Molybdenum L‐Edge XAS Spectra of MoFe Nitrogenase

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 MoIII assignment for the molybdenum atom in FeMoco. The L3-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 L3-edges. The calculated results reproduce well the relative spectral trends that are observed experimentally. Ultimately, these results give further support for the MoIII assignment in protein-bound FeMoco, as well as isolated FeMoco.

X‐ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin‐Coupled Molybdenum(III) Center

“…Moving to the Mo L 3 -edge XMCD,t he relative concentration of the absorber is decreased by af actor of about 15, making the experiments prohibitively challenging.M oL 3 -edge XMCD measurements are further complicated by the large number of sulfur atoms in the protein (72 in total). [12] Here it is important to note that the SK-emission lines are not resolvable from the Mo L-emission lines within the resolution of the present setup.I nt he present study,w ew ere able to measure the Mo L 3 -edge XAS of the MoFep rotein consistent with the previously published Mo L 3 -edge XAS data of FeMoco ( Figure S4 in the Supporting Information). However,because of the issues noted above,w ew ere not able to obtain as atisfactory Mo XMCD signal-to-noise ratio for the MoFe protein.…”

QM/MM Study of the Nitrogenase MoFe Protein Resting State: Broken-Symmetry States, Protonation States, and QM Region Convergence in the FeMoco Active Site

“…Due to resolution limitations and the fact that homocitrate was severely truncated and approximated as a glycolate group in their study, the validity of this assignment was unclear. In previous studies 28,29,30,31,32 by one of us (RB) we have assigned the alkoxide group of the homocitrate as protonated but have not until now given a complete justification for this as the question of how well the protein environment is described surrounding the homocitrate remained unclear. The only exception to this is the 4ND8 crystal structure, recently published by Rees & Howard et al 85 which is a crystal structure solved under basic (pH 9.5) conditions, where the O-O distance is on average 2.77 Å as shown in Figure 6d.…”

“…Recently Adamo 26 et al used QM/QM' calculations to propose new mechanisms and Ryde et al studied protonation states by QM/MM 27 . In previous articles by one of us (RB) we utilized large 225-atom cluster models in our studies of the spectroscopic properties of the FeMo cofactor 28,29,30,31,32 . Large cluster models become difficult to work with and require many constraints to keep the correct active site geometry and may not describe correctly the structural flexibility of the cofactor in the enzyme active site pocket.…”

Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates

“…The soft X-ray energy range between 100 and 1000 eV constitutes a very sensitive probe for the local electronic structure of 3 d transition metal complexes as well as of organic compounds 17,33–37 . The presented setup is a versatile tool to study systems relevant in different fields of photochemistry like catalysis, 38 biology 39 and energy 40 . As a case study, we present time-resolved NEXAFS data on iron(II)-trisbipyridine $false[\text{Fe}{(\text{bpy})}_3^{2+}false]$ in aqueous solution, where the inter-system-crossing processes following a photo-excitation 41,42 have been subject to various studies employing a wide range of spectroscopic methods, 43,44 including X-ray absorption measurements at the ligand 45 and metal center 46 .…”