Copyright © 2003 BioChem Press Density Functional Study on the Highest and Lowest Spin

Motivation. Water oxidation mechanism of oxygen-evolving complex (OEC) in photosystem II (PSII) has not been established yet. Recently, the X-ray structure of PSII has been reported and it has been suggested that OEC contains cubane-like Mn3CaO4 cluster linked to Mn by mono-µ-oxo bridge. The Mn–Mn and Mn–Ca in cubane-like cluster are connected by di-µ-oxo bridge to form a face of cubane-like structure. As the first step to examine the mechanism of dioxygen generation by OEC, the geometrical and electronic structures of model complexes [Mn2O2(H2O)8]q+ (q = 0, 2, 4) are investigated for the highest and lowest spin configurations from theoretical viewpoint in this work. Method. Model complexes were constructed by placing eight H2O molecules as ligands around Mn–Mn core with di-µ-oxo bridges to form six-coordinated Mn sites. Full geometry optimizations were carried out by the hybrid-type DFT method with B3LYP functional. For Mn, the LanL2DZ basis set with double-zeta quality was used and the effective core potential was applied. The unrestricted and broken-symmetry wave functions were obtained for the highest and lowest spin states, respectively. Results. The coordinated positions of H2O molecules in Mn(II)–Mn(II) complex are different from those in Mn(III)–Mn(III) or Mn(IV)–Mn(IV) complex, while Mn(III)–Mn(III) and Mn(IV)–Mn(IV) complexes hav