxygen evolution, photo-system II, photoinhibition, structure-function relationships},
	pages = {342--350},
}

Recent evidence suggests that after selective perturbation of the protein structure in photosystem II, hydrogen peroxide is formed at the water oxidation catalytic site instead of molecular oxygen. In this communication, we review the interpretation of these observations and elaborate on the hypothesis that an essential factor in determining the end-product of photosynthetic water oxidation is one of substrate accessibility. It is argued that normally the access of water to the catalytic site is controlled by a hydrophobic domain in the surrounding protein matrix and that the production of O2 is optimized by an ordered binding of the two substrate water molecules. It is proposed that upon perturbation of the hydrophobic domain (for example, through the removal of various extrinsic proteins) the catalytic site becomes exposed to excess water from the external solvent phase. As a consequence, additional water binds at the catalytic site during intermediate oxidation steps and undergoes a partial oxidation reaction to form hydrogen peroxide. The importance of water accessibility to the structure/function relationships of photosystem II is discussed, particularly with respect to photoinhibitory damage through the formation of hydrogen peroxide.

@article{messinger_detection_1995,
	title = {Detection of one slowly exchanging substrate water molecule in the {S3} state of photosystem {II}.},
	volume = {92},
	url = {https://www.pnas.org/doi/10.1073/pnas.92.8.3209},
	doi = {10.1073/pnas.92.8.3209},
	abstract = {The exchangeability of the substrate water molecules at the catalytic site of water oxidation in photosystem II has been probed by isotope-exchange measurements using mass spectrometric detection of flash-induced oxygen evolution. A stirred sample chamber was constructed to reduce the lag time between injection of H2(18)O and the detecting flash by a factor of more than 1000 compared to the original experiments by R. Radmer and O. Ollinger [(1986) FEBS Lett. 195, 285-289]. Our data show that there is a slow (t1/2 approximately 500 ms, 10 degrees C) and a fast (t1/2 {\textless}25 ms, 10 degrees C) exchanging substrate water molecule in the S3 state of photosystem II. The slow exchange is coupled with an activation energy of about 75 kJ/mol and is discussed in terms of a terminal manganese oxo ligand, while the faster exchanging substrate molecule may represent a water molecule not directly bound to the manganese center.},
	number = {8},
	urldate = {2024-11-28},
	journal = {Proceedings of the National Academy of Sciences},
	publisher = {Proceedings of the National Academy of Sciences},
	author = {Messinger, J and Badger, M and Wydrzynski, T},
	month = apr,
	year = {1995},
	pages = {3209--3213},
}

The exchangeability of the substrate water molecules at the catalytic site of water oxidation in photosystem II has been probed by isotope-exchange measurements using mass spectrometric detection of flash-induced oxygen evolution. A stirred sample chamber was constructed to reduce the lag time between injection of H2(18)O and the detecting flash by a factor of more than 1000 compared to the original experiments by R. Radmer and O. Ollinger [(1986) FEBS Lett. 195, 285-289]. Our data show that there is a slow (t1/2 approximately 500 ms, 10 degrees C) and a fast (t1/2 \textless25 ms, 10 degrees C) exchanging substrate water molecule in the S3 state of photosystem II. The slow exchange is coupled with an activation energy of about 75 kJ/mol and is discussed in terms of a terminal manganese oxo ligand, while the faster exchanging substrate molecule may represent a water molecule not directly bound to the manganese center.

