SPIN COUPLING MODEL FOR TETRAMERIC IRON CLUSTERS IN FERREDOXINS .1. THEORY, EXCHANGE LEVELS, G-FACTORS

A Heisenberg exchange model of the [4Fe-4S]+ clusters of ferredoxins, enzymes and of synthetic analog compounds is developed. The proposed a pproach gives the generalized solution of the exchange problem which l eads to the results of different Kambe models in particular cases. The exchange mixing of the states with fixed intermediate spins determine s the splittings and transformation of levels, the features of EPR spe ctra of these systems with [Fe3(II)Fe(III)] clusters. The problem of c alculation of all types (diagonal and non-diagonal) matrix elements of one- and two-particle (exchange) spin operators was solved on the bas e of the technique of the irreducible tensor operators. The analytical expressions for the exchange variations of the factors K(i)=[s[iz]/[S (Z)], which form the relationship g(mol) = SIGMA(i=1)4 K(i)g(i) betwee n molecular and local g-factors, were derived. Exchange variations exp lain the values and the change of the molecular g-factors. The charact eristic ''g(av) < 2'' values g(mol) = 1.94-1.98 of the [4Fe-4S]+ clust ers of the majority of ferredoxins, enzymes and of synthetic analog co mpounds with S(gr) = 1/2 are explained by strong delta1/DELTA ''distor tions''. The small deformations lead to g(av) > 2.