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 structur e and oxidation state of the water-oxidizing Mn complex of photosystem II h as been obtained for all four semi-stable intermediate states of its cataly tic cycle. We summarize our recent XAS results and discuss their mechanisti c implications. The following aspects are covered: (a) information content of X-ray spectra (pre-edge feature, edge position, extended X-ray absorptio n fine-structure (EXAFS), dichroism in the EXAFS of partially oriented samp les); (b) S-1-state structure; (c) X-ray edge results on oxidation state ch anges; (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 wor king 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 d ioxygen formation during the S-3-(S-4)-S-0 transition. According to this hy pothesis, water oxidation is facilitated by manganese reduction that is cou pled to proton transfer from a substrate water to bridging oxides. (C) 2001 Elsevier Science B.V. All rights reserved.