Theoretical illumination of water-inserted structures of the CaMn ₄O₅ cluster in the S₂ and S₃ 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 ₄O₄XYZ(H₂O)₂ (X, Y, Z = H ₂O, OH^- or O^2-), 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 Mn_a-X-Mn_d bond in CaMn₄O₄XYZ(H₂O)₂. The right-elongated (R) Mn_a-X⋯Mn_d and left-elongated (L) Mn_a⋯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 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.