The Radiation Chemistry of Aqueous Solutions of Hydrogen Cyanide in the Megarad Dose Range

The radiolytic behavior of HCN was studied in dilute (0.1 M) oxygen-free solutions at their natural pH and at pH 2.4. The solutions were exposed to high doses of 60 Co gamma rays (up to 16 Mrad) causing large decompositions (up to 90%). The decomposition yield, G(-HCN), and the radiation-chemical yields of formation of NH3, HCHO, H2, CO2, and of seven amino acids were determined. Also, the total yields of nitrogen in nitrogen-containing nonvolatile compounds, G(N), were measured. The irradiated solutions exhibited positive biuret reaction and the absorbancies depend on both the dose and the initial pH. Histidine, aspartic acid, threonine, serine, glutamic acid, alanine, and glycine were found in irradiated samples. Glycine was found to be the most abundant among the radiolytically produced amino acids; strong acid hydrolysis increases its yield by one or two orders of magnitude in samples irradiated initially at pH 2.4 as well as at their natural pH. The release of amino acids on hydrolysis and the positive biuret reaction suggest the presence of peptidic material in irradiated samples both at acid and neutral pH's. To account for these findings the formation of polymers (or their segments) having a peptidic structure was taken into consideration. The polymerization reactions were assumed to be initiated by the free radicals which were produced by HCN reactions with H, OH, and $e_{{\rm aq}}{}^{-}$ species. It has been concluded that the radiolysis of dilute aqueous solutions of HCN offers significant information on the eventual role of ionizing radiation in prebiotic chemical evolution in aqueous media.