Variation of average g values and effective exchange coupling constants among [2Fe-2S] clusters:: A density functional theory study of the impact of localization (trapping forces) versus delocalization (double-exchange) as competing factors

A phenomenological model aimed at rationalizing variations in both average g-tensor values (g(av) equivalent to 1/3 Sigma(i)g(i)) and effective exchange coupling constants J(eff) (defined as two-thirds of the energy difference between the S = 3/2 and S = 1/2 spin states) has been derived in order to describe the great variety of magnetic properties exhibited by reduced [2Fe-2S] clusters in proteins. The key quantity in the present analysis is the ratio Delta E/B computed from two competing terms. Delta E comprises various effects that result in trapping-site asymmetries: vibronic coupling and the chemical nature (S/N/O) and conformations of the ligands on the one hand and solvation terms, the hydrogen bonding network, etc., on the other. Ail of these additive terms (in a bottom-up approach) favor valence localization of the reducing electron onto one of the two iron sites. In contrast, the B term is the double-exchange term, which favors electronic delocalization. Both g(av) and J(eff) can be expressed as functions of Delta E/B. We have also shown that electronic localization generally favors small g(av) and large J(eff) values (while the opposite is true for electronic delocalization) in a comparative study of the spectroscopic features of plant-type ferredoxins (Fd's) and Rieske centers (and related mutants). Two other types of problems were particularly challenging. The first of these involved deprotonated Rieske centers and the xanthine oxidase clusters 11, which are characterized by very small Jeff values (40-45 cm(-1) with a J (S) over cap (A). (S) over cap (B) model) correlated with unusually large g(av) values (in the range 1.97-2.01) as a result of an antisymmetric exchange coupling mechanism. The second concerned the analogous Fd's from Clostridium pasteurianum (Cp) and Aquifex aeolicus (Aa). Detailed Mossbauer studies of the C56S mutant of the Cp system revealed a mixture of clusters with valence-localized S = 1/2 and valence-delocalized S = 9/2 ground states. We relied on crystallographic structures of wild-type and mutant Aa Fd's in order to explain such a distribution of spin states.