Rs. Czernuszewicz et al., RESONANCE RAMAN-SPECTROSCOPY OF IRON(III) TETRATHIOLATE COMPLEXES - IMPLICATIONS FOR THE CONFORMATION AND FORCE-FIELD OF RUBREDOXIN, Journal of the American Chemical Society, 116(16), 1994, pp. 7134-7141
Vibrational spectra are analyzed for a series of iron(III) tetrathiola
te complexes, including [Fe(SMe)(4)](-), [Fe(SEt)(4)](-), and [Fe(S-2-
o-xyl)(2)](-) (SMe = methylthiolate, SEt = ethylthiolate, and S-2-o-xy
l = o-xylene-alpha,alpha'-dithiolate), using resonance Raman (RR) and
infrared spectra of isotopomers (Fe-54, S-34, and H-2). Assignments ar
e made with the aid of normal coordinate analysis calculations, using
a consistent force field for all three species. These results permit r
eanalysis 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 stretc
hing mode, due to the S-C bonds being oriented out of the S-Fe-S plane
s; (2) further splitting, in the case of [Fe(SMe)(4)](-), due to inequ
ivalence 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 t
he chelate ring constraints in [Fe(S-2-o-xyl)(2)](-). The rubredoxin R
R 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 degre
es FeS-CC dihedral angles for two of the cysteinate ligands. Proper ca
lculation of the yl frequency of rubredoxin requires a significant red
uction 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.