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 S-2 to S-3 and S-4 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 S-4 state of the path 1 is remarkably stable (16.1 kcal/mol) rather than the path 2. [Mn4-O(W2)] (S-3) and [Mn4-OOH(W2,W6)] (S-4) structures are recommended for the S-2 -> S-3 -> S-4 transitions. (C) 2014 Elsevier B.V. All rights reserved.