Max-Planck-Institut für Chemische Energiekonversion Stiftstr. 34-36, 45470 Mülheim an der Ruhr, Germany.
Published: January 2015
AI Article Synopsis
Article Abstract
A molybdenum L-edge X-ray absorption spectroscopy (XAS) study is presented for native and oxidized MoFe protein of nitrogenase as well as Mo-Fe model compounds. Recently collected data on MoFe protein (in oxidized and reduced forms) is compared to previously published Mo XAS data on the isolated FeMo cofactor in NMF solution and put in context of the recent Mo K-edge XAS study, which showed a Mo assignment for the molybdenum atom in FeMoco. The L-edge data are interpreted within a simple ligand-field model, from which a time-dependent density functional theory (TDDFT) approach is proposed as a way to provide further insights into the analysis of the molybdenum L-edges. The calculated results reproduce well the relative spectral trends that are observed experimentally. Ultimately, these results give further support for the Mo assignment in protein-bound FeMoco, as well as isolated FeMoco.
A molybdenum L-edge X-ray absorption spectroscopy (XAS) study is presented for native and oxidized MoFe protein of nitrogenase as well as Mo-Fe model compounds. Recently collected data on MoFe protein (in oxidized and reduced forms) is compared to previously published Mo XAS data on the isolated FeMo cofactor in NMF solution and put in context of the recent Mo K-edge XAS study, which showed a Mo assignment for the molybdenum atom in FeMoco. The L-edge data are interpreted within a simple ligand-field model, from which a time-dependent density functional theory (TDDFT) approach is proposed as a way to provide further insights into the analysis of the molybdenum L-edges.
Departamento de Físico-Química and Departamento de Química Inorgânica, Instituto de Química, UFRJ, Ilha do Fundão, CT, Bloco A, Rio de Janeiro, RJ, CEP 21949-900, Brazil.
Carburized molybdenum catalysts supported on a dealuminated NaH-Y zeolite were prepared by carburization under a 20% methane in hydrogen flow of two precursors obtained by adsorption of molybdenum hexacarbonyl, one containing 5 wt % and the other 10 wt % Mo, and a third one was prepared by impregnation with aqueous ammonium heptamolybdate, containing 5 wt % Mo. The three catalysts displayed very distinct behaviors in the benzene hydrogenation reaction at atmospheric pressure and 363 K. By using XANES spectroscopy at the molybdenum L edge, EXAFS and XANES spectroscopy at the molybdenum K edge, and 27Al solid-state NMR spectroscopy, it was shown that different carburized molybdenum species exist in each sample.
The utility of using X-ray absorption spectroscopy (XAS) to study metalloproteins and, specifically, the enzyme complex nitrogenase, is highlighted by this study comparing both the structural and Mo-localized electronic features of the iron-molybdenum cofactor (FeMoco) in isolated MoFe protein and in the ADP.AlF4--stabilized complex of the MoFe protein with the Fe protein. No major differences are found at Mo between the two protein forms.
Mo L-edge and S K-edge X-ray absorption spectroscopy were applied to investigate the charge distribution between Mo and S in a series of Mo thiolate compounds, which serve as amide-sulfur H-bonding models and exhibit different redox potentials arising from polar group effects and ligand hydrogen bonds near the redox center. For all oxidized complexes, the S K-edge spectra exhibit a thiolate-based pre-edge feature centered at 2470.2 eV and the inflection point oCCurs at 2472.
