Theoretical Study on the Mechanism of Dioxygen Evolution in Photosystem II. I. Molecular and Electronic Structures at the S-0, S-1, and S-2 States of Oxygen-Evolving Complex

摘要

Based on the structure at 1.9 angstrom resolved OEC (PDBid=3ARC), geometries and electronic structures of the S-0, S-1, and S-2 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 S-0 state are Mn-4[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 S-1 state, the oxidation states are Mn-4[III, IV, IV, III]. The hydrogen-bonding pattern of the lowest state is similar to that of the higher energy state of the S-0 state. The H2O molecule coordinated to the Mn-4 atom loses a proton to yield an OH- anion. At the S-2 state, the oxidation states are Mn-4[III, IV, IV, IV]. For S-0 -> S-1 -> S-2 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.