title: Comparative assessment of the composition and charge state of nitrogenase FeMo-cofactor.
abstract: A significant limitation in our understanding of the molecular mechanism of biological nitrogen fixation is the uncertain composition of the FeMo-cofactor (FeMo-co) of nitrogenase. In this study we present a systematic, density functional theory-based evaluation of spin-coupling schemes, iron oxidation states, ligand protonation states, and interstitial ligand composition using a wide range of experimental criteria. The employed functionals and basis sets were validated with molecular orbital information from X-ray absorption spectroscopic data of relevant iron-sulfur clusters. Independently from the employed level of theory, the electronic structure with the greatest number of antiferromagnetic interactions corresponds to the lowest energy state for a given charge and oxidation state distribution of the iron ions. The relative spin state energies of resting and oxidized FeMo-co already allowed exclusion of certain iron oxidation state distributions and interstitial ligand compositions. Geometry-optimized FeMo-co structures of several models further eliminated additional states and compositions, while reduction potentials indicated a strong preference for the most likely charge state of FeMo-co. Mössbauer and ENDOR parameter calculations were found to be remarkably dependent on the employed training set, density functional, and basis set. Overall, we found that a more oxidized [Mo(IV)-2Fe(II)-5Fe(III)-9S(2-)-C(4-)] composition with a hydroxyl-protonated homocitrate ligand satisfies all of the available experimental criteria and is thus favored over the currently preferred composition of [Mo(IV)-4Fe(II)-3Fe(III)-9S(2-)-N(3-)] from the literature.
authors.affiliation: NAI Astrobiology Biogeocatalysis Research Center, Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, USA.
authors.forename: Travis V
authors.initials: TV
authors.lastname: Harris
authors.forename: Robert K
authors.initials: RK
authors.lastname: Szilagyi
mesh_terms: D018360:Crystallography, X-Ray; D008958:Models, Molecular; D009591:Nitrogenase / Q000737:chemistry* / Q000378:metabolism; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D009591:Nitrogenase