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IR‐Monitored Photolysis of CO‐Inhibited Nitrogenase: A Major EPR‐Silent Species with Coupled Terminal CO Ligands
Author(s) -
Yan Lifen,
Pelmenschikov Vladimir,
Dapper Christie H.,
Scott Aubrey D.,
Newton William E.,
Cramer Stephen P.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201202072
Subject(s) - photodissociation , azotobacter vinelandii , electron paramagnetic resonance , chemistry , nitrogenase , infrared , fourier transform infrared spectroscopy , recombination , analytical chemistry (journal) , crystallography , photochemistry , stereochemistry , nuclear magnetic resonance , physics , nitrogen , biochemistry , organic chemistry , quantum mechanics , nitrogen fixation , chromatography , optics , gene
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 $\tilde \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 $\tilde \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 $\tilde \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 $\tilde \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.