Organics Produced by Irradiation of Frozen and Liquid HCN Solutions: Implications for Chemical Evolution Studies

Abstract

Hydrogen cyanide (HCN), an important precursor of organic compounds, is widely present in extraterrestrial environments. HCN is also readily synthesized in prebiotic simulation experiments. To gain insight into the radiation chemistry of one of the most important and highly versatile constituents of cometary ices, we examined the behavior of over-irradiated frozen and liquid HCN solutions under ionizing radiation. The samples were exposed to gamma radiation at a dose range from 0 up to 419 kGy. Ultraviolet spectroscopy and gas chromatography were used to follow the process. The analyses confirmed that gamma-ray irradiation of liquid HCN solutions generates several organic products. Many of them are essential to life; we verified the presence of carboxylic acids (some of them members of the Krebs cycle) as well as free amino acids and urea. These are the first studies to reveal the presence of these compounds in experiments performed at low temperatures and bulk irradiation. Organic material was produced even at low temperatures and low radiation doses. This work strongly supports the presumption that, as a parent molecule, HCN played a central essential role in the process of chemical evolution on early Earth, comets, and other extraterrestrial environments.

Detection and formation scenario of citric acid, pyruvic acid, and other possible metabolism precursors in carbonaceous meteorites

“…A series of experiments employing the irradiation of various starting compounds, including HCN, were shown to produce some compounds also found in meteorites (19,20). The list includes some of the compounds we report here such as tricarballyic acid and citric acid (20). Tricarballyic acid (and other tricarboxylic acids) is also produced by aqueous reactions of pyruvate (outlined below).…”

“…The exposure of interstellar grains and comets to various forms of radiation inspired research into how such phenomena would affect the production of organic compounds (19). A series of experiments employing the irradiation of various starting compounds, including HCN, were shown to produce some compounds also found in meteorites (19,20). The list includes some of the compounds we report here such as tricarballyic acid and citric acid (20).…”

Heterogeneous radiolysis of urea. Implications in astrobiology and prebiotic chemistry

“…In addition, the irradiation with protons [5], beta rays [6], and X rays [7] yield urea from a mixtures of gases (methane, ammonia, and water). It is also easily synthesized from the irradiation of hydrogen cyanide (HCN) in a liquid or solid state [8]. Additionally, it has been suggested that the participation of urea in different reactions could have favored the synthesis of more complex organic compounds.…”

Chemical Evolution: An Approach From Radiation Chemistry

“…In the context of chemical evolution [68], irradiated propane and methane or carbon monoxide with high energy protons in the presence of water and ammonia, after an acid hydrolysis, detected the formation of amino acids. The behavior of over-irradiated frozen HCN solutions has been studied by Colin-Garcia et al [69,70]. Their analysis confirmed that gamma-ray irradiation generates several organic products, among them carboxylic acids, amino acids, and urea, all of which have biological relevance.…”