Hydrogen cyanide polymerization: A preferred cosmochemical pathway

Abstract

In the presence of a base such as ammonia, liquid HCN polymerizes spontaneously at room temperature to a brown-black solid from which a yellow-brown powder can be extracted by water and further hydrolyzed to yield a-amino acids. Two types of structural units appear to be present in these polymeric products, stable ladder polymers with conjugated -C=N- bonds, and polyamidines, readily converted by water to polypeptides.

Several kinds of investigations, including electric discharge experiments which produce HCN from methane and ammonia, give results consistent with the hypothesis that the original polypeptides on Earth were synthesized directly from such HCN polymers and water without the intervening formation of α-amino acids. In the absence of water - on land - the intermediate polyamidines could have been the original condensing agents directing the synthesisis of nucleosides and nucleotides from available sugars, phosphates and nitrogen bases. Most significant would have been the parallel synthesis of polypeptides and polynucleotides arising from the dehydrating action of these polyamidines on nucleotides.

The expected predominance on cometary nuclei of frozen volatiles such as methane, ammonia, and water subjected to high energy sources makes them ideal sites for the formation and condensed-phase polymerization of HCN. Dust emanating from the nucleus, contributing to the coma and tail, would also arise partly from the polymer. Results of the recent Halley missions support this view, particularly the detection of cyanide radicals, HCN itself, and particles consisting only of H, C and N. HCN polymerization could account, too, for the dark material detected on some asteroids and for much of the yellow-brown-orange coloration of Jupiter and Saturn, as well as for the orange haze high in Titan's stratosphere.

In sum, laboratory and extraterrestrial studies increasingly suggest that hydrogen cyanide polymerization is a truly universal process that accounts not only for the past synthesis of protein ancestors on Earth but also for chemistry proceeding elsewhere today within our solar system, on satellites around other stars, and in the dusty molecular clouds of the Milky Way and other spiral galaxies. The existence of this preferred pathway adds greatly to the probability that life is widespread in the universe.