Structure of the Mn complex in photosystem II: Insights from x-ray spectroscopy

We have used Mn K-edge absorption and Kb emission spectroscopies to determine the oxidation states of the Mn complex in the various S-states. We have started exploring the new technique of resonant inelastic X-ray scattering spectroscopy (RIXS); this technique can be characterized as a Raman process that uses K-edge energies (1s to 4p, ~6550 eV) to obtain L-edge-like spectra (2p to 3d, ~650 eV). The relevance of these data to the oxidation states and structure of the Mn complex is presented. We have obtained EXAFS data from the S0 and S3 states and observed heterogeneity in the Mn-Mn distances, leading us to conclude that there may be three rather than two di-(mu)-oxo bridged units present per tetranuclear Mn cluster. In addition, we have obtained data using Ca/Sr X-ray spectroscopy that provide evidence for a heteronuclear Mn/Ca cluster. The possibility of three di-(mu)-oxo-bridged Mn Mn moieties and the proximity of Ca is incorporated into developing structural models for the Mn cluster. The involvement of bridging and terminal O ligands of Mn in the mechanism of oxygen evolution is discussed in the context of our X-ray spectroscopy results