Z. D. Draganić

Aldehydes, Ketones, and Carboxylic Acids Formed Radiolytically in Aqueous Solutions of Cyanides and Simple Nitriles

A systematic search for aldehydes, ketones, and carboxylic acids was carried out in aqueous solutions of HCN, NH/sub 4/CN, CH/sub 3/CN, and C/sub 2/H/sub 4/CN, that had received multikilogray doses of /sup 60/Co ..gamma.. radiation. About 30 radiolytic products were identified, among them a large variety of dicarboxylic and tricarboxylic acids. Some of them might be of significant interest in molecular evolution studies of prebiotic processes. They originate in the free-radical-initiated chemical reactions where the additional oligomerization processes are particularly important. Most of the radiolytic products appear in both cyanides and nitriles and point to the importance of reactions involving the carbon-nitrogen triple bond.

Radiolysis of aqueous solutions of ammonium bicarbonate over a large dose range

Oxygen-free aqueous solutions of 0.05 mol dm-3 ammonium and sodium bicarbonate were studied after receiving various doses of 60Co gammas (0.01–400 kGy) or 0.5–20 Gy pulses of 10 Mev electrons. Formate and oxalate were found to be the main radiolytic products, in addition to trace amounts of formaldehyde and an unidentified polymer. A large initial yield of formate in the γ-radiolysis, G(HCOO-)=2.2, is due to the reaction COO- + HCO-3⤦HCOO-+CO-3. The efficiency of organic synthesis within the large dose range studied is low and is explained by efficient pathways leading to the reformation of bicarbonate, where the reaction COO-+CO-3 is particularly significant. Computer fitting of the data obtained gives k(COO- + HCO-3)=(2±0.4) x 103 dm3 mol-1 s-1, k(COO-+CO-3) = (5±1) x 107 dm3 mol-1 s-1, k(NH2+HCO-3)< 104 dm3 mol-1 s-1, and k(NH2+CO-3) = (1.5±0.5) x 109 dm3 mol-1 s-1.

The Radiolysis of Aqueous Ammonium Cyanide: Compounds of Interest to Chemical Evolution Studies

SummaryOxygen-free aqueous solutions of NH4CN (0.1 M, pH 9) were exposed to gamma rays from a60Co source, the mixture of nonvolatile products was fractionated, and the fractions were analyzed. The procedures were chosen to make effective investigations of radiolytic products, and to minimize the contributions of chemical changes which are known to occur in aqueous solution in the absence of ionizing radiation. It has been found that the main constituents are: urea, 25.9%; an oligomer, very likely oligoimine (18.4%); and several fractions (about 50%) which release amino acids on hydrolysis. These fractions differ considerably, as shown by amino acid assay, enzymatic digestion, IR spectra, and biuret reaction. All these tests were found to be positive for two fractions; in two further fractions the enzymatic cleavage was absent, but other tests were positive. Negative enzymatic and biuret tests, and no bands characteristic of amide or peptide, were found for a fraction whose hydrolysate consisted of 55% glycine. Although most of the isolated materials were found to be composite, the results of the analyses were sufficient for getting a reliable over-all picture of the chemical action of the ionizing radiation. The role of free radicals in reactions leading to the formations of radiolytic products was considered.

Radiation chemistry of a multicomponent aqueous system relevant to chemistry of cometary nuclei

SummaryWe have examined a water-dominated multicomponent system after irradiation in the multimegarad dose range with gamma rays from a60Co source at both 77 and 310 K. The constituents were simple organic compounds in the proportions in which they appear in a dense interstellar cloud: HCN/CH3OH/CH3CN/C2H5CN/HCOOH=1∶0.6∶0.2∶0.1∶0.05. The total amounts were adjusted to correspond to a carbon to nitrogen ratio of 1.8 and a water content of about 50% in a cometary nucleus where the dust to volatiles ratio is 1; the total amount of CN-bearing compounds was taken to correspond to 0.4% of the cometary mass. In experiments at 310 K about 40 radiolytic products are identified, among them aldehydes and amino and carboxylic acids. Abundant polymeric material (Mw up to 80,000 daltons) is formed. The basic aspects of radiolysis of the liquid system are present also at 77 K, although at radiation-chemical yields that are lower by one to two orders of magnitude. We have considered the relevance of the present findings to the chemistry of a liquid-water core and the icy layers of a cometary nucleus.

