Most of the oxygen in the atmosphere which supports aerobic life on earth is generated by plants and cyanobacteria by the photo-induced oxidation of water to dioxygen. The oxygen-evolving complex (OEC), of the photosynthetic apparatus that catalyzes this reaction contains a cluster of four Mn atoms. Water oxidation in Photosystem II (PSII) 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. 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. Mn X-ray spectroscopy studies provide direct information on the microstructure of the Mn 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 all of the S-states. The objective of this proposal is to study the structure of the Mn complex in the various S-states, and understand the mechanism by which water is oxidized to dioxygen at the metal site.