Full geometry optimizations of several inorganic model clusters, CaMn 4O4XYZ(H2O)2 (X, Y, Z = H 2O, OH- or O2-), by the use of the B3LYP hybrid density functional theory (DFT) have been performed to illuminate plausible molecular structures of the catalytic site for water oxidation in the S 0, S1, S2 and S3 states of the Kok cycle for the oxygen-evolving complex (OEC) of photosystem II (PSII). Optimized geometries obtained by the energy gradient method have revealed the degree of symmetry breaking of the unstable three-center Mna-X-Mnd bond in CaMn4O4XYZ(H2O)2. The right-elongated (R) Mna-X⋯Mnd and left-elongated (L) Mna⋯X-Mnd structures appear to occupy local minima on a double-well potential for several key intermediates in these states. The effects of insertion of one extra water molecule to the vacant coordination site, Mnd (Mna), for R (L) structures have also been examined in detail. The greater stability of the L-type structure over the R-type has been concluded for key intermediates in the S2 and S 3 states. Implications of the present DFT structures are discussed in relation to previous DFT and related results, together with recent X-ray diffraction results for model compounds of cubane-like OEC cluster of PSII.
ASJC Scopus subject areas
- Inorganic Chemistry