title: IR-monitored photolysis of CO-inhibited nitrogenase: a major EPR-silent species with coupled terminal CO ligands.
abstract: Fourier transform infrared spectroscopy (FTIR) was used to observe the photolysis and recombination of a new EPR-silent CO-inhibited form of α-H195Q nitrogenase from Azotobacter vinelandii. Photolysis at 4 K reveals a strong negative IR difference band at nu = 1938 cm(-1), along with a weaker negative feature at 1911 cm(-1). These bands and the associated chemical species have both been assigned the label "Hi-3". A positive band at nu = 1921 cm(-1) was assigned to the "Lo-3" photoproduct. By using an isotopic mixture of (12)C (16)O and (13)C (18)O, we show that the Hi-3 bands arise from coupling of two similar CO oscillators with one uncoupled frequency at approximately nu = 1917 cm(-1). Although in previous studies Lo-3 was not observed to recombine, by extending the observation range to 200-240 K, we found that recombination to Hi-3 does indeed occur, with an activation energy of approximately 6.5 kJ mol(-1). The frequencies of the Hi-3 bands suggest terminal CO ligation. This hypothesis was tested with DFT calculations on models with terminal CO ligands on Fe2 and Fe6 of the FeMo-cofactor. An S = 0 model with both CO ligands in exo positions predicts symmetric and asymmetric stretches at nu = 1938 and 1909 cm(-1), respectively, with relative band intensities of about 3.5:1, which is in good agreement with experiment. From the observed IR intensities, Hi-3 was found to be present at a concentration about equal to that of the EPR-active Hi-1 species. The relevance of Hi-3 to the nitrogenase catalytic mechanism and its recently discovered Fischer-Tropsch chemistry is discussed.
authors.affiliation: Department of Chemistry, University of California, Davis, CA 95616, USA.
authors.forename: Lifen
authors.initials: L
authors.lastname: Yan
authors.forename: Vladimir
authors.initials: V
authors.lastname: Pelmenschikov
authors.forename: Christie H
authors.initials: CH
authors.lastname: Dapper
authors.forename: Aubrey D
authors.initials: AD
authors.lastname: Scott
authors.forename: William E
authors.initials: WE
authors.lastname: Newton
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Cramer
mesh_terms: D016948:Azotobacter vinelandii / Q000737:chemistry*; D002248:Carbon Monoxide / Q000737:chemistry*; D002384:Catalysis; D004795:Enzyme Stability; D008024:Ligands; D008983:Molybdoferredoxin / Q000737:chemistry*; D009591:Nitrogenase / Q000737:chemistry*; D010782:Photolysis; D011789:Quantum Theory; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D008024:Ligands; D008983:Molybdoferredoxin; D002248:Carbon Monoxide; D009591:Nitrogenase