Reactions of free radicals in γ-irradiated DNA in the presence of oxygen

Reactions of free radicals with oxygen have been studied by means of the ESR method in γ-irradiated at 77 K dry and moistened preparations of native DNA (NDNA) and single-stranded DNA (SDNA). At the temperatures above 180 K peroxide radicals have been found to form as a result of oxidation of thymine and cytosine radicals. Peroxide radicals are formed more readily in SDNA than in NDNA and in a moistened biopolymer. The concentration of peroxide radicals is maximal at 220-230 K; and at the temperature above 250 K peroxide radicals disappear. Under the effect of light with lambda <300 mμ at 77 K, part of peroxide radicals enters the reactions which are fatal for them. In addition to peroxide radicals, a singlet, having a width of approximately 13 Oe, has been observed in the ESR spectrum of DNA in the presence of oxygen. The part that radiooxidation reactions of radicals in DNA play in the oxygen radiobiological effect is discussed