@incollection{messinger_heterogeneity_1995,
	title = {Heterogeneity in substrate water binding to {Photosystem} {II}},
	volume = {II},
	booktitle = {P. {Mathis} ({Ed}.) {Photosynthesis}: {From} {Light} to {Biosphere}},
	publisher = {Kluwer Academic Publishers},
	author = {Messinger, Johannes and Hillier, Warwick and Badger, Murray R. and Wydrzynski, Tom},
	year = {1995},
	pages = {283--286},
}

@article{kebekus_structural_1995,
	title = {Structural {Changes} in the {Water}-{Oxidizing} {Complex} {Monitored} via the {pH} {Dependence} of the {Reduction} {Rate} of {Redox} {State} {S1} by {Hydrazine} and {Hydroxylamine} in {Isolated} {Spinach} {Thylakoids}},
	volume = {34},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi00018a021},
	doi = {10.1021/bi00018a021},
	number = {18},
	urldate = {2024-11-28},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Kebekus, U. and Messinger, J. and Renger, G.},
	month = may,
	year = {1995},
	pages = {6175--6182},
}

@article{messinger_analyses_1994,
	title = {Analyses of {pH}-{Induced} {Modifications} of the {Period} {Four} {Oscillation} of {Flash}-{Induced} {Oxygen} {Evolution} {Reveal} {Distinct} {Structural} {Changes} of the {Photosystem} {II} {Donor} {Side} at {Characteristic} {pH} {Values}},
	volume = {33},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi00202a008},
	doi = {10.1021/bi00202a008},
	number = {36},
	urldate = {2024-11-28},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Messinger, Johannes and Renger, Gernot},
	month = sep,
	year = {1994},
	pages = {10896--10905},
}

@article{renger_structure-function_1994,
	title = {Structure-function relationships in photosynthetic water oxidation},
	volume = {22},
	issn = {0300-5127},
	url = {https://doi.org/10.1042/bst0220318},
	doi = {10.1042/bst0220318},
	number = {2},
	urldate = {2024-11-28},
	journal = {Biochemical Society Transactions},
	author = {Renger, G. and Bittner, T. and Messinger, J.},
	month = may,
	year = {1994},
	pages = {318--322},
}

@article{messinger_generation_1993,
	title = {Generation, oxidation by the oxidized form of the tyrosine of polypeptide {D2}, and possible electronic configuration of the redox states {S0}, {S}-1, and {S}-2 of the water oxidase in isolated spinach thylakoids},
	volume = {32},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi00087a017},
	doi = {10.1021/bi00087a017},
	number = {36},
	urldate = {2024-11-28},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Messinger, J. and Renger, G.},
	month = sep,
	year = {1993},
	pages = {9379--9386},
}

@article{messinger_structure-function_1993,
	title = {Structure-function relations in photosystem {II}. {Effects} of temperature and chaotropic agents on the period four oscillation of flash-induced oxygen evolution},
	volume = {32},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi00081a009},
	doi = {10.1021/bi00081a009},
	number = {30},
	urldate = {2024-11-28},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Messinger, Johannes and Schroeder, Wolfgang P. and Renger, Gernot},
	month = aug,
	year = {1993},
	pages = {7658--7668},
}

@incollection{schroder_effects_1992,
	address = {Netherlands},
	title = {Effects of {BSA}, fatty acids and lipase treatment on {PSII}},
	volume = {II},
	booktitle = {N. {Murata} ({Ed}.) {Research} in {Photosynthesis}},
	publisher = {Kluwer Academic Publishers},
	author = {Schröder, Wolfgang P and Messinger, Johannes and Tremolieres, A and Renger, Gernot},
	year = {1992},
	pages = {159--162},
}

@incollection{renger_ph_1992,
	address = {Netherlands},
	title = {{PH} induced structural changes of {PSII} affect the reaction properties of the water oxidase},
	volume = {II},
	booktitle = {N. {Murata} ({Ed}.) {Research} in {Photosynthesis}},
	publisher = {Kluwer Academic Publishers},
	author = {Renger, Gernot and Messinger, Johannes and Wacker, U.},
	year = {1992},
	pages = {329--332},
}

