Theoretical illumination of water-inserted structures of the CaMn4O5 cluster in the S2 and S3 states of oxygen-evolving complex of photosystem II: full geometry optimizations by B3LYP hybrid density functional

Abstract

Full geometry optimizations of several inorganic model clusters, CaMn₄O₄XYZ(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 S₀, 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 S₃ 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.