Radiation-induced Hydroxylation of Thymine Promoted by Electron-affinic Compounds

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Abstract
The effect of 20 electron-affinic compounds including nitroimidazoles, nitro-furans, 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 [1.0364 .cntdot. 10-7 mol material formed or destroyed/J absorbed] of thymine glycol increased in sigmoidal form with increasing 1-electron reduction potential of the electron-affinic compounds and attained the ultimate values of .apprx. 1.1 and 1.8 in deaerated and N2O-saturated solutions, respectively. The results are in accord with 1-electron oxidation of the hydroxythymylradical, produced by the reaction of thymine with .OH, to the corresponding cation by electron-affinic compounds. The so-formed hydroxy-thymine 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.

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