The radiolysis of aqueous solutions of ethylene

The products of the radiolysis of deaerated aqueous solutions of ethylene at atmospheric pressure are butane, hexane, octane, decane, acetaldehyde, butyraldehyde, ethanol, butanol, and hydrogen perioxide. Product yields, especially those of the aldehydes and hydrogen peroxide, increase with increasing ethylene concentration and decrease with increasing pH and radiation dose. Addition of efficient electron scavengers increases the yields of oxidized products and decreases the yields of reduced products. The results are compatible with a mechanism in which OH radicals add to rather than abstract from ethylene. Computer-aided analysis of the kinetics of competing reactions indicates that the low yields of aldehydes are related to the ability of these compounds to act as internal scavengers of solvated electrons.     

The effect of oxygen on the radiolysis of tyrosine in aqueous solutions

The effect of oxygen on the radiolysis of tyrosine in aqueous solutions was investigated by using gamma and pulsed electron irradiation. Steady-state radiolysis was reexamined and extended to include the effect of pH and determination of hydrogen peroxide. The loss of tyrosine, G(-Tyr), during irradiation and yields of 3,4-dihydroxyphenylalanine, G(DOPA), and hydrogen peroxide, G(H2O2), are determined in the pH range from 1 to 9. In the whole pH range used G(-Tyr) equals G(DOPA), and a higher G(H2O2) than expected was observed. In slightly acid and neutral media, both G(-Tyr) and G(DOPA) equal the yield of hydroxyl radicals, GOH, formed in the radiolysis of water, while the excess of hydrogen peroxide equals 1/2 GOH. Hence it was concluded that all tyrosine OH-adducts react with oxygen yielding peroxy radicals. In acid and alkaline media all measured yields decrease. This is caused by formation of tyrosine phenoxyl radicals (TyrO), which react with superoxide anion (O2-) and hydroperoxy (HO2) radicals regenerating tyrosine. By using pulse radiolysis K(TyrO + O2) less than or equal to 2 X 10(5) mol-1 dm3 s-1 and k(TyrO + O2-) = (1.7 +/- 0.2) X 10(9) mol-1 dm3 s-1 were determined. On the basis of the results, a reaction mechanism is proposed.     

Pulsed radiolysis of aqueous solutions of serum albumin containing naphthoquinones

As was shown by the pulse radiolysis method the simultaneous presence of naphthoquinone and human serum albumin molecules in an aqueous solution leads to the adsorption of the former on the surface of the latter. It is suggested that in these conditions the protein tertiary structure changes. New conformation reduces the reactivity of albumin toward the hydrated electron.     

The pulse radiolysis pH-jump in aqueous solutions: applications to lanthanide(III)-methyl red systems

Use of the pulse radiolytic technique for studying acid-base and complex formation reactions in aqueous solutions is described. The method is based on the conversion of the hydrated electron e⁻_aq to the anion of a strong acid, thereby obtaining a fast pH-jump. Rate constants are calculated from relaxation times measured with either spectrophotometric or conductometric detection systems. The technique has been validated with previously studied chemical systems. Rate constants for the complex formation between La³⁺ and Gd³⁺ with Methyl Red indicator have also been determined.     

Osmometric studies on self-association of pyrimidines in aqueous solutions: evidence for involvement of hydrophobic interactions.

Vapour pressure osmometric studies were performed on stacking self-association of 25 uracil derivatives variously C- and N-substituted with polar and alkyl groups in aqueous solution at various temperatures. The respective equilibrium association constants Kst were computed on the assumption of the isodesmic model of self-association (K2=K3=...=Kn=Kst). Enthalpies of association for most of the compounds studied were obtained from the temperature-dependence of Kst, according to the van 't Hoff equation. Analysis of the equilibrium and thermodynamic parameters in terms of the association mechanism demonstrated the involvement of classical hydrophobic interactiors in the stabilization of complexes of di-and higher alkylated uracils. Data for the derivatives substituted with polar groups proved consistent with the predominant involvement of dipole-induced dipole forces in the association.     

Final products obtained from the gamma radiolysis of frozen aqueous solutions of thymidine.

The final radiation products obtained by gamma-irradiation of frozen aqueous solutions of thymidine have been identified as 5,6-dihydro-5,6-dihydroxythy-midine, 5,6-dihydrothymidine, thymidine dimers, 1-(2-deoxy-beta-D-threo-pento-furanosyl)-thymine, 1-(2-deoxy-alpha-L-threo-pentofuranosyl)-thymine, thymine and 5,6-dihydrothymine. The nature of the radiation products could be explained on the basis of the radical structures reported earlier.     

