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 ?-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.

SUBMITTER: Yan L 

PROVIDER: S-EPMC4497518 | biostudies-literature | 2012 Dec

REPOSITORIES: biostudies-literature