Theoretical illumination of water-inserted structures of the CaMn4O5 cluster in the S-2 and S-3 states of oxygen-evolving complex of photosystem II: full geometry optimizations by B3LYP hybrid density functional

Full geometry optimizations of several inorganic model clusters, CaMn(4)O(4)XYZ(H2O)(2) (X, Y, Z = H2O, 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, S-1, S-2 and S-3 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 Mn-a-X-Mn-d bond in CaMn(4)O(4)XYZ(H2O)(2). The right-elongated (R) Mn-a-X center dot center dot center dot Mn-d and left-elongated (L) Mn-a center dot center dot center dot X-Mn-d 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, Mn-d (Mn-a), 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 S-2 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.