Sonolysis, radiolysis, and hydrogen peroxide photolysis of pyrimidine derivatives in aqueous solutions: a spin-trapping study

The sonolysis of aqueous solutions of various dihydropyrimidines and substituted pyrimidines was investigated by ESR and spin trapping with the nonvolatile, water soluble spin trap, 3,5-dibromonitrosobenzene sulfonate (DBNBS) and its deuterated analog to examine the possibility of detecting new radicals specifically generated in the high temperature zones produced by collapsing cavitation bubbles. Similar ESR spectra were obtained from sonolysis of argon-saturated aqueous solutions, from uv photolysis of aqueous solutions containing H2O2, and from gamma radiolysis of nitrous oxide saturated solutions, although sonolysis of aqueous solutions leads to the formation of pyrimidine radicals by H atom as well as OH radical addition to the 5,6 double bond of pyrimidines. No evidence for specific new radicals formed in the high temperature regions induced by cavitation could be found. For the reactions of dihydropyrimidines with hydroxyl radicals additional spin adducts could be detected and identified with the spin trap DBNBS compared to 2-methyl-2-nitrosopropane which was used in previous studies; however, for alkylpyrimidines fewer spin adducts were observed. The use of the deuterated analog of DBNBS is helpful for unambiguous radical structure assignment.     

Radiation-induced conversion of 5,6-dihydropyrimidines to the corresponding pyrimidines in N2O-saturated aqueous solutions

The effect of transition metal salts on the radiation-induced conversion of 5,6-dihydropyrimidines to the corresponding parent pyrimidines was studied in N2O-saturated aqueous solution at pH 7.0. The yield of the pyrimidines increased in sigmoidal forms with the increased one-electron reduction potential of the transition metal salts. The radiolysis of 5,6-dihydroorotic acid suggested that the 6-yl radical of the acid undergoes oxidation by transition metal salts to give orotic acid, whereas the corresponding 5-yl radical readily liberates CO2 to give uracil radical anion.     

E.s.r. of free radicals in aqueous solutions of substituted pyrimidines.

Reactions of OH radicals with methyl and ethyl derivatives of uracil, cytosine and thymine in aqueous solutions have been investigated. Photolysis of H2O2 was used to generate OH radicals and the radicals on the base derivatives were spin-trapped using t-nitrosobutane and identified with the help of e.s.r. spectroscopy. Addition of OH radicals was found to take place predominantly to the C(5)--C(6) double bond of the bases. H-abstraction from the methyl group occurred in the N(1) methyl derivatives of uracil, cytosine and thymine. Radicals formed by H-abstraction from the methyl group were also detected for 3-methyluracil, thymine, 1-methylthymine and 1-ethylthymine. Introduction of a methyl or ethyl group at the N(1) position of uracil, cytosine and thymine causes an increase in the C(6) proton coupling and a decrease in the N(1) splitting for radicals formed by OH addition at the C(5) position.     

The radiolysis of aqueous propionitrile: Compounds of interest to chemical evolution studies

Summary Oxygen-free aqueous solution of CH₃CH₂CN (0.1 M, pH 6) were exposed to gamma rays from a⁶⁰Co source and the mixture of radiolytic products fractionated. The separated fractions were analysed by magnetic resoncance methods (EPR, NMR), spectrophotometry (UV-VIS-IR), gas chromatography and amino acids analysis. About 70% of radiolytic products consist of non volatile material. A large variety of compounds was detected: propionaldehyde, acetaldeyde, sixteen carboxylic acids and, in the hydrolysate, eleven protein and nonprotein amino acids. About 20% of the nonvolatile radiolytic products have a hydrophobic character and can be extracted with chloroform. Among them is a long-living nitroxide free radical which is stable for months at room temperature. It has been suggested that the nitroso compounds are formed as radiolytic products, and that they act as spin-traps by converting some of the short living radicals to the observed nitroxide radical. This and other experimental findings are discussed in the light of free radical reactions induced by ionizing radiation.     

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

Summary Oxygen-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 radiolysis of aqueous ammonium cyanide: polymers of interest to chemical evolution studies

Some of the polymers isolated from the mixture of products obtained upon irradiation of dilute oxygen-free solutions of NH4CN with gamma rays from a 60Co source were examined. Their molecular weights were estimated to be in the range 4000-20000 daltons; elements of their structure were inferred from chemical and spectroscopic studies; and their interactions with nucleic acid components were examined. Significance of these results for chemical evolution studies is summarized.