Theoretical and computational investigations of geometrical, electronic and spin structures of the CaMn O (X = 5, 6) cluster in the Kok cycle S (i = 0-3) of oxygen evolving complex of photosystem II.

The optimized geometries of the CaMn O (X = 5, 6) cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) by large-scale quantum mechanics (QM) and molecular mechanics (MM) calculations are compared with recent serial femtosecond crystallography (SFX) results for the S (i = 0-3) states. The valence states of four Mn ions by the QM/MM calculations are also examined in relation to the experimental results by the X-ray emission spectroscopy (XES) for the S intermediates. Geometrical and valence structures of right-opened Mn-hydroxide, Mn-oxo and Mn-peroxide intermediates in the S state are investigated in detail in relation to recent SFX and XES experiments for the S state. Interplay between theory and experiment indicates that the Mn-oxo intermediate is a new possible candidate for the S state. Implications of the computational results are discussed in relation to possible mechanisms of the oxygenoxygen bond formation for water oxidation in OEC of PSII.