Radiation-chemical aspects of chemical evolution and radiation chemistry of simple cyano compounds

Abstract The role of ionizing radiation as one of energy sources for chemical evolution processes is examined. The discovery of fossile nuclear reactors in old uranium deposits of Oklo (Africa) suggest that natural nuclear reactors could have have been the sources of abundant, localized, radiation in the past of our planet. A brief survey of radiation chemistry of aqueous solutions of cyanides and simple nitriles is given. Some results obtained at BKI (Vinca) suggest that simple cyano compounds could have been an important raw material for primordial organic synthesis in aqueous medium and ionizing radiation as the source of energy. It has been concluded that the studies of chemical evolution can be a rewarding domain for radiation chemistry to transfer some of its achievements.

Radiolysis of aqueous solutions of hydrogen cyanide (pH∼6): Compounds of interest in chemical evolution studies

SummaryOxygen-free aqueous solutions of hydrogen cyanide, 0.1 M and pH∼6, were exposed to gamma rays from a60Co source, the mixture of nonvolatile products was fractionated and the fractions were analysed. It has been found that the complex mixture contains oligomers and polymers with molecular weights up to 20,000 daltons, mainly polyamides with urea and peptidic fragments. Among the constituents are carbamyl glycinonitrile and carbamyl glycinamide that represent 6.4% and 3.1% of the total of unfractionated material respectively. Urea content is 2.6%, but the derivatives of urea are more abundant. Acid hydrolysis releases several amino acids. Glycine is the most abundant (75% or more of total amino acid content), and its concentration considerably increases in some fractions when the hydrolysis is carried out at 130°C. The role of free radicals in reactions leading to the formations of radiolytic products is considered. Some comparisons are made between findings in the present work, at initial pH∼6, and an earlier study of ammonium cyanide at pH 9.

Evidence for amino acids in hydrolysates of compounds formed by ionizing radiations. I. Aqueous solutions of HCN, NH4CN, and NaCN.

Dilute, O2-free aqueous cyanides were exposed to multikilorad doses of a radioactive cobalt source. After the removal of unreacted cyanides and of volatile radiolytic products, the residue was ydrolyzed and the resulting material analyzed for amino acids. The results show the presence of five protein amino acids and five amino acids which do not occur in natural proteins. The amino acids of enantiomeric derivatives separated on an optically active column, appeared to consist of approximately equal amounts of D and L isomers. Radiation-chemical yields of amino acids were determined at various radiation doses. The results obtained support the previous findings that the free-radical initiated process is the source of oligomers which on hydrolysis release the amino acids.

The radiolysis of aqueous acetonitrile - Compounds of interest to chemical evolution studies

SummaryOxygen-free aqueous solutions of CH3CN (0.1 M, pH 6) were exposed to gamma rays from a60Co source, the mixture of nonvolatile radiolytic products was fractionated and the fractions were analysed. Succinic, maleic, fumaric, malonic and pyruvic acids were identified. Glycol aldehyde, glucose and probably ribose were observed in the hydrolysate of fractionated material. It has been suggested that an oligomer is formed which has a fragment with the polyhydroxy structure and on hydrolysis releases the carbohydrates. Radiolytic products which release amino acids on hydrolysis were found in several fractions. The amino acid contents of the hydrolysates were up to about 2.8% of the fraction mass. The presence of several protein and nonprotein amino acids suggests that their origin should be in a peptidic structure, which is probably a fragment of an oligomer radiolytically produced. A direct analysis of the irradiated solution shows the presence of acetaldehyde, propionaldehyde, glyoxal and of biacetyl. Experimental findings are discussed and a free-radical mechanism is proposed to account for the chemical changes observed.

The Radiation Chemistry of Aqueous Solutions of Ammonium and Sodium Cyanides in the Megarad Dose Range

to large doses of 60Co -y rays. Radiation-chemical yields were determined for the decomposition of cyanide molecules, as were the total amount of nitrogen in nonvolatile nitrogen-containing radiolytic products, the free ammonia, and the yields of several amino acids released on acid hydrolysis of irradiated samples. Positive biuret reaction was found and its intensity used for an estimation of amide bond yields. Up to 18% of the nitrogen from nonvolatile radiolytic products appears in the amide bond, and up to 14% appears in natural amino acids. The origin of peptidic material is attributed to the polymerization initiated by cyano radicals produced in reactions of cyanides with OH, H, and eaq- species from radiolyzed water.

Infrared spectral characterization of peptidic material produced by ionizing radiation in aqueous cyanides

SummaryOligomers formed by ionizing radiation in aqueous cyanide solutions, under various experimental conditions, have been characterized by infrared spectroscopy. IR bands appear in the region known to be characteristic for amides and peptides. The results are discussed in relation to radiation-induced formation of peptidic material and the potential role of ionizing radiation as an energy source for some processes in prebiotic molecular evolution.