Isolation and identification of thymine products from DNA γ irradiated in oxygen-free aqueous solutions

A solution of Escherichia coli DNA 14CH3 labelled in the thymine moiety was deaerated under vacuum by repeated cycles of freezing and pumping; sample was irradiated in sealed vial by 60Co γ rays with a dose rate of 70 Gy/m at Gy(gy = gray). The resulting polymeric material was separated from the low-molecular weight material by dialysis. Formic acid hydrolysis of the high-molecular weight material yielded thymine, 5,6-dihydrothymine, cis- and trans-5,6-dihydroxythimine, and 5-hydroxy-5,6-dihydrothymine. The initial step of a proposed mechanism involves the formation of hydroxyl radicals, hydrogen atoms, and hydrated electrons, which are formed when water absorbs gamma rays. These species are short-lived and react with DNA. The hydroxyl radicals attack both the C-5 and C-6 sites of the thymine ring; the hydrated electrons react pyrimidine to form ions which then undergo protonation; and hydrogen atoms add preferentially at C-6. The pyrimidine radicals are then able to undergo electron transfer through a dismutation reaction. Solvolytic substitution of the pyrimidic ions and hydrogen atom exchange produce the final reaction products