L-Edge spectroscopy of molybdenum compounds and enzymes

Abstract

Spectra at the molybdenum L2 and L3 edges have been recorded by use of synchrotron radiation and analyzed in terms of ligand field theory. Four distinct p -» d transitions were observed in the derivative spectra of molybdenum oxychloride complexes. Comparison with optical data for the same compounds, as well as for Tc analogues, showed that L2 3-edge spectra qualitatively reflect the unfilled Mo d-level splittings. A semiempirical correlation scheme, using Racah parameters to correct for exchange and Coulomb interactions, predicted optical splittings with an accuracy of better than 5%. This capability was used to reject certain interpretations of the Mo042~, MoOCl4(H20)~, and MoOClf spectra. Single-crystal spectra for [N(Et)4][MoOCl4(H20)] helped confirm the assignments. Chemical effects on Mo L-edge spectra were surveyed for LMoOXY compounds, where L represents hydrotris(3,5-dimethyl-1 -pyrazolyl)borate and X and Y are various ligands. Spectral sensitivity to oxidation state, terminal oxo vs terminal sulfido ligands, and different halide ions are also compared. Preliminary spectral analysis of several molybdenum enzymes is presented. L3-edge splittings of 1.72 and 1.40 eV were observed for nitrogenase and active xanthine oxidase, respectively. Oxidized sulfite oxidase gave L3-edge splittings of 1.05, 2.14, and 3.05 eV. L-edge spectroscopy is a useful technique for studying molybdenum in small molecules, enzymes, and catalysts, especially if such materials are available in oriented forms.Molybdenum K-edge X-ray absorption has been used for the study of molybdenum enzymes,1 4aqueous solutions,2 amorphous materials,3,4 and catalysts.5 Although the experimental conditions at 20 keV are ideal for EXAFS, utilization of the K-edge XANES

The distribution of the doped holes in La2−xSrxCu1−yRuyO4−δ

“…Compared with the situation as regards 3d TM L 2 and L 3 XAS spectra [28], both the experimental and theoretical study of 4d TM L 2;3 XAS spectra lag far behind. It is usually assumed that the 4d TM L 2;3 XAS spectra reflect directly the unoccupied 4d orbitals, and thus the spectra have often been interpreted in terms of crystal-field or molecular-orbital theories [29][30][31][32]. In the case of O h local symmetry, it is then expected that the intensity ratio of transitions into the crystal-fieldsplit t 2g and e g states Iðt 2g Þ=Iðe g Þ is 0.5 for Ru(IV) [4d 4 ] and increases to 0.75 for Ru(V) [4d 3 ] at both the L 2 and the L 3 edges.…”

Multiplet effects in the RuL2,3x-ray-absorption spectra of Ru(IV) and Ru(V) compounds

“…1,2 In contrast, it has often been assumed that the L 2,3 spectra of the 4d TM compounds reflect directly unoccupied 4d orbitals influenced by the local symmetry of the metal ion, and spectra have been interpreted in terms of crystal-field or molecularorbital theories. [3][4][5][6] In this context, the single peak observed at the Ru-L 2 edge and the two peaks at the Ru-L 3 edge of Ru͑NH 3 ͒ 6 Cl 3 ͑which has a Ru 4d 5 configuration and octahedral symmetry͒ were explained in terms of vanishing matrix elements for the transitions at the L 2 edge into the t 2g orbitals resulting from consideration of the 4d spin-orbit coupling within a single-particle treatment. 7 On the other hand, detailed CFMC resulted in multiplet spectra with a single peak at the L 2 and a double peak at the L 3 edge for a 4d 5 configuration.…”

Detection of the optical bands of molybdenum(V) in DMSO reductase (Rhodobacter capsulatus) by low‐temperature MCD spectroscopy

“…The cofactor in R. sphaeroides DMSO reductase is also covalently linked through a pyrephosphate bond to guanosine monophosphate [5]. Although no X-ray structure is available of any molybdoenzyme there is considerable spectroscopic data including EPR of the Me(V) state [3,6] and EXAFS studies [7]. Optical methods have not been widely applied to the study of molybdoenzymes since the presence of additional chromophores such as flavin, ironAbbreviations: EPR, electron paramagnetic resonance; MCD, magnetic circular dichroism; EXAFS, extended X-ray absorption fine structure; SDS-PAGE, sodium dodecylsulphate-polyacrylamide gel elcetrophoresis; EDTA, ethylene diaminetetraacetic acid; Trig, Tris(hydroxymethyl)aminomethane; Bicine, N.N-bis(2.hydroxyethyl).…”