@article{messinger_flash_1991,
	title = {The {Flash} {Pattern} of {Photosynthetic} {Oxygen} {Evolution} after {Treatment} with {Low} {Concentrations} of {Hydroxylamine} as a {Function} of the {Previous} {S1}/{S0}-{Ratio} {Further} {Evidence} that {NH2OH} {Reduces} the {Water} {Oxidizing} {Complex} in the {Dark}},
	volume = {46},
	copyright = {De Gruyter expressly reserves the right to use all content for commercial text and data mining within the meaning of Section 44b of the German Copyright Act.},
	issn = {1865-7125},
	url = {https://www.degruyter.com/document/doi/10.1515/znc-1991-11-1217/html?srsltid=AfmBOor9MtfXHjm6QfyYb8fiR7ztJBjnzKANWtqfqyD4MEJByTTrJ2vF},
	doi = {10.1515/znc-1991-11-1217},
	abstract = {Flash induced oxygen evolution patterns of isolated PS II complexes from the cyanobacterium Synechococcus were measured with a Joliot-type electrode. By suitable preflash and dark adaptation procedures, samples were prepared in the state S 1 (100\%), as well as enriched in S 0 (60\% S0, 40\% S,). After treatment with low concentrations of NH 2 OH (≤ 100 μм), the two flash patterns were identical. This is further evidence for a reduction of the water oxidizing complex by hydroxylamine in the dark. Two reduced states (S -1 and S -2 ) below S 0 are formed by this reduction.},
	language = {en},
	number = {11-12},
	urldate = {2024-11-28},
	journal = {Zeitschrift für Naturforschung C},
	publisher = {De Gruyter},
	author = {Messinger, J. and Pauly, S. and Witt, H. T.},
	month = dec,
	year = {1991},
	pages = {1033--1037},
}

Flash induced oxygen evolution patterns of isolated PS II complexes from the cyanobacterium Synechococcus were measured with a Joliot-type electrode. By suitable preflash and dark adaptation procedures, samples were prepared in the state S 1 (100%), as well as enriched in S 0 (60% S0, 40% S,). After treatment with low concentrations of NH 2 OH (≤ 100 μм), the two flash patterns were identical. This is further evidence for a reduction of the water oxidizing complex by hydroxylamine in the dark. Two reduced states (S -1 and S -2 ) below S 0 are formed by this reduction.

@article{messinger_unusual_1991,
	title = {Unusual low reactivity of the water oxidase in redox state {S3} toward exogenous reductants. {Analysis} of the {NH2OH}- and {NH2NH2}-induced modifications of flash-induced oxygen evolution in isolated spinach thylakoids},
	volume = {30},
	issn = {0006-2960},
	url = {https://doi.org/10.1021/bi00245a027},
	doi = {10.1021/bi00245a027},
	number = {31},
	urldate = {2024-11-28},
	journal = {Biochemistry},
	publisher = {American Chemical Society},
	author = {Messinger, J. and Wacker, U. and Renger, G.},
	month = aug,
	year = {1991},
	pages = {7852--7862},
}

@incollection{messinger_temperature_1990,
	address = {Dordrecht},
	title = {Temperature {Dependence} of {O2}-{Oscillation} {Pattern} of {Spinach} {Thylakoids}},
	isbn = {978-94-009-0511-5},
	url = {https://doi.org/10.1007/978-94-009-0511-5_196},
	doi = {10.1007/978-94-009-0511-5_196},
	abstract = {Photosynthetic water cleavage into dioxygen and metabolically bound hydrogen in the form of plastohydroquinone takes place within a polypeptide complex referred to as system II (for details see ref.1). It is now widely assumed that a heterodimer of polypeptides D1 and D2 forms the matrix that carries the functional redox groups participating in the overall reaction sequence. Accordingly, this matrix determines the reaction coordinates of all individual redox steps. However, in addition to this protein matrix a number of polypeptides are associated that probably act as regulatory subunits. Furthermore, structural effects of functional relevance could also arise from the interaction of the proteins with the surrounding lipid environment of the membrane. Differential scanning calorimetry measurements (2) indicate, that in spinach thylakoids, five to ten per cent of the polar lipids undergo a phase transition in the temperature range of 10 to 30°C.},
	language = {en},
	urldate = {2024-11-25},
	booktitle = {Current {Research} in {Photosynthesis}: {Proceedings} of the {VIIIth} {International} {Conference} on {Photosynthesis} {Stockholm}, {Sweden}, {August} 6–11, 1989},
	publisher = {Springer Netherlands},
	author = {Messinger, J. and Renger, G.},
	editor = {Baltscheffsky, M.},
	year = {1990},
	pages = {849--852},
}

