Abstract
Abstract A theoretical method to extract structural information on spin-exchange-coupled manganese tetramers from the EPR spectroscopy data is presented. This method has been applied to two EPR data, i.e. the ground-state spin (S = 1/2) and the first excitation energy (30—37 cm−1), for the S2-state Mn tetramer in the photosynthetic oxygen-evolving complex, which exhibits a “g = 2 multiline” EPR signal. Based on the EXAFS data and the manganese chemistry, a simplified model spin Hamiltonian to describe the S2-state Mn cluster will be presented, such that two spin-exchange interactions due to 2.7—2.8 Å and/or 3.2—3.5 Å Mn–Mn bonds can vary from weak to strong coupling, sensitively, depending upon the bridge structure, except for a strong-antiferromagnetic interaction due to a 2.7 Å Mn(III)–Mn(IV) bond and the other weak ones. By computer-search of the possible spin-exchange structures with respect to these two parametric interactions, it was found that (1) a dimer of di-μ2-oxo bridged Mn dimers, a propeller-type tetramer in which the central Mn ion is chelated by three di-μ-oxo Mn cores and some other models are highly unlikely, (2) the most promising cluster is a trimer-plus-monomer type of distorted cubane, and (3) S* = 5/2 excited states are higher than the first excited state with S* = 3/2 (majority) or S* = 1/2 (minority).
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