Water oxidation chemistry of photosystem II

Abstract

Photosystem II (PSII) uses light energy to split water into protons, electrons and oxygen. In this reaction, Nature has solved the difficult chemical problem of efficient four‐electron oxidation of water to yield O2 without significant side reactions. In order to use Nature's solution for the design of materials that split water for solar fuel production, it is important to understand the mechanism of the reaction. The recent 1.9 Å X‐ray crystal structure of cyanobacterial PSII provides information on the structure of the Mn and Ca ions, the redox‐active tyrosine called tyrosine‐Z, chloride and the surrounding amino acids that comprise the oxygen‐evolving complex (OEC). However, reduction of the high‐valent Mn ions in the OEC during X‐ray exposure makes the redox state of the OEC represented in the crystal structure uncertain. Computational modeling has been used to refine the structure of the OEC in specific redox states by using spectroscopic data as a benchmark. In addition, EPR and ENDOR spectroscopy has been used to probe the binding of the substrate waters to Mn in the OEC by using 17O‐labeled water. The structure of the OEC in the intermediate oxidation states of the catalytic cycle, the binding of substrate water molecules to the OEC and the water oxidation chemistry of PSII will be discussed in the light of biophysical and computational studies, inorganic chemistry and X‐ray crystallographic information.