One-electron oxidation of DNA by ionizing radiation: competition between base-to-base hole-transfer and hole-trapping.

The distance of hole migration through DNA determines the degree to which radiation-induced lesions are clustered. It is the degree of clustering that confers to ionizing radiation its high toxicity. The migration distance is governed by a competition between hole transfer and irreversible trapping reactions. An important type of trapping is reactions that lead to the formation of deoxyribose radicals, which are precursors to free base release (fbr). Using HPLC, fbr was measured in X-irradiated films of d(CGCGCGCGCG)(2) and d(CGCGAATTCGCG)(2) as well as three genomic DNAs: M. luteus, calf thymus, and C. perfringens. The level of DNA hydration was varied from Gamma = 2.5 to 22 mol waters/mol nucleotide. The chemical yields of each base, G(base), were measured and used to calculate the modification factor, M(base). This factor compensates for differences in the GC/AT ratio, providing a measure of the degree to which a given base influences its own release. In the DNA oligomers, M(Gua) > M(Cyt), a result ascribed to the previously observed end effect in short oligomers. In the highly polymerized genomic DNA, we found that M(Cyt) > M(Gua) and that M(Thy) is consistently the smallest of the M factors. For these same DNA films, the yields of total DNA trapped radicals, G(tot)(fr), were measured using EPR spectroscopy. The yield of deoxyribose radicals was calculated using