SPIN COUPLING MODEL FOR TETRAMERIC IRON CLUSTERS IN FERREDOXINS .2. HYPERFINE INTERACTIONS, MAGNETISM, HIGH-SPIN SYSTEMS

A Heisenberg model of the [4Fe-4S]+ clusters of reduced ferredoxins, e nzymes and synthetic analog compounds is being developed. This model o f the exchange mixing of the states with different fixed intermediate spins explains the magnetic properties and the features of EPR, ENDOR, and Mossbauer spectra of these systems containing [Fe3(II)Fe(III)] cl usters. Analytical expressions for the exchange variations of the effe ctive hyperfine constants A(i) are derived. Variations of J(ij) transf orm the values and the signs of all effective hyperfine constants. The A(i) constants of Fe3+ and Fe2+ ions become equal under the action of exchange variations. The strong delta1/DELTA ''deformations'' of S(gr ) = 1/2 clusters determine the two types of A(i) of the opposite sips and g(mol) < 2, which are observed usually. Weak deformations form sma ll A(i) constants and g(mol) > 2. Exchange interactions determine the positions and distributions of hyperfine lines in the EPR spectra, the frequencies of ENDOR transitions, and also the magnitudes of superhyp erfine interactions. The exchange mixing model for the S(gr) = 3/2 [4F e-4S]+ clusters explains the existence of the only negative A(i) const ants observed in experiments on the tetrameric iron-sulfur systems wit h S(gr) = 3/2. The magnetic behavior of the [Fe4S4(SR)4] compounds is described by the exchange model. The four different effective hyperfin e constants of the active center of aconitase are explained in the mod el with complex distortions.