DNA Damage by the Direct Effect of Ionizing Radiation: Products Produced by Two Sequential One-Electron Oxidations

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It has long been assumed that the population of radicals trapped in irradiated DNA (that is, the radicals escaping recombination) would quantitatively account for the lesions observed in DNA. Recent results indicate that this is not the case. The yield of DNA lesions exceed the yield of trappable radicals. To account for a portion of this shortfall, it is thought that some of the initially formed 2′-deoxyribose radicals undergo a second oxidation by nearby base cation radicals to form 2′-carbocations. The carbocations react to give strand breaks and free base release. Schemes are presented to account for the major oxidation products observed including 8-oxoGua, 8-oxoAde, 5-OHMeUra, and free base release. Theoretical calculations were performed to ascertain the likelihood of the second oxidation step in these reaction pathways actually occurring, and to account for base sequence dependence and various levels of hydration.