Free radicals in pyrimidines:single crystals of dihydro-6-methyl uracil irradiated and observed at 77 K.

Electron-spin-resonance measurements have been made on single crystals of dihydro-6-methyl uracil, which were irradiated by 4-0 MeV electrons at 77 K. At low temperature, four radicals have been identified. Two, the C5 and C6 hydrogen abstraction radicals, were also studied in a previous work. On exposure of U.V., the C6 abstraction radical was found to convert into the C5 abstraction radical. Annealing at room temperature reversed the process. The other radicals identified are that formed after H-atom addition to O4, and that formed after H-atom abstractions from N1, C5, and C6. The latter was not present in deuterated crystals. Both radicals were found to be unstable at intermediate temperatures. Thermoluminescence glow-curves were found to correlate to the formation and the decay of these radicals. Semi-empirical INDO MO-calculations have been performed for identification purposes.

E.s.r. studies of sulphur-substituted pyrimidines. 2-thio-5-carboxyuracil at 77 K.

Single crystals of 2-thio-5-carboxyuracil were irradiated and studied at 77 K with e.s.r. spectroscopy. Five resonances were observed and related to the sulphur atom in the 2 position of the pyrimidine ring. Three of the resonances have been assigned to three conformations of a radical formed by hydrogen abstraction from N1. The principal values for the nitrogen coupling are 9-7, 0-0 and 0-0 gauss. The g tensor principal values are 2-173, 1-997 and 1-990 for the dominant conformation of this radical. Two other radicals could not be identified unambiguously.

E.s.r. of spin-trapped radicals in gamma-irradiated polycrystalline amino acids, N-acetyl amino acids and dipeptides.

The radicals produced in several polycrystalline amino acids, N-acetyl amino acids and dipeptides by gamma-radiolysis at room temperature were investigated by spin-trapping. After irradiation in the solid state, the samples were dissolved in aqueous solutions f t-nitrosobutane and the trapped radicals identified by e.s.r. For alpha-amino acids, deamination radicals were found, and in some cases H-abstraction radicals were also observed. No decarboxylation radicals could be detected. For N-acetyl amino acids, except for N-acetylglycine, the major radical was the decarboxylation radical. For N-acetyglycine the H-abstraction radical from the glycine residue was observed. For dipeptides of the x-glycine, the radical formed by removal of H from the alpha-carbon of the carboxyl-terminal residue was always spin-trapped. Some primary deamination radicals and minor amounts of decarboxylation radicals could also be observed. For dipeptides of the type x-alanine, glycine-x and alanine-x, the decarboxylation radical was always the major spin-trapped radical. Some primary and secondary deamination radicals were also detected.

Free radicals in dicarboxylic acids: an e.s.r. study of radical conversions in gamma-irradiated single crystals of glutaric acid and glutaric-2,2,4,4-d4 acid.

The gamma-radiation-induced free radicals in single crystals of glutaric acid and glutaric-2,2,4,4-d4 acid were studied in the temperature range 77-300 K by e.s.r. techniques. At 77 K the decarboxylation radical and the anion are stabilized. At higher temperatures the decarboxylation radical is found to be converted into a hydrogen abstraction radical with an activation energy of 6.3 +/- 0.5 kcal/mole for the non-deuterated crystal. This radical is stable at room temperature. The anion seems be be converted to an unidentified intermediate radical which in turn is converted to the gamma-acyl radical. An analysis of the g-value anisotropy and of the 13C hyperfine splitting variation for this radical in the deuterated crystal is consistent with the assigned radical structure. By heat treatment the alpha-acyl radical is converted to another form of the hydrogen abstraction radical with an activation energy of 9.6 +/- 0.6 kcal/mole in the deuterated crystal. U.V.-light (gamma = 254 nm) transforms one of the room temperature radicals into the other.

E.s.r. of spin-trapped radicals in aqueous solutions of deuterated amino acids and alcohols.

Selectively deuterated compounds can be utilized in spin-trapping to obtain conclusive evidence for the structures of the trapped radicals. Radicals formed from DL-alanine-2-d1, DL-alanine-3, 3, 3-d3, DL-alanine-d4, glycine-d5, isopropyl-2-d1 alcohol, isopropyl-d7 alcohol and t-butyl-d9 alcohol were studied. The hydrogen abstraction, deamination and decarboxylation radicals generated from deuterated alanines were investigated by spin-trapping using t-nitrosobutane. Since nitroxides of the type XYZC-CH2-N(O)-tBu which contain an asymmetric centre next to the CH2 group exhibit unequal beta-proton splittings and temperature-dependent line width alternations, the effects of temperature on the e.s.r. spectra were studied. By these methods the structures of the trapped radicals were verified and the hyperfine coupling constants could be assigned without ambiguity. In gamma-irradiated polycrystalline amino acids, the observed deuterium exchange can be conveniently surveyed by dissolution in aqueous solutions containing t-nitrosobutane.

E.s.r. of free radicals in irradiated uracil-beta-D-arabinofuranoside.

Electron-spin-resonance measurements have been made on single crystals of uracil-beta-D-arabinofuranoside, which were irradiated by 4-0 MeV electrons at 77 K. At low temperatures, two radicals have been identified, one attributed to a hydrogen abstraction from 05' in the sugar moiety and the other to a radical anion located on the pyrimidine ring. The former is very unstable and seems to act as a precursor to other unidentified radical species stable at 77K. At room temperature, the main resonance is due to hydrogen addition to C5 and is probably produced by protonation of the anion. This same radical is also produced by X-irradiation at room temperature.

Free radicals in dicarboxylic acids: an e.s.r. study of gamma-irradiated single crystals of glutaric acid at 77 K.

Electron spin resonance techniques were used to study the gamma-radiation-induced free radicals in single crystals of glutaric acid in the temperature range from 77 K to 300 K. Three different radicals are stabilized at 77 K. The decarboxylation radical is the dominant species and the other two radicals are assigned to the anion and to the substituted acetyl sigma-radical. When the temperature of the crystal is raised, these radicals disappear and the previously studied room temperature radicals appear. E.S.R.-data and the results from semi-empirical INDO-MO calculations were compared in order to elucidate the structures of the various radicals.

Free radicals in pyrimidines: 5-thymyl radical nitrogen splitting in irradiated thymidine.

Single crystals of thymidine irradiated with 4.0 MeV electrons exhibit the well-known eight-line e.s.r. spectrum due to the 5-thymyl radical. A careful analysis of these lines revealed an extra hyperfine splitting, which by a combination of INDO-MO calculations and experiments, was attributed to an interaction between the unpaired electron and the N3-H group.