Theoretical studies of O-O bond formation in photosystem II.

The most critical part of dioxygen evolution in photosystem II is the O-O bond formation step. In order to reach an efficient mechanism, nature uses a unique oxygen-evolving complex (OEC) having four manganese and one calcium center. Even though the structure of the OEC has become much more clear during recent years, it has still been difficult to find a transition state (TS) for O-O bond formation with a sufficiently low barrier. However, about a year ago, a quite surprising type of TS was found. With the latest X-ray ligand assignment, the local barrier for this TS is only 5.1 kcal/mol. It can be described as an attack by an oxygen radical, held by a dangling manganese, on a bridging oxo ligand in the Mn3Ca cube. In the present short Article, energy diagrams describing the entire process of dioxygen formation will be presented. An important conclusion drawn from these diagrams is that the major features of dioxygen formation remain the same irrespective of which one of the experimentally suggested structures the diagram is built on. Compared to earlier presentations of the same type, a slightly different approach has been used for setting up the diagrams. Results from a recent experimental study of the pressure dependence of oxygen release have been used to define the final energy levels. The loss of energy in the electron transfer from Tyrz to P680 has also been incorporated into the diagrams. A good agreement with experimental observations is demonstrated.