RESONANCE RAMAN-SPECTROSCOPY OF IRON(III) TETRATHIOLATE COMPLEXES - IMPLICATIONS FOR THE CONFORMATION AND FORCE-FIELD OF RUBREDOXIN

Vibrational spectra are analyzed for a series of iron(III) tetrathiolate complexes, including [Fe(SMe)(4)](-), [Fe(SEt)(4)](-), and [Fe(S-2-o-xyl)(2)](-) (SMe = methylthiolate, SEt = ethylthiolate, and S-2-o-xyl = o-xylene-alpha,alpha'-dithiolate), using resonance Raman (RR) and infrared spectra of isotopomers (Fe-54, S-34, and H-2). Assignments are made with the aid of normal coordinate analysis calculations, using a consistent force field for all three species. These results permit reanalysis and modeling of previously published RR spectra of oxidized rubredoxin in which Fe3+ is bound by four cysteinate side chains. The spectra of the analog complexes reveal (1) symmetry lowering from Td, manifested in the splitting of the triply degenerate nu(3) Fe-S stretching mode, due to the S-C bonds being oriented out of the S-Fe-S planes; (2) further splitting, in the case of [Fe(SMe)(4)](-), due to inequivalence of the S-Fe-S angles; (3) elevation of the vl Fe-S breathing frequency via interaction with methyl torsional modes in [Fe(SEt)(4)](-); and (4) mixing of Fe-S stretching and S-C-C bending modes due to the chelate ring constraints in [Fe(S-2-o-xyl)(2)](-). The rubredoxin RR bands and Fe-54 isotope shifts are modeled with the same force field, revealing a dominant influence of Fe-S/S-C-C mixing due to 180 degrees FeS-CC dihedral angles for two of the cysteinate ligands. Proper calculation of the yl frequency of rubredoxin requires a significant reduction of the Fe-S stretching force constant, relative to that of the analog complexes. This reduction is proposed to reflect the influence of H-bonding to the cysteinate S atoms in the protein.