This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Most of the oxygen in the atmosphere which supports aerobic life on earth is generated by plants and cyanobacteria by the photoinduced oxidation of water to dioxygen. This is one of the most important chemical processes occurring on such a large scale in the biosphere. The oxygen-evolving complex (OEC), of the photosynthetic apparatus that catalyzes this reaction contains a cluster of four Mn atoms. Water oxidation in photosystemII (PS II) is a stepwise process wherein each of 4 sequential photons absorbed by the reaction center powers the advance of the OEC through the S-state intermediates S0-S4. Upon reaching the hypothesized S4 state, the complex releases O2 and returns to S0. The critical questions related to this process are the oxidation state and structural changes in the Mn complex as the OEC proceeds through the S-state cycle, and the mechanism by which four electrons are removed from two water molecules by the Mn complex to produce an O2 molecule. It is generally accepted that the Mn cluster is the active site for water oxidation, but the chemical mechanism by which this is achieved is inadequately understood. In addition to Mn, Cl- and Ca2+ are essential cofactors that are required for activity, but their exact structural and functional role is not yet clear. The structure and chemistry of the Mn complex and the partially oxidized intermediates of the donor complex, the S-states of the Mn-OEC(S0-S3),have been the subject of intense interest and study. Mn X-ray spectroscopy studies provide direct information on the microstructure of the complex and on the oxidation states of the Mn atoms in the S0, S1, S2 and S3 states of the OEC. Few other spectroscopic techniques provide such specificity for studying the structure of Mn in the OEC, and at present this is the only method suitable to monitor Mn in the S-states.
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