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 + H{sub 2}O {leftrightarrow} XO + 2H{sup +} + 2e{sup {minus}} involving substrate and product X/XO. All such enzymes contain one or two pterin dithiolene ligands 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-ray diffraction, that are relevant to the active sites of several families of enzymes containing two pterin dithiolenes: [M{sup IV}O(bdt){sub 2}]{sup 2{minus}} (1, 6), [M{sup V}(bdt){sub 2}]{sup 1{minus}} (3, 8), [M{sup VI}O{sub 2}(bdt){sub 2}]{sup 2{minus}} (4, 9), and [M{sup VI}O(OSiBu{sup t}Ph{sub 2})(bdt){sub 2}]{sup 1{minus}} (5, 10) [M = Mo (1-5), W (6-10); bdt = benzene-1,2-dithiolate(2-)]. In particular, complexes 3 and 5 simulate the reduced and oxidized sites, respectively, of one DMSO reductase. This set of complexes of accurately known structure provides a heretofore unavailable opportunity to examine by X-ray absorption 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{sub 2,3}-edge spectra and EXAFS analysis (using the GNXAS protocol) are reported for 1--10. Systematic shifts of edge energies 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 substantially advanced by protein crystallography, there exist variances in metal-dithiolene interactions for a given enzyme and structural differences among closely related enzymes. Further, protein crystallographic results are not always consistent with conclusions from XAS and other spectroscopic studies. These molecules serve as benchmarks or calibrants with respect to the corresponding structural properties of enzyme sites containing two pterin dithiolene cofactor ligands and one or two oxygen-based ligands such as members 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 crystallography results, these complexes will prove useful for future comparisons with enzymes of both known and unknown structure and may help to clarify discrepancies seen in protein crystallography.
- Research Organization:
- Stanford Univ., CA (US)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 20013685
- Journal Information:
- Journal of the American Chemical Society, Vol. 121, Issue 44; Other Information: PBD: 10 Nov 1999; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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