Photosynthetic water cleavage into dioxygen and metabolically bound hydrogen in the form of plastohydroquinone takes place within a polypeptide complex referred to as system II (for details see ref.1). It is now widely assumed that a heterodimer of polypeptides D1 and D2 forms the matrix that carries the functional redox groups participating in the overall reaction sequence. Accordingly, this matrix determines the reaction coordinates of all individual redox steps. However, in addition to this protein matrix a number of polypeptides are associated that probably act as regulatory subunits. Furthermore, structural effects of functional relevance could also arise from the interaction of the proteins with the surrounding lipid environment of the membrane. Differential scanning calorimetry measurements (2) indicate, that in spinach thylakoids, five to ten per cent of the polar lipids undergo a phase transition in the temperature range of 10 to 30°C.

@article{messinger_reactivity_1990,
	title = {The reactivity of hydrazine with photosystem {II} strongly depends on the redox state of the water oxidizing system},
	volume = {277},
	copyright = {FEBS Letters 277 (1990) 1873-3468 © 2015 Federation of European Biochemical Societies},
	issn = {1873-3468},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1016/0014-5793%2890%2980829-8},
	doi = {10.1016/0014-5793(90)80829-8},
	abstract = {The decay kinetics of the redox states S2 and S3 of the water-oxidizing enzyme have been analyzed in isolated spinach thylakoids in the absence and presence of the exogenous reductant hydrazine. In control samples without NH2NH2 a biphasic decay is observed. The rapid decline of S2 and S3 with yD as reductant exhibits practically the same kinetics with t 1/2 = 6-7 s at pH = 7.2 and 7°C. The slow reduction (order of 5-10 min at 7°C) of S2 and S3 with endogenous electron donors other than yD is about twice as fast for S2 as for S3 under these conditions. In contrast, the hydrazine-induced reductive shifts of the formal redox states Si (i = 0⋯3) are characterized by a totally different kinetic pattern: (a) at 1 mMNH2NH2 and incubation on ice the decay of S2 is estimated to be at least 25 times faster (t 1/2⩽0.4 min) than the corresponding reaction of S3 (t 1/2≈13 min); (b) the NH2NH2-induced decay of S2 is even slower (about twice) than the transformation of S1 into the formal redox state ‘S−1’ (t 1/2≈6 min), which gives rise to the two-digit phase shift of the oxygen-yield pattern induced by a flash train in dark adapted thylakoids. (c) the NH2NH2-induced transformation S0→‘S−2’ [Renger, Messinger and Hanssum (1990) in: Curr. Res. Photosynth. (Baltscheffsky, M., ed). Vol. 1, pp. 845-848, Kluwer, Dordrecht] is about three times faster (t 1/2≈ min) than the reaction. Based on these results, the following dependence on the redox state Si of the reactivity towards NH2NH2 is obtained: S3 {\textless} S1 {\textless} S0 ⪡ S2. The implications of this surprising order of reactivity are discussed.},
	language = {en},
	number = {1-2},
	urldate = {2024-11-25},
	journal = {FEBS Letters},
	author = {Messinger, J. and Renger, G.},
	year = {1990},
	note = {\_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793\%2890\%2980829-8},
	keywords = {Hydrazine, Hydroxylamine, Photosystem II, Si-state lifetimes, Water oxidation},
	pages = {141--146},
}

