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

•Oxidation proceeds through the [Mn4O] structure at the S3 state.•Oxidation proceeds through the [Mn4OOH] structure at the S4 state.•[Mn4(OH)2] structure is ruled out at the S3 state.•[Mn1O, Mn4O5] structure is ruled out at the S4 state.

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.

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