Electronic Structure of the CaMn4O5 Cluster in the PSⅡ System Refined to the 1.9 (A) X-ray Resolution. Possible Mechanisms of Photosynthetic Water Splitting

摘要

Broken-symmetry (BS) UB3LYP calculations have been performed for the CaMn4O5 cluster (1) in the oxygenevolving complex (OEC) of the PSⅡ system refined to 1.9 A resolution by Umena, Kawakami, Kamiya, Shen to elucidate its electronic structure that is crucial for consideration of possible mechanisms of photosynthetic water splitting. Our UB3LYP computations have elucidated the position of protonated oxygen of the CaMn(Ⅲ)2Mn(Ⅳ)2O4(OH) cluster (1a) at the S1 stage of Kok cycle. Starting from the newly elucidated S1 structure of la, we have calculated the electronic structure of proton and electron released CaMn(Ⅳ)4O5 cluster (1b) that mimics the S4 stage of the cycle. The LUMOs of 1b are depicted for pictorial understanding of electrophilic oxygen sites that are responsible for nucleophilic attack of hydroxide anion (or water) for the O-O bond formation. Implications of present computational results are discussed in relation to possible mechanisms of photosynthetic water splitting.