Science 334, 974–977 (2011);

Science 334, 940 (2011)

Breaking dinitrogen's triple bond, one of the strongest known chemical bonds, is extremely difficult. Industrially, it is only done by the energy-intensive and high-pressure Haber–Bosch process. In nature, the only enzyme that performs the reaction is nitrogenase, through the use of the most complex biological metal centre known. This iron–molybdenum cofactor contains seven iron atoms, one molybdenum atom and nine sulfur atoms. Its structure was elucidated in 1992, but in 2002 a light atom was found to be present at the centre of the cluster. This was suggested to be nitrogen, although the resolution was not good enough to confirm this and rule out oxygen and carbon. Now, two teams of researchers have independently confirmed, using different experimental techniques, that this interstitial atom in the iron–molybdenum cofactor is in fact carbon.

Credit: © 2011 AAAS

Kyle Lancaster and colleagues used iron Kβ X-ray emission spectroscopy to examine the isolated cofactor as well as a protein that does not contain it, but which does have a structure similar to nitrogenase's 'P cluster'. This allowed them to isolate the contribution of the mysterious interstitial atom to the spectrum. This contribution was discovered to be lower in energy than would be expected for oxygen or nitrogen, suggesting it is carbon, which was supported by extensive calculations.

Taking a different approach, Oliver Einsle and colleagues obtained a higher-resolution crystallography dataset. The improved resolution meant that they could compare the measured electron density of the unknown atom at an improved probe radius. Together with improved thermal factors, Einsle and colleagues found that the evidence pointed towards carbon. They also used improved 13C- and 15N-enriched resonance spectroscopy to further confirm their findings.