K. Hasegawa et al., How to evaluate the structure of a tetranuclear Mn cluster from magnetic and EXAFS data: Case of the S-2-state Mn-cluster in photosystem II, B CHEM S J, 72(5), 1999, pp. 1013-1023
A theoretical method to extract structural information on spin-exchange-cou
pled 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 tet
ramer in the photosynthetic oxygen-evolving complex, which exhibits a "g =
2 multiline" EPR signal. Based on the EXAFS data and the manganese chemistr
y, 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 s
trong antiferromagnetic interaction due to a 2.7 Angstrom Mn(III)-Mn(IV) bo
nd and the other weak ones. By computer-search of the possible spin-exchang
e structures with respect to these two parametric interactions, it was foun
d that (1) a dimer of di-mu(2)-oxo bridged Mn dimers, a propeller-type tetr
amer in which the central Mn ion is chelated by three di-mu-oxo Mn cores an
d 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 st
ates are higher than the first excited state with S* = 3/2 (majority) or S*
= 1/2 (minority).