MOSSBAUER STUDY OF ZNFE3S4 AND NIFE3S4 CLUSTERS IN PYROCOCCUS-FURIOSUS FERREDOXIN

Pyrococcus furiosus ferredoxin is a small protein (MW 7500) containing an Fe4S4 Cluster which readily converts into an Fe3S4 form. Incubation of Fe3S4 ferredoxin with Zn2+ or Ni2+ yields the mixed-metal clusters [ZnFe3S4]+ and [NiFe3S4]+. Here we report a systematic Mossbauer study of [Fe3S4]0, [ZnFe3S4]+, [NiFC3S4]+, and [NiFe3S4]+ complexed with CN-. Two sites of [Fe3S4]0 form a delocalized Fe3+.Fe2+ pair whereas the third site has a localized Fe3+ valence state. Upon incorporation of a divalent heterometal the Fe3S4 fragment of the core can accommodate one additional electron, leading to a localized Fe2+ site in the [ZnFe3S4]+ (cluster spin S = 5/2) ferredoxin. We have analyzed the Mossbauer spectra taken between 1.5 and 150 K in fields up to 6.0 T in the framework of a spin Hamiltonian formalism. While the parameter sets obtained for the sites of the delocalized pair are similar to that obtained for the pair of [Fe3S4]0, the data suggest unusual magnetic hyperfine interactions, perhaps caused by a low-lying orbital state (almost-equal-to 200 cm-1), for the Fe2+ site. In contrast to [ZnFe3S4]+, the S = 3/2 states of [NiFe3S4]+ and its CN- complex lack a localized Fe2+ site; rather, the three Fe sites display the same quadrupole splittings and isomer shifts. Analysis of the magnetic hyperfine interactions, however, reveals distinguishable sites. Isomer shift data suggest a shift of d-electron density from the Fe3S4 core fragment to the Ni site.