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

Summary

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.

• thymine
• thymine glycol
• electron-affinic compounds
• OH radical
• radiation-induced oxidation