Molybdenum L-Edge XAS Spectra of MoFe Nitrogenase

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(III) assignment for the molybdenum atom in FeMoco. The L(3)-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(3)-edges. The calculated results reproduce well the relative spectral trends that are observed experimentally. Ultimately, these results give further support for the Mo(III) assignment in protein-bound FeMoco, as well as isolated FeMoco.