Mn K-Edge XANES Spectroscopy of a Photosynthetic O2-Evolving Complex. High-Quality Pre-Edge Features and Distinct Fine Structures in the S1- and S2-States1

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Abstract
High-resolution XANES (X-ray Absorption Near Edge Structure) spectroscopy for Mn in the S1 and S2 states of the spinach photosynthetic O2-evolving complex revealed distinct features in K-edge spectra, when a high signal-to-noise (S/N) ratio of ca. 80 with a low and constant background-to-signal (B/S) ratio of 0.15 to 0.18 was attained. Six features resolved in each S-state spectrum involve a pre-edge feature due to 1s→3d transitions, a main-edge feature possibly due to 1s→4s transitions and four fine structures superimposed on the principal absorption bands due to 1s→4p* transitions. The high-quality pre-edge features were analyzed according to a parametric ligand-field theory in comparison with those of some typical authentic Mn complexes. It was deduced that i) all of the four Mn ions in the S1 -state are octahedrally coordinated and two of them constitute a di-μ-oxo bridged Mn(III, III) dimeric subunit; ii) the bridged Mn(III) ions are further bridged by a deprotonated water dimer, (HOHOH)−, and coordinated by imidazole-N and carboxylate-O− on the opposite side of the Mn atom from the di-μ-oxo bridge; iii) the other two Mn ions exist in the form of Mn(III) monomeric subunits; and iv) upon the S1→S2 transition, only the bridged Mn(III,III) is oxidized to Mn(III,IV). The distinct change in the principal absorption band shape upon the S1→S1 transition is briefly discussed to obtain the XANES evidence for a tetrameric Mn-cluster.

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