The decay kinetics of the redox states S2 and S3 of the water-oxidizing enzyme have been analyzed in isolated spinach thylakoids in the absence and presence of the exogenous reductant hydrazine. In control samples without NH2NH2 a biphasic decay is observed. The rapid decline of S2 and S3 with yD as reductant exhibits practically the same kinetics with t 1/2 = 6-7 s at pH = 7.2 and 7°C. The slow reduction (order of 5-10 min at 7°C) of S2 and S3 with endogenous electron donors other than yD is about twice as fast for S2 as for S3 under these conditions. In contrast, the hydrazine-induced reductive shifts of the formal redox states Si (i = 0⋯3) are characterized by a totally different kinetic pattern: (a) at 1 mMNH2NH2 and incubation on ice the decay of S2 is estimated to be at least 25 times faster (t 1/2⩽0.4 min) than the corresponding reaction of S3 (t 1/2≈13 min); (b) the NH2NH2-induced decay of S2 is even slower (about twice) than the transformation of S1 into the formal redox state ‘S−1’ (t 1/2≈6 min), which gives rise to the two-digit phase shift of the oxygen-yield pattern induced by a flash train in dark adapted thylakoids. (c) the NH2NH2-induced transformation S0→‘S−2’ [Renger, Messinger and Hanssum (1990) in: Curr. Res. Photosynth. (Baltscheffsky, M., ed). Vol. 1, pp. 845-848, Kluwer, Dordrecht] is about three times faster (t 1/2≈ min) than the reaction. Based on these results, the following dependence on the redox state Si of the reactivity towards NH2NH2 is obtained: S3 \textless S1 \textless S0 ⪡ S2. The implications of this surprising order of reactivity are discussed.

@incollection{renger_thermodynamic_1990,
	address = {Dordrecht},
	title = {Thermodynamic, {Kinetic} and {Mechanistic} {Aspects} of {Photosynthetic} {Water} {Oxidation}},
	isbn = {978-94-009-0511-5},
	url = {https://doi.org/10.1007/978-94-009-0511-5_195},
	doi = {10.1007/978-94-009-0511-5_195},
	abstract = {Water cleavage into dioxygen and four protons requires the cooperation of four redox equivalents of sufficient oxidizing power. In photosynthesis the 1-electron oxidant is generated by electron ejection from the excited singlet state of a special chlorophyll a complex (P680) with pheophytin a (Pheo) as primary acceptor and the indispensible stabilization through Pheo− reoxidation by a specifically bound plastoquinone (QA). The oxidizing equivalents are transferred from P680+ via a tyrosine residue (Yz) into a manganese containing hole storage unit referred to as HSU(Mn). After accumulation of four holes by a sequential univalent storage process (described by S0→S1→S2→S3→S4) oxygen is evolved and HSU(Mn) returns to state S0 (for review see ref. 1,2).},
	language = {en},
	urldate = {2024-11-28},
	booktitle = {Current {Research} in {Photosynthesis}: {Proceedings} of the {VIIIth} {International} {Conference} on {Photosynthesis} {Stockholm}, {Sweden}, {August} 6–11, 1989},
	publisher = {Springer Netherlands},
	author = {Renger, G. and Messinger, J. and Hanssum, B.},
	editor = {Baltscheffsky, M.},
	year = {1990},
	pages = {845--848},
}

Water cleavage into dioxygen and four protons requires the cooperation of four redox equivalents of sufficient oxidizing power. In photosynthesis the 1-electron oxidant is generated by electron ejection from the excited singlet state of a special chlorophyll a complex (P680) with pheophytin a (Pheo) as primary acceptor and the indispensible stabilization through Pheo− reoxidation by a specifically bound plastoquinone (QA). The oxidizing equivalents are transferred from P680+ via a tyrosine residue (Yz) into a manganese containing hole storage unit referred to as HSU(Mn). After accumulation of four holes by a sequential univalent storage process (described by S0→S1→S2→S3→S4) oxygen is evolved and HSU(Mn) returns to state S0 (for review see ref. 1,2).

@article{renger_tribromotoluquinone_1989,
	title = {Tribromotoluquinone {Induced} {Modifications} of the {Oscillation} {Pattern} of {Oxygen} {Evolution} and of {Herbicide} {Binding} in {Thylakoids} and {PS} {II} {Membrane} {Fragments} from {Spinach}},
	volume = {44},
	copyright = {De Gruyter expressly reserves the right to use all content for commercial text and data mining within the meaning of Section 44b of the German Copyright Act.},
	issn = {1865-7125},
	url = {https://www.degruyter.com/document/doi/10.1515/znc-1989-5-614/html},
	doi = {10.1515/znc-1989-5-614},
	abstract = {In the present study the effect of TBTQ on PS II and its mutual interaction with DCMU was analyzed by measurements of the oxygen yield oscillation pattern and of DCMU binding. It was found: 1)TBTQ in its reduced form is able to induce the reduction of D ox which gives rise to an accelerated decay of S 2 and S 3 of the wateroxidizing complex. 2) Triton X-100 treatment used for isolation of PS II membrane fragments does not significantly affect the lateral mobility of p lastoquinone within the membrane. TBTQ bound to the thylakoid membrane does not enhance the electron pool capacity in PS II membrane fragments. 3) Preincubation of thylakoids with TBTQ diminishes the blockage of O2-evolution by DCMU significantly. In correspondence with previous findings [18, 19] the effect strongly depends on the order of addition of TBTQ and DCMU . 4) Excitation with a single saturating flash causes enhanced DCMU binding in TBTQ pretreated samples leading to the inhibition of flash induced oxygen evolution. The rate of the latter process depends on the DCMU concentration. 5) In thylakoids pretreated in the d ark with TBTQ the oxygen yield of the 3rd flash slowly declines as a function o f dark incubation time at constant DCMU concentration. Based on the above mentioned findings it is inferred that a mutual interaction between TBTQ and DCMU takes place at the PS II acceptor side. Two alternative mechanisms are discussed: i) TBTQ tightly (covalently?) bound at the Q B -site (or very close to it) is modified in its function by DCMU via structural effects (allosteric type), or ii) there occurs a TBTQ /DCMU exchange that is fast in the light and slow in the dark.},
	language = {en},
	number = {5-6},
	urldate = {2024-11-25},
	journal = {Zeitschrift für Naturforschung C},
	publisher = {De Gruyter},
	author = {Renger, G. and Messinger, J. and Fromme, R.},
	month = jun,
	year = {1989},
	keywords = {Binding Sites, Halogenated p-B enzoquinones, Oxygen Yield Oscillation, Photosystem II, Quinone/Herbicide Interaction},
	pages = {423--430},
}

In the present study the effect of TBTQ on PS II and its mutual interaction with DCMU was analyzed by measurements of the oxygen yield oscillation pattern and of DCMU binding. It was found: 1)TBTQ in its reduced form is able to induce the reduction of D ox which gives rise to an accelerated decay of S 2 and S 3 of the wateroxidizing complex. 2) Triton X-100 treatment used for isolation of PS II membrane fragments does not significantly affect the lateral mobility of p lastoquinone within the membrane. TBTQ bound to the thylakoid membrane does not enhance the electron pool capacity in PS II membrane fragments. 3) Preincubation of thylakoids with TBTQ diminishes the blockage of O2-evolution by DCMU significantly. In correspondence with previous findings [18, 19] the effect strongly depends on the order of addition of TBTQ and DCMU . 4) Excitation with a single saturating flash causes enhanced DCMU binding in TBTQ pretreated samples leading to the inhibition of flash induced oxygen evolution. The rate of the latter process depends on the DCMU concentration. 5) In thylakoids pretreated in the d ark with TBTQ the oxygen yield of the 3rd flash slowly declines as a function o f dark incubation time at constant DCMU concentration. Based on the above mentioned findings it is inferred that a mutual interaction between TBTQ and DCMU takes place at the PS II acceptor side. Two alternative mechanisms are discussed: i) TBTQ tightly (covalently?) bound at the Q B -site (or very close to it) is modified in its function by DCMU via structural effects (allosteric type), or ii) there occurs a TBTQ /DCMU exchange that is fast in the light and slow in the dark.