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 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 A and/or 3.2—3.5 A 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 A 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 tet...