Radiation-induced reduction of thymine derivatives in aqueous solution. Part 4. Promoted transformation of thymine glycol into thymine by aromatic amines and low-valent transition metal salts

Abstract

The radiation-induced reduction of thymine glycol [(1a), cis-5,6-dihydroxy-5,6-dihydrothymine] in aqueous solution containing sodium formate (pH 7.0) has been studied under deaerated and N₂O-saturated conditions at room temperature. For comparison, the radiolysis of (1a) has also been performed with alcohols as a scavenger of ˙OH and H˙, instead of formate. γ-Irradiation of the deaerated formate solution gave thymine (2), 5,6-dihydrothymine (3), and 6-hydroxy-5,6-dihydrothymine (4) along with a minor product, 5-methylbarbituric acid (5). The radiolytic transformation of (1a) into these products was inefficient under N₂O, indicating that CO₂^–˙ as a primary active species produced by irradiation has much less ability to reduce (1a) compared with e_aq^–. The presence of either inorganic salts (6a–c) or aromatic amines (7e–f) greatly promoted the total formation of (2)+(3), which depressed those of (4) and (5). The sum of the G values (= molecules/100 eV of absorbed energy) of (2) and (3){G[(2)+(3)]} increased and that of (4){G[(4)]} decreased, respectively, in the sigmoidal forms with decreasing one-electron oxidation potential of the amines. The ratio G[(2)+(3)]/G[(4)] increased upon increasing the concentration of o-phenylenediamine (7e). In the absence of added reductants, (6a–e) or (7a–g), the yields of (2)+(3) and (4) changed either with the formate concentration or with variation of alcohols used as radical scavengers. The mechanism involving consecutive one-electron reductions of (1a) and an intermediate 6-hydroxythymin-5-yl radical (8) is discussed and compared with the reduction of 5-bromo-6-hydroxy-5,6-dihydrothymine (1b).