Theory of chemical bonds in metalloenzymes VI: Manganese-oxo bonds in the photosynthesis II system

Electronic and spin structures of high-valent manganese-oxo bonds in the photosynthesis II system (oxygen evolving center, OEC) are investigated by the use of spin polarized hybrid DFT (HDFT) method. Theoretical calculations of a high-valent manganese-oxo porphyrin complex are also performed to elucidate common characteristic of the active Mn{double bond, long}O bonds in both native OEC and artificial systems. The oxygen site of the high-valent Mn{double bond, long}O is found to be electrophilic in nature, in accord with our previous work, where the SE₂, ¹O- and ³O-models have been presented for theoretical understanding of complex behaviors of oxygenation reactions by metal-oxo species. The ¹O- and ³O-models are applicable to model complexes examined here, since the manganese-oxo bonds exhibit strong biradical character. Possibility of the SE₂-like transition structure model for OEC is also discussed on both the theoretical and experimental grounds. Implications of present computational results are discussed in relation to hydroxylation reaction by MMO and P450.