High-Resolution XANES Study of Molecular Structure of the Manganese Clusters in S_1 and S_2 States in Photosynthetic O_2-Evolving Complex

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Highly-resolved low-background XANES (X-ray Absorption Near Edge Structure) spectra of Mn in an oxygen-evolving complex (OEC) in S_1 and S_2 states were measured by use of a solid state detector to record mainly the Mn fluorescence alone under the stable operation of synchrotron orbital radiation. The raw Mn K-edge spectra characterized by extremely high signal-to-noise ratio (S/N = 80) and very low background-to-signal ratio (B/S = 0.15 - 0.18) revealed that a number of superposed principal absorption bands (corresponding to electronic transitions from 1 s core to various antibonding 4 p* states) can be resolved into four bands in S_1 state and five bands in S_2 state. Comparison of the peak positions of these bands with those observed for some related model Mn complexes (especially involving ligands like carboxylate, H_2O, and imidazolate) is assumed to establish the assignment to a unique L1-Mn-L2 axial bond or to some possible ones, along which the 4 p* state wave function extends. Correct normalization of the spectra by a new integral-equalizing method following careful removal of backgound counts, was accomplished. This yielded, from the light-minus-dark spectrum, information about the total number of Mn ions in an OEC, the change of the Mn oxidation states and the quantity of each K-edge shift. Further, a comparative study of pre-edge absorption bands due to the ls to antibonding 3 d* transition revealed that there is only one rhombic di-μ_2-oxo bridge in the Mn cluster and a third bridge ligand which can break the inversion symmetry of the 3 d* orbital as much as a μ_2-oxo bridge can do must exist. All the results obtained were found to lead to the best model for the molecular structure of the Mn cluster which are consistent with both EPR and EXAFS data. That is, in S_1 state, the Mn cluster consists of a di-μ_2-oxo bridge Mn(III, III) dimeric subunit and a pair of Mn(III) monomeric subunits being mutually connected by three flexible di-μ_2-carboxylato bridges. These flexible bridges can give rise to either ferromagnetic or antiferromagnetic exchange couplings whose intensities can sensitively vary by a conformational change of the surrounding proteinaceous medium. Two histidine residues are also involved as in-plane ligands for the Mn(III, III) with C_2 or C_s symmetry to form a "catalytic microsurface" on which a deprotonated water dimer, (HOHOH)^-, is bound as mono-μ_2-anionic water bridge. Further, two carboxylates are involved as a chelator for each monomeric Mn(III) ion, which is evidently in a resonance state of electron-shuttling between O ligands. Hence, the whole structure of this complex has either C_2 or C_s symmetry; while, in S_2 state, only the "catalytic" Mn(III, III) dimeric subunit has been oxidized to Mn(III, IV). At the same time, the resonant electron-shuttling in a "cocatalytic" Mn(III) monomeric subunit (perhaps the one directly bridged with Mn(IV)) appears to have evidently stopped upon S_1 to S_2 transition. The magnetic property of this S_2-state model can be quite inhomogeneous because of flexible three di-μ_2-carboxylato bridges connecting two monomeric Mn ions, which can give rise to a sample-dependent distribution of the energy levels of the s = 1/2 and s = 3/2 spin states. Experimental data on the multiline and g = 4.1 EPR signals are discussed along this line.
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High-Resolution XANES Study of Molecular Structure of the Manganese Clusters in S_1 and S_2 States in Photosynthetic O_2-Evolving Complex

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