Rationalising the geometric variation between the a and b monomers in the 1.9 å crystal structure of photosystem II

Density functional theory calculations are reported on a set of models of the water-oxidising complex (WOC) of photosystem II (PSII), exploring structural features revealed in the most recent (1.9 Å resolution) X-ray crystallographic studies of PSII. Crucially, we find that the variation in the Mn-Mn distances seen between the A and B monomers of this crystal structure can be entirely accounted for, in the low oxidation state (LOS) paradigm, by consideration of the interplay between two hydrogen-bonding interactions involving proximate amino acid residues with the oxo bridges of the WOC, that is, His337 with O3 (which leads to a general elongation in the Mn-Mn distances between Mn1, Mn2 and Mn3) and Arg357 with O2 (which results in a specific elongation of the Mn2-Mn3 distance).