Mechanism and energy diagram for O-O bond formation in the oxygen-evolving complex in photosystem II

The recent finding of a transition state with a significantly lower barrier than previously found, has made the mechanism for O-O bond formation in photosystem II much clearer. The full mechanism can be described in the following way. Electrons and protons are ejected from the oxygen-evolving complex (OEC) in an alternating fashion, avoiding unnecessary build-up of charge. The S-0-S-1 and S-1-S-2 transitions are quite exergonic, while the S-2-S-3 transition is only weakly exergonic. The strong endergonic S-3-S-4 transition is a key step in the mechanism in which an oxygen radical is produced, held by the dangling manganese outside the Mn3Ca cube. The O-O bond formation in the S-4-state occurs by an attack of the oxygen radical on a bridging oxo ligand in the cube. The mechanism explains the presence of both a cube with bridging oxo ligands and a dangling manganese. Optimal orbital overlap puts further constraints on the structure of the OEC. An alternating spin alignment is necessary for a low barrier. The computed rate-limiting barrier of 14.7 kcal mol(-1) is in good agreement with experiments.