Metalloproteins containing Mn in a redox active role are involved in a variety of physiologically important reactions involving dioxygen metabolism. These include, amongst others, a superoxide dismutase that detoxifies superoxide radicals to O(2) and peroxide, a catalase that disproportionates peroxide to O(2) and H(2)O, a ribonucleotide reductase that catalyzes synthesis of deoxyribonucleotide, and perhaps the most complex and important, the Mn-containing oxygen-evolving complex (Mn-OEC) that is involved in the oxidation of water of dioxygen in Photosystem II. Oxygen, that supports all aerobic life, is abundant in the atmosphere because of its constant regeneration by photosynthetic water oxidation by the Mn-OEC. The light-induced oxidation of water to dioxygen is one of the most important chemical processes occurring on such a large scale in the biosphere. Photosynthetic water oxidation involves removal of four electrons, in a stepwise manner by light-induced oxidation, from two water molecules by the Mn-OEC to produce a molecule of oxygen. Central questions that need to be resolved and the overall objective of this proposal are as follows: 1) Determine the oxidation states of Mn in the four intermediate or S-states, 2) Characterize the structural changes of the oxo-bridged tetranuclear Mn complex as it advances through the enzymatic cycle, 3) Determine the mechanism of water oxidation and oxygen evolution, 4) Elucidate the structural and functional role of the cofactors C1(-) and Ca(2+). The interplay between XAS and EPR has played an essential role in our understanding of the structural and mechanistic aspects of O(2) evolution. The oxidation states of Mn and the structural changes of the Mn complex as it advances through the enzymatic cycle are determined by X-ray absorption spectroscopy using samples characterized by EPR spectroscopy. The oxidation states are determined by Mn K- and L-edge spectroscopy. The structure of the intermediates are characterized by XAS using samples prepared by flash illumination. The role of cofactors Ca(2+)(Sr(2+) and C1(-) (Br-) is addressed by Ca(Sr) and Cl (Br) K-edge XAS respectively.
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