HEISENBERG EXCHANGE MODEL FOR [FE3(II)FE(III)] CLUSTERS OF FERREDOXINS - A GENERALIZED SOLUTION OF THE EXCHANGE PROBLEM

A generalized model of the Heisenberg exchange interaction for the [4Fe-4S]1+ clusters of ferredoxins, enzymes and synthetic analogs is solved. The generalized analytical solution for the S = 1/2 state explains the mixing of exchange levels and the transformation of the spin expectation value [s(i)] on each ion under cluster deformation. The proposed model joins two Kambe solutions for iron clusters of different C2v and C3 symmetry. Exchange variations determine the correlations: (1) g(mol) = SIGMA(i=1)4, K(i)g(i) between molecular and local g factors (K(i) = [s(iz)]/[S(Z)]), and (2) A(i) = alpha(i)K(i) between the effective and local hyperfine constants. The exchange variations lead to equalization of the effective hyperfine constants of the Fe2+-Fe3+ pair.