Electronic Structure Studies of Oxomolybdenum Tetrathiolate Complexes: Origin of Reduction Potential Differences and Relationship to Cysteine-Molybdenum Bonding in Sulfite Oxidase

摘要

Electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies have been used to determine the nature of oxomolybdenum-thiolate bonding in (PPh_4)[MoO(SPh)_4] (SPh=phenylthiolate) and (HNEt_3)[MoO(SPh-PhS)_2] (SPh-PhS=biphenyl-2,2'-dithiolate). These compounds, like all oxomolybeenum tetraarylthiolate complexes previously reported, display an intense low-energy charge-transfer feature that we have now shown to be comprised of multiple S->Mo d_(xy) transitions. The integrated intensity of this low-energy band in [MoO(SPh)_4]~- is approximately twice that of [MoO(SPh-PhS)_2]~-, implying a greater covalent reduction of the effective nuclear charge localized on the molybdenum ion of the former and a concomitant negative shift in the Mo(V)/Mo(IV) reduction potential brought about by the differential S->Mo d_(xy) charge donation. However, this is not observed experimentally; the Mo(V)/Mo(IV) reduction potential of [MoO(SPh)_4]~- is approx 120 mV more positive than that of [MoO(SPh-PhS)_2]~-(-783 vs -900 mV). Additional electronic factors as well as structural reorganizational factors appear to play a role in these reduction potential differences. Density functional theory calculations indicate that the electronic contribution results from a greater #sigma#-mediated charge donation to unfilled higher energy molybdenum acceptor orbitals, and this is reflected in the increased energies of the [MoO(Sph-PhS)_2]~- ligand-to-metal char4ge-transfer transitions relative to those of [MoO(SPh)_4]~-. The degree of S-Mo d_(xy)covalency is a function of the O ident to Mo-S-C dihedral angle, with increasing charge donation to Mo d_(xy) and increasing charge-transfer intensity occurring as the dihedral angle decreases from 90 to 0dg. These results have implications regarding the role coordinated cysteine residue in sulfite oxidase. Although the O ident to Mo-S-C dihedral angles are either approx59 or approx121deg in these oximolybdenum tetraarylthiolate complexes, the crystal structure of the enzyme reveals an O ident to Mo-Sc_(ys)-C angle of approx90deg. Thus, a significant reduction in Sc_(ys) Mo d_(xy) covalency is anticipated in sulfite oxidase. This is postulated to preclude the direct involvement of coordinated cysteine in coupling the active site into efficient superexchange pathways for electron transfer, provided the O ident to Mo-Sc_(ys)-C angle is not dynamic during the course of catalysis. Therefore, we propose that a primary role for coordinated cysteine in sulfite oxidase is to statically poise the reduced molybdenum center at more negative reduction potentials in order to thermodynamically facilitate electron transfer from Mo(IV) to the endogenous b-type heme.