Branched Alkyl Chain Found In Space

Astronomers are reporting the first definitive detection of a branched alkyl molecule, isopropyl cyanide (i-C₃H₇CN), in the interstellar medium (Science 2014, DOI: 10.1126/science.1256678).

The discovery, from a team led by Arnaud Belloche, an astronomer at the Max Planck Institute for Radio Astronomy, in Bonn, Germany, confirms what scientists have long suspected: Cold interstellar space is a hotbed of complex chemistry. It also increases the likelihood that prebiotic molecules such as amino acids may be formed there.

Over the years, astronomers have found more than 180 molecules in space. But other than fullerenes, these molecules have all had linear backbones. In fact, Belloche’s group had recently detected the straight-chain isomer of C₃H₇CN (n-propylcyanide).

The detection of isopropyl cyanide was made possible by the unprecedented resolving power and sensitivity of the new Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile. The spectra of branched molecules were lost in a sea of spectral lines for previous generations of telescopes. ALMA is opening the door to more complicated molecules.

The authors found isopropyl cyanide in a star-forming region, and it is as abundant as the normal isomer.

“This bodes well for the presence of amino acids, since these are branched chain species,” says Eric Herbst, professor in the chemistry, astronomy, and physics departments at the University of Virginia.

Amino acids have been found on meteorites and even a comet, and it’s been expected that they could form in star forming-regions of interstellar space. In particular, astronomers have been searching for glycine, the simplest amino acid, for decades without success.

David E. Woon, astrochemist at the University of Illinois, Urbana-Champaign, notes that the branching of this particular molecule, with its CN group in the middle of the chain, “isn’t directly relevant to the formation of biological amino acids.” α-Amino acids have NH₂ side chains and other groups.

But Woon and others agree that it’s likely that more branched chains will be discovered in the interstellar medium.

As the complexity of discovered molecules increases, the bar for what constitutes a definitive detection rises. Dozens of characteristic spectral lines must be detected consistently for larger molecules. In this new report, the authors found 50 lines in patterns unique to isopropyl cyanide.

“It certainly reads like a very nice detection,” notes Geoffrey A. Blake, professor of cosmochemistry and planetary science and of chemistry at California Institute of Technology.