Journal of the American Chemical Society
Communication
amount of the interstitial carbide, further facilitating the
enrichment of carbide in the hydrocarbon products. However,
contrary to the observation of labeled products when 13CO was
turned over by the unlabeled cofactor, no labeled product could
be detected when 12CO was turned over by the 13C-labeled
cofactor, suggesting that the interstitial carbide was not
exchanged into the hydrocarbon products upon CO reduction
(Figure 3).
ASSOCIATED CONTENT
* Supporting Information
Materials and Methods. This material is available free of charge
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S
AUTHOR INFORMATION
Corresponding Author
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Author Contributions
†J.A.W. and C.C.L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by National Institutes of Health
Grant GM-67626 (M.W.R.). We thank Prof. Joseph Jarrett
(University of Hawaii at Manoa) for his generous supply of
[13C-methyl]SAM.
REFERENCES
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Figure 3. GC−MS analysis of hydrocarbon products generated from
the turnover of (A) 12CO by unlabeled FeMoco, (B) 13CO by
unlabeled FeMoco, and (C) 12CO by 13C-labeled FeMoco. The
relative intensity of each hydrocarbon product traced at a given mass
was arbitrarily set at 100%.
(8) Lee, H. I.; Benton, P. M.; Laryukhin, M.; Igarashi, R. Y.; Dean, D.
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The question of whether the interstitial atom is involved in
substrate turnover was tackled even before this atom was
identified as a carbide ion, and early electron−nuclear double
resonance (ENDOR)/electron spin echo envelope modulation
(ESEEM) analyses demonstrated that this atom was not an
exchangeable nitrogen atom.8 Here we have provided direct
evidence that the interstitial carbide can neither be exchanged
during turnover nor used as a substrate and incorporated into
the products. These results point to a role of this interstitial
atom in stabilizing the structure of the cofactor, providing a
certain “rigidity” to the metal−sulfur core through symmetrical
coordination of this atom to the six core Fe atoms of the
cofactor (Figure 1).
(10) Xie, H.; Wu, R.; Zhou, Z.; Cao, Z. J. Phys. Chem. B 2008, 112,
11435−11439.
(11) Moret, M. E.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 18118−
18121.
Interestingly, previous density functional theory calculations
indicated that a more stable structure of FeMoco could be
achieved by having an interstitial nitrogen or oxygen species
rather than an interstitial carbon species.9,10 Consistent with
these calculations, a recent study of N2 activation on iron
metallaboratranes suggested that the presence of a carbide atom
in the center of the FeMoco could allow variations of the Fe−C
bond distances and adjustments of the overall geometry of the
metal−sulfur core during the substrate turnover process.11
Thus, a possible function of this interstitial atom in nitrogenase
catalysisbe it indirect in tuning the reactivity of the cofactor
or direct in interacting with the substratescannot be
excluded. The exact role of the interstitial carbide in
nitrogenase mechanism merits further investigation.
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