Modeling for the Active Site of Sulfite Oxidase: Synthesis, Characterization, and Reactivity of [Mo < sup > VI < /sup > O < inf > 2 < /inf > (mnt) < inf > 2 < /inf > ] < sup > 2- < /sup > (mnt < sup > 2- < /sup > = 1,2-Dicyanoethylenedithiolate)

Abstract

The complexes, [Bu4N]2[MoVI02(mnt)2] (1), [Bu4N]2[MoIVO(mnt)2] (2), and [Ph3PNPPh3] [Et4N] [Mov-OCl(mnt)2] (3) (mnt2- = 1,2-dicyanoethylenedithiolate) have been synthesized as possible models for active sites of sulfite oxidase which is proposed to contain molybdenum cofactor with dithiolene coordination around molybdenum. The structure of the [Bu4P]+ salt of complex anion of 1 has been determined by X-ray crystallography. The compound crystallizes in space group P21/c, with a = 14.200(3) Å, b = 19.402(4) Å, c = 18.967(3) Å, β = 95.48(1)°, and Z = 4. [MoVIO2(mnt)2]2- is a distorted octahedron with the oxo groups cis to each other and trans to the dithiolene sulfur atoms. The complexes 1-3 have been characterized by IR, UV-visible, 13C NMR, and negative ion FAB mass spectra. Complex 1 shows a quasireversible reduction and proton coupled electron transfer reaction. Complex 2 undergoes an one-electron reversible oxidation; but on the coulometric time scale it disproportionates to a tris dithiolene complex, [MoIV(mnt)3]2- and M0O3. Complex 2 in the presence of CI- is oxidized irreversibly with the appearance of a new quasireversible couple corresponding to the electrochemical detection of [MovOCl(mnt)2]2-/[MoIVOCl(mnt)2]3-. The EPR parameters of 3 and [MovO(mnt)2]1- are reported. The 35,37Cl superhyperfine splitting of the chloro complex 3 is shown in relevance to Mo-Cl interaction in native sulfite oxidase. Complex 1 oxidizes HSO3- to HSO4- with the formation of 2 and without forming the biologically irrelevant m-oxo Mo(V) dimer. This reaction follows enzymatic substrate saturation kinetics with apparent Km (Michaelis-Menten constant) = 0.010(±0.001) M and k2 (kObs at substrate saturation concentration and is proportional to VmRX) = 0.87(±0.04) s-1 in MeCN/H2O (1:1) medium at 20 °C. © 1994, American Chemical Society. All rights reserved.