Does adenine, a key organic molecule, occur in interstellar dust clouds? If so, those clouds could have delivered the molecule to Earth billions of years ago, a possibility interesting not only in terms of life’s formation on this planet but, of course, on other worlds as well. And as University of Missouri chemist Rainer Glaser notes, the idea of space-borne adenine is not implausible, for adenine is known to occur in meteorites and was identified in 1986 in the organic mantle surrounding comet Halley’s core.
Could adenine have been synthesized on the early Earth? Perhaps, but there are reasons for finding an outside delivery mechanism provocative. Note this from the paper on this work, which appeared in the journal Astrobiology (internal references omitted for brevity). HCN refers to hydrogen cyanide, which can flag the presence of adenine:
The idea of prebiotic adenine synthesis on Earth remains controversial. The HCN-based syntheses rely on the presence of a reducing atmosphere, and there is growing evidence that Earth did start out with a reducing atmosphere. But even with the substrates present, questions remain regarding the kinetic and thermodynamic feasibilities and probabilities of adenine synthesis under various conditions on an early Earth.
Clearly, space-borne adenine would be prime fodder for astrobiologists. The nucleotide adenosine triphosphate (ATP) is formed when phosphates are added to adenosine (a nucleoside formed from adenine and ribose), enabling the transfer of chemical energy in cellular metabolism. If such an essential organic molecule were found in interstellar space, it could actually become a diagnostic tool, helping us identify areas where life is more likely to occur in our galaxy. Here’s Glaser on the possibilities:
“There is a lot of sky with a few areas that have dust clouds. In those dust clouds, a few of them have HCN. A few of those have enough HCN to support the synthesis of the molecules of life. Now, we have to look for the HCN concentrations, and that’s where you want to look for adenine.”
And then this:
“Chemistry in space and ‘normal chemistry’ can be very different because the concentrations and energy-exchange processes are different. These features make the study of chemistry in space very exciting and academically challenging; one really must think without prejudice.”
A University of Missouri news release is here. The paper is Glaser et al., “Adenine Synthesis in Interstellar Space: Mechanisms of Prebiotic Pyrimidine-Ring Formation of Monocyclic HCN-Pentamers,” Astrobiology Vol. 7 No. 3 (2007), pp. 455-470 (available online).