Theoretical Study on the Mechanism of Dioxygen Evolution in Photosystem II. I. Molecular and Electronic Structures at the S0, S1, and S2 States of Oxygen-Evolving Complex

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Based on the structure at 1.9 Å resolved OEC (PDBid=3ARC), geometries and electronic structures of the S0, S1, and S2 states in the Kok cycle have been theoretically investigated by the broken-symmetry B3LYP method. The exchange coupling constants of spins in three states were also estimated. The oxidation states of four manganese atoms at the S0 state are Mn4[III, III, IV, III]. It was found that the hydrogen-bonding patterns of water molecules are influenced by the electronic structure of the CaMn4O5 core. At the S1 state, the oxidation states are Mn4[III, IV, IV, III]. The hydrogen-bonding pattern of the lowest state is similar to that of the higher energy state of the S0 state. The H2O molecule coordinated to the Mn4 atom loses a proton to yield an OH− anion. At the S2 state, the oxidation states are Mn4[III, IV, IV, IV]. For S0 → S1 → S2 states, the released electrons have the same direction of spins. Only Mn atoms are oxidized not the substrate (H2O). Our theoretical results are consistent with the observed results.