Resonance Raman Studies of Nickel Tetrathiolates and Nickel-Substituted Rubredoxins and Desulforedoxin

Low-temperature resonance Raman spectroscopy has been used to investigate the nickel coordination geometry in the [Ni(SPh)₄]²⁻ anion in solution, Ni(II)-substituted rubredoxins from Desulfovibrio gigas and Pyrococcus furiosus, and Ni(II)-substituted desulforedoxin from D. gigas. Ni-S vibrational modes have been assigned on the basis of ⁵⁸Ni/⁶⁰Ni isotope shifts, polarization studies, and comparison with structurally characterized tetrathiolate Ni(II) complexes. The results indicate tetragonally elongated tetrahedral [NiS₄] cores in the [Ni(SPh)₄]²⁻ anion in solution and in both Ni(II)-rubredoxins and a distorted square planar [NiS₄] core in Ni(II)-desulforedoxin. Cysteine deformation modes that are enhanced via coupling with Ni-S stretching modes are observed near 400 cm⁻¹ in the spectra of both native and Ni(II)-substituted rubredoxins. The potential of resonance Raman for investigating Ni-cysteinate centers in other metalloproteins and for elucidating the conformation of coordinating cysteine residues is discussed in light of these new results.