An X-ray spectroscopic investigation of bis(dithiolene)molybdenum(IV,V,VI)and -tungsten(IV,V,VI) complexes: Symmetrized structural representations of the active sites of molybdoenzymes in the DMSO reductase family and of tungstoenzymes in the AOR and F(M)DH families

Molybdenum and tungsten oxotransferase and hydroxylase enzymes catalyze the generalized reaction X + H2O <----> XO + 2H(+) + 2e(-) involving substrate and product X/XO. All such enzymes contain one or two pterin dithiolene li gands bound to a molybdenum or tungsten atom in the enzyme cofactor. Recent investigations in these laboratories together with earlier work by others have afforded a set of 10 complexes, all structurally characterized by X-ra y diffraction, that are relevant to the active sites of several families of enzymes containing two pterin dithiolenes: [(MO)-O-IV(bdt)(2)](2-) (1, 6), [M-V(bdt)(2)](1-) (2, 7), [M-IV((OSiBuPh2)-Ph-t)(bdt)(2)](1-) (3, 8), [(MO 2)-O-VI(bdt)(2)](2-) (4, 9), and [(MO)-O-VI((OSiBuPh2)-Ph-t)(bdt)(2)](1-) ( 5, 10) [M = Mo (1-5), W (6-10); bdt = benzene-1,2-dithiolate(2-)]. In parti cular, complexes 3 and 5 simulate the reduced and oxidized sites, respectiv ely, of one DMSO reductase. This set of complexes of accurately known struc ture provides a heretofore unavailable opportunity to examine by X-ray abso rption spectroscopy (XAS) the effects of different oxidation states, ligand types, and coordination geometries on absorption edge and EXAFS features. Molybdenum K-edge or tungsten L-2,L-3-edge spectra and EXAFS analysis (usin g the GNXAS protocol) are reported for 1-10. Systematic shifts of edge ener gies over the M(IV,V,VI) oxidation states are observed, and features in the second derivative edge spectra are correlated with the number (0-2) of oxo ligands. While the field of molybdenum and tungsten enzymes has been subst antially advanced by protein crystallography, there exist variances in meta l-dithiolene interactions for a given enzyme and structural differences amo ng closely related enzymes. Further, protein crystallographic results are n ot always consistent with conclusions from XAS and other spectroscopic stud ies. These molecules serve as benchmarks or calibrants with respect to the corresponding structural properties of enzyme sites containing two pterin d ithiolene cofactor ligands and one br two oxygen-based ligands such as memb ers of the DMSO family of molybdoenzymes and the AOR and F(M)DH families of tungstoenzymes. Because of the internal consistency of EXAFS and X-ray cry stallography results, these complexes will prove useful for future comparis ons with enzymes of both known and unknown structure and may help to clarif y discrepancies seen in protein crystallography.