The Nonenzymatic Decomposition of 5-Methylbarbituric Acid

Radiation-induced hydroxylation of thymine promoted by electron-affinic compounds

The effect of 20 electron-affinic compounds including nitroimidazoles, nitrofurans, nitrobenzenes, and quinones on the radiation-induced reaction of thymine in aqueous solution was studied under deaerated and N2O-saturated conditions. The radiolysis of thymine in aerated aqueous solution was also performed for comparison. Thymine decomposition was depressed to some extent by the addition of electron-affinic compounds in both deaerated and N2O-saturated solutions, while promoted in aerated solution. The radiolyses with varying concentration of misonidazole indicated that the depression of thymine decomposition can be attributed to a competition between thymine and electron-affinic compounds for the reactions with .OH. Among the radiolysis products, the formation of thymine glycol was remarkably promoted by the addition of electron-affinic compounds. Irrespective of structures of the electron-affinic compounds, the G-value of thymine glycol increased in sigmoidal form with increasing one-electron reduction potential of the electron-affinic compounds and attained the ultimate values of ca. 1.1 and 1.8 in deaerated and N2O-saturated solutions, respectively. The results are in accord with one-electron oxidation of the hydroxythymyl radical, produced by the reaction of thymine with .OH, to the corresponding cation by electron-affinic compounds. The so-formed hydroxythymine cation undergoes solvolytic substitution to give thymine glycol. Based on the ultimate G-values of thymine glycol, the difference in reactivity between hydroxythymine-5-yl and 6-yl radicals toward electron-affinic compounds is discussed.

Gamma radiolysis of thymine in oxygen-free aqueous solution in the presence of electron affinic radiosensitizers: identification of stable products

Radiosensitizers react with nucleic radicals by addition and by electron transfer reactions. We report the steady-state gamma radiolysis of 1 mM thymine in oxygen-free aqueous solutions containing different classes of radiosensitizing drugs: N-oxyl-free radicals (TAN and TMPN), quinones (menadione and naphtoquinone), nitroheterocyclic compounds (metronidazole and 5-nitro-2-furoic acid) and N-ethylmaleimide. Two classes of thymine degradation products were isolated by thin-layer chromatography and characterized by spectroscopic measurements. The main products, irrespective of radiosensitizers, resulting from oxidation reaction were identified as the cis and trans isomers of 5,6-dihydroxy-5,6-dihydrothymine, N-pyruvyl-N'-formylurea, 6-hydroxy-5,6-dihydrothymine and 5-hydroxy-5,6-dihydrothymine. In the experimental conditions used only N-oxyls and to a lesser extent NEM reacted with 5-hydroxy-5,6-dihydrothymin-6-yl radical, giving stable covalently-bonded addition products with a high yield. TAN showed a higher binding ability with respect to TMPN, which is in good agreement with the rate-constants previously reported for these bimolecular reactions.