EPR studies of oxo-molybdenum(V) complexes with sulfur donor ligands: Implications for the molybdenum center of sulfite oxidase

A series of six-coordinate compounds containing a chelating dithiolate coordinated to the [LMo(V)O](2+) unit (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) have been characterized by EPR spectroscopy as models for the molybdenum centers of pterin-containing molybdenum enzymes. The structure of LMoO(bdt) (1) (bdt = 1,2-benzenedithiolate) has been determined by X-ray crystallography; the space group is P2(1)/n with a = 10.727(1) Angstrom, b = 14.673(2) Angstrom, c = 15.887(2) Angstrom, beta = 100.317(4)degrees and Z = 4. Compound 1 exhibits distorted octahedral stereochemistry; the terminal oxo group and the sulfur atoms are mutually cis to one another. The Mo=O distance is 1.678(4) Angstrom, and the average Mo-S distance is 2.373(2) Angstrom. The EPR parameters for 1, determined from simulation of the frozen-solution spectrum, are g(1) = 2.004, g(2) = 1.972, g(3) = 1.934 and A(1)(Mo-95,Mo-97) = 50.0 X 10(-4), A(2) = 11.4 x 10(-4), A(3) = 49.7 x 10(-4) cm(-1). The EPR parameters for several LMo(V)O{S(CH2)(x)S} compounds (x = 2-4) with saturated chelate skeletons are similar to those of 1, indicating that it is the coordinated S atoms and not unsaturation of the chelate skeleton that gives rise to the large g values for 1. The presence of g components larger than the free-electron value is ascribed to low-energy charge transfer transitions from the filled sulfur pi orbitals to half-filled Mo d orbitals. The EPR spectrum of [LMo(V)O{S2P(OEt)(2)}](+) shows an unusually large isotropic P-31 hyperfine splitting of 66.1 x 10(-4) cm(-1) from the noncoordinated phosphorus atom. The frozen-solution EPR spectra of the low-pH and high-pH forms of sulfite oxidase have been reinvestigated in D2O and the anisotropic g and A(Mo-95,Mo-97) parameters determined by simulation of the spectrum arising from the naturally abundant Mo isotopes (75% I = 0, 25% I = 5/2). The EPR parameters for the low-pH form are g(1) = 2.007, g(2) = 1.974, g(3) = 1.968 and A(1) = 56.7 x 10(-4), A(2) = 25.0 x 10(-4), A(3) = 16.7 x 10(-4) cm(-1). The EPR parameters for the high-pH form are g(1) = 1.990, g(2) = 1.966, g(3) = 1.954 and A(1) = 54.4 x 10(-4), A(2) = 21.0 x 10(-4), A(3) = 11.3 x 10(-4) cm(-1). These are the first determinations of the complete A(Mo-95,Mo-97) hyperfine components for an enzyme that possesses an [(MoO2)-O-VI](2+) core in its fully oxidized state.