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, enzymes and synthetic analog compounds is being developed. This model of 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)] clusters. Analytical expressions for the exchange variations of the effective hyperfine constants A(i) are derived. Variations of J(ij) transform 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 small 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 superhyperfine interactions. The exchange mixing model for the S(gr) = 3/2 [4Fe-4S]+ clusters explains the existence of the only negative A(i) constants observed in experiments on the tetrameric iron-sulfur systems with S(gr) = 3/2. The magnetic behavior of the [Fe4S4(SR)4] compounds is described by the exchange model. The four different effective hyperfine constants of the active center of aconitase are explained in the model with complex distortions.