IR-monitored photolysis of CO-inhibited nitrogenase: a major EPR-silent species with coupled terminal CO ligands.
Chemistry
; 18(51): 16349-57, 2012 Dec 14.
Article
em En
| MEDLINE
| ID: mdl-23136072
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 4â
K reveals a strong negative IR difference band at nu = 1938â
cm(-1), along with a weaker negative feature at 1911â
cm(-1). These bands and the associated chemical species have both been assigned the label "Hi-3". A positive band at nu = 1921â
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 = 1917â
cm(-1). Although in previous studies Lo-3 was not observed to recombine, by extending the observation range to 200-240â
K, we found that recombination to Hi-3 does indeed occur, with an activation energy of approximately 6.5â
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 1909â
cm(-1), respectively, with relative band intensities of about 3.51, 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.
Texto completo:
1
Coleções:
01-internacional
Base de dados:
MEDLINE
Assunto principal:
Monóxido de Carbono
/
Azotobacter vinelandii
/
Molibdoferredoxina
/
Nitrogenase
Tipo de estudo:
Prognostic_studies
Idioma:
En
Revista:
Chemistry
Assunto da revista:
QUIMICA
Ano de publicação:
2012
Tipo de documento:
Article
País de afiliação:
Estados Unidos