Alternative mechanisms for O2 release and O–O bond formation in the oxygen evolving complex of photosystem II

Abstract

In a previous detailed study of all the steps of water oxidation in photosystem II, it was surprisingly found that O₂ release is as critical for the rate as O–O bond formation. A new mechanism for O₂ release has now been found, which can be described as an opening followed by a closing of the interior of the oxygen evolving complex. A transition state for peroxide rotation forming a superoxide radical, missed in the previous study, and a structural change around the outside manganese are two key steps in the new mechanism. However, O₂ release may still remain rate-limiting. Additionally, for the step forming the O–O bond, an alternative, experimentally suggested, mechanism was investigated. The new model calculations can rule out the precise use of that mechanism. However, a variant with a rotation of the ligands around the outer manganese by about 30° will give a low barrier, competitive with the old DFT mechanism. Both these mechanisms use an oxyl–oxo mechanism for O–O bond formation involving the same two manganese atoms and the central oxo group (O5).