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Simulation of the isotropic EXAFS spectra for the S2 and S3 structures of the oxygen evolving complex in photosystem II.

ABSTRACT: Most of the main features of water oxidation in photosystem II are now well understood, including the mechanism for O-O bond formation. For the intermediate S2 and S3 structures there is also nearly complete agreement between quantum chemical modeling and experiments. Given the present high degree of consensus for these structures, it is of high interest to go back to previous suggestions concerning what happens in the S2-S3 transition. Analyses of extended X-ray adsorption fine structure (EXAFS) experiments have indicated relatively large structural changes in this transition, with changes of distances sometimes larger than 0.3 Å and a change of topology. In contrast, our previous density functional theory (DFT)(B3LYP) calculations on a cluster model showed very small changes, less than 0.1 Å. It is here found that the DFT structures are also consistent with the EXAFS spectra for the S2 and S3 states within normal errors of DFT. The analysis suggests that there are severe problems in interpreting EXAFS spectra for these complicated systems.

SUBMITTER: Li X

PROVIDER: S-EPMC4386341 | biostudies-literature | 2015 Mar

REPOSITORIES: biostudies-literature

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Simulation of the isotropic EXAFS spectra for the S2 and S3 structures of the oxygen evolving complex in photosystem II.

Li Xichen X Siegbahn Per E M PE Ryde Ulf U

Proceedings of the National Academy of Sciences of the United States of America 20150316 13

Most of the main features of water oxidation in photosystem II are now well understood, including the mechanism for O-O bond formation. For the intermediate S2 and S3 structures there is also nearly complete agreement between quantum chemical modeling and experiments. Given the present high degree of consensus for these structures, it is of high interest to go back to previous suggestions concerning what happens in the S2-S3 transition. Analyses of extended X-ray adsorption fine structure (EXAFS ...[more]

PMID: 25775575

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Project description:The oxygen-evolving complex of Photosystem II in plants and cyanobacteria catalyzes the oxidation of two water molecules to one molecule of dioxygen. A tetranuclear Mn complex is believed to cycle through five intermediate states (S0-S4) to couple the four-electron oxidation of water with the one-electron photochemistry occurring at the Photosystem II reaction center. We have used X-ray absorption spectroscopy to study the local structure of the Mn complex and have proposed a model for it, based on studies of the Mn K-edges and the extended X-ray absorption fine structure of the S1 and S2 states. The proposed model consists of two di-?-oxo-bridged binuclear Mn units with Mn-Mn distances of ~2.7 Å that are linked to each other by a mono-?-oxo bridge with a Mn-Mn separation of ~3.3 Å. The Mn-Mn distances are invariant in the native S1 and S2 states. This report describes the application of X-ray absorption spectroscopy to S3 samples created under physiological conditions with saturating flash illumination. Significant changes are observed in the Mn-Mn distances in the S3 state compared to the S1 and the S2 states. The two 2.7 Å Mn-Mn distances that characterize the di-?-oxo centers in the S1 and S2 states are lengthened to ~2.8 and 3.0 Å in the S3 state, respectively. The 3.3 Å Mn-Mn and Mn-Ca distances also increase by 0.04-0.2 Å. These changes in Mn-Mn distances are interpreted as consequences of the onset of substrate/water oxidation in the S3 state. Mn-centered oxidation is evident during the S0?S1 and S1?S2 transitions. We propose that the changes in Mn-Mn distances during the S2?S3 transition are the result of ligand or water oxidation, leading to the formation of an oxyl radical intermediate formed at a bridging or terminal position. The reaction of the oxyl radical with OH(-), H2O, or an oxo group during the subsequent S state conversion is proposed to lead to the formation of the O-O bond. Models that can account for changes in the Mn-Mn distances in the S3 state and the implications for the mechanism of water oxidation are discussed.

Dataset Information

Simulation of the isotropic EXAFS spectra for the S2 and S3 structures of the oxygen evolving complex in photosystem II.

Publications

Simulation of the isotropic EXAFS spectra for the S2 and S3 structures of the oxygen evolving complex in photosystem II.

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