Water Oxidation for Simplified Models of the Oxygen-Evolving Complex in Photosystem II

For the main parts of the mechanism for water oxidation in photosystem II there has recently been very strong experimental support for the mechanism suggested by theoretical model studies. The question addressed in the present study is to what extent this knowledge can be used for the design of artificial catalysts. A major requirement for a useful artificial catalyst is that it is small enough to be synthesized. Small catalysts also have the big advantage that they could improve the catalysis per surface area. To make the mechanism found for PSII useful in this context, it needs to be analyzed in detail. A small model system was therefore used and the ligands were replaced one by one by water-derived ligands. Only the main chemical step of O-O bond formation was investigated in this initial study. The energetics for this small model and the larger one previously used for PSII are remarkably similar, which is the most important result of the present study. This shows that small model complexes have a potential for being very good water oxidation catalysts. It was furthermore found that there is a clear correlation between the barrier height for O-O bond formation and the type of optimal structure for the S-3 state. The analysis shows that a flexible central part of the complex is the key for efficient water oxidation.