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.     

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 from X-irradiated single crystals of uridine-5'-phosphate disodium salt

X-irradiation of single crystals of uridine-5'-phosphate (disodium salt) between 10 and 300 K as well as storage of irradiated crystals at 300 K produces at least seven different radical species. Between 10 and 77 K, the uracil base anion and a secondary alkoxy radical at the ribose-O3'-site are formed. The latter transforms into a C5'-centred alkylphosphate species between 110 and 130 K which in turn decays between 180 and 220 K under formation of a base 5-yl hydrogen addition radical. Irradiation at 300 K additionally produces the base-located 6-yl radical together with a radical tentatively assigned to the doubly protonated base anion. Storage of crystals for several months results in decay of most of these species leaving a radical possibly located at c5' of the ribose. The spectral parameters of these radicals are given and discussed.     

Radical formation in salts of pyrimidines. I. 1-methyluracil. HBr crystals.

Radicals produced by X-irradiation at 77 K of 1-methyluracil. HBr crystals have been analysed by electron spin resonance spectroscopy. The results are compared with those obtained on 1-methyluracil crystals. Four radicals have been identified, two of which are present only in 1-methyluracil. HBr crystals: the C6-addition radical and the pyrimidine--anion radical. Warming-up experiments have been performed in order to study the secondary radical reactions. The different mechanisms proposed for the radiolysis of DNA constituents in the solid state are discussed in connection with these findings.     

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.     

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.     

Free radicals trapped in methyl alpha-D-mannopyranoside X-irradiated at 77 K: an ESR/ENDOR study

Four free radicals are trapped in methyl alpha-D-mannopyranoside X-irradiated and maintained at 77 K. All four have been identified, with high confidence levels, using ESR and ENDOR spectroscopy. One, an alkoxy radical located at O4, is characterized by a gmax of 2.059, an isotropic beta hydrogen hyperfine coupling (hfc) of 98 MHz, and small interactions due to gamma or delta hydrogens. The second, a secondary dioxyalkyl radical due to loss of hydrogen from C1 is characterized by one beta hfc with an isotropic component of 19.03 MHz. The third, a secondary hydroxyalkyl radical due to loss of hydrogen from C2 is characterized by two nonexchangeable hydrogens with isotropic beta interactions of 22.45 and 6.44 MHz and one exchangeable hydrogen with an isotropic beta interaction of 9.88 MHz. The fourth is a .CH2OR radical that is formed by the net loss of hydrogen from the methyl group.     

Free radicals in malonic acid: A study of radical conformations at 77 K and 295 K by the ESR-technique

Summary Single crystals of malonic acid (CH₂(COOH)₂), have been irradiated with 4.0MeV electrons at liquid nitrogen temperature. The induced radicals have been identified by electron spin resonance technique. ESR lines due to different conformations of the H₂(COOH) radical are observed. ESR lines due to the OOH radical are also seen. It is suggested that the fragmentation into these radicals takes place preferentially on one side of the molecule and subsequent to the ionization event. Upon warming of the crystal to room temperature, the OOH radical disappears and the H(COOH)₂ radical is formed. The latter radical seems to exist in two slightly different modifications.     

Psoralen-photosensitized formation of free lipid radicals at 77 degrees K

It is shown by ESR method that 8-methoxypsoralen (8-MOP) under low temperature UV-irradiation has no influence on free radical formation of saturated fatty acids (FA). Under the same conditions the quantum yield of free radical photogeneration for unsaturated FA (oleate and limolenate) after addition of 8-MOP increased 7.5-15 times. The ESR spectrum of free radicals generated after interaction of 8-MOP with polyunsaturated FA, may be presented as superposition of signals-CH-(CH = CH)2-, -CH2CHCH2- and -CH2CH2. The molecular mechanism of photosensitized reactions is discussed.