ELECTRONIC-STRUCTURE OF THE [FE4SE4]3+ CLUSTERS IN C-VINOSUM HIPIP AND ECTOTHIORHODOSPIZA-HALOPHILA HIPIP-II THROUGH NMR AND EPR STUDIES

The selenium-for-sulfur substitutions in the [4Fe-4S] core of the high-potential iron protein (HiPIP) from Chromatium vinosum and HiPIP II from Ectothiorhodospiza halophila were carried out, and the modified proteins were characterized by electronic EPR and NMR spectroscopies. EPR spectra showed a small increase in anisotropy and average g values. These results were interpreted as an indication that the Se-for-S substitution does not influence significantly the properties of the spin ground state. The assignment of the hyperfine shifted signals corresponding to protons of the four cysteine residues bound to the iron ions of the [4Fe-4Se] core was performed for both proteins using difference NOE, NOESY, EXSY, and TOCSY H-1 NMR methods. The changes induced by Se-for-S substitution on the magnetic coupling constants operative within the cluster core were inferred from the temperature dependence behavior of the paramagnetically shifted signals using a simple Heisenberg exchange model. The experimental data could be interpreted using a J(Se) congruent-to 0.8J(S) and an overall decrease of J(ij), which seems to be more pronounced on the ''mixed-valence'' pair. This suggests a relative increase of the importance of double exchange on the ''mixed-valence'' pair upon Se-for-S substitution. Finally, the electronic distribution within the cluster core of HiPIP II from E. halophila was not modified by the presence of Se instead of S, whereas in HiPIP from C vinosum a slight curvature of the temperature dependence of hyperfine shifted signals was interpreted as a further indication that an equilibrium exists between two orientations of the ''ferric'' and ''mixed-valence'' pairs within the cluster core.