title: Fluorine-19 chemical shifts as probes of the structure and reactivity of the iron-molybdenum cofactor of nitrogenase.
abstract: The reaction of the iron-molybdenum cofactor with thiolate and the redox behavior of the iron-molybdenum cofactor-thiolate complex have been studied by 19F NMR using p-CF3C6H4S- as the reporter ligand. These experiments give results different from those produced by other methods which have been performed near 4 K rather than at ambient temperature. Specifically, these data show that the iron-molybdenum cofactor-thiolate complex is not the product of an irreversible reaction. Rather, the complex is in dynamic equilibrium with the free iron-molybdenum cofactor and free thiolate. Models of the reactions of nitrogenase may need to take this temperature-dependent difference into account because the lability of the iron-molybdenum thiolate bond means its making and breaking could be involved in substrate binding or reduction. The 19F NMR results reported here also show that the S = 3/2 state of the iron-molybdenum cofactor-thiolate complex can be easily and reversibly oxidized by one electron. However, electron exchange between the oxidized and reduced states of the complex is quite slow at approximately 1 mM. Based on low temperature spectroscopic studies, the oxidized iron-molybdenum cofactor-thiolate complex was expected to be diamagnetic. Isotropically shifted NMR spectra of the oxidized cofactor samples at 240-320 K, however, indicate at least partial population of a paramagnetic state, possibly with S = 1.
authors.affiliation: Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138.
authors.forename: S D
authors.initials: SD
authors.lastname: Conradson
authors.forename: B K
authors.initials: BK
authors.lastname: Burgess
authors.forename: R H
authors.initials: RH
authors.lastname: Holm
mesh_terms: D001665:Binding Sites; D004578:Electron Spin Resonance Spectroscopy; D005288:Ferredoxins / Q000378:metabolism*; D005461:Fluorine; D005559:Formamides; D009682:Magnetic Resonance Spectroscopy; D008983:Molybdoferredoxin / Q000378:metabolism*; D009591:Nitrogenase / Q000378:metabolism*; D010084:Oxidation-Reduction; D013438:Sulfhydryl Compounds / Q000378:metabolism
chemical_list: D005288:Ferredoxins; D005559:Formamides; D008983:Molybdoferredoxin; D013438:Sulfhydryl Compounds; D005461:Fluorine; D009591:Nitrogenase; C002950:methylformamide