Structural change of the Mn cluster during the S2 {yields} S3 state transition of the oxygen-evolving complex of Photosystem II. Does it reflect the onset of water/substrate oxidation? Determination by Mn X-ray absorption spectroscopy
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-mu-oxo bridged binuclear Mn units with Mn-Mn distances of {approximately}2.7 Angstrom that are linked to each other by a mono-mu-oxo bridge with a Mn-Mn separation of {approximately}3.3 Angstrom. The Mn-Mn distances are invariant in the 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 Angstrom Mn-Mn distances that characterize the di-mu-oxo centers are both lengthened to {approximately}2.8 and 3.0 Angstrom. The 3.3 Angstrom Mn-Mn and Mn-Ca distance also increases by 0.04-0.2 Angstrom. 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 to S1 and S1 to S2 transitions. During the S2 to S3 transition, we propose that the changes in Mn-Mn distances are the consequence of ligand or water oxidation. Models that can account for such changes and the implications for the mechanism of water oxidation are discussed.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director, Office of Science. Office of Basic Energy Studies. Division of Energy Biosciences; National Institutes of Health (US)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 771800
- Report Number(s):
- LBNL-45297; JACSAT; R&D Project: 448150; TRN: AH200104%%128
- Journal Information:
- Journal of the American Chemical Society, Vol. 122, Issue 14; Other Information: PBD: 15 Jul 1999; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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