How to evaluate the structure of a tetranuclear Mn cluster from magnetic and EXAFS data:: Case of the S2-state Mn-cluster in photosystem II

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 S-2-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 S-2-state Mn cluster will be presented, such that two spin-exchange interactions due to 2.7-2.8 Angstrom and/or 3.2-3.5 Angstrom Mn-Mn bonds can vary from weal; to strong coupling, sensitively, depending upon the bridge structure, except for a strong antiferromagnetic interaction due to a 2.7 Angstrom 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-mu(2)-oxo bridged Mn dimers, a propeller-type tetramer in which the central Mn ion is chelated by three di-mu-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).