The tetra-manganese complex of photosystem II during its redox cycle - X-ray absorption results and mechanistic implications.

Using X-ray absorption spectroscopy (XAS), relevant information on structure and oxidation state of the water-oxidizing Mn complex of photosystem II has been obtained for all four semi-stable intermediate states of its catalytic cycle. We summarize our recent XAS results and discuss their mechanistic implications. The following aspects are covered: (a) information content of X-ray spectra (pre-edge feature, edge position, extended X-ray absorption fine-structure (EXAFS), dichroism in the EXAFS of partially oriented samples); (b) S(1)-state structure; (c) X-ray edge results on oxidation state changes; (d) EXAFS results on structural changes during the S-state cycle; (e) a structural model for the Mn complex in its S(3)-state; (f) XAS-based working model for the S(2)-S(3) transition; (g) XAS-based working model for the S(0)-S(1) transition; (h) potential role of hydrogen atom abstraction by the Mn complex. Finally, we present a specific hypothesis on the mechanism of dioxygen formation during the S(3)-(S(4))-S(0) transition. According to this hypothesis, water oxidation is facilitated by manganese reduction that is coupled to proton transfer from a substrate water to bridging oxides.