Theoretical study on mechanism of dioxygen evolution in photosystem II. II. Molecular and electronic structures at the S3 and S4 states of oxygen-evolving complex

Two reaction paths from S2 to S3 and S4 states of OEC have been studied by the hybrid density functional method. The path 1 where the added H2O molecule is irrelevant to the substrate of oxidation and the path 2 where the added H2O molecule is relevant to the substrate have been investigated. The S3 states of the paths 1 and 2 are isoenergetic, while the S4 state of the path 1 is remarkably stable (16.1 kcal/mol) rather than the path 2. [Mn4O(W2)] (S3) and [Mn4OOH(W2,W6)] (S4) structures are recommended for the S2 → S3 → S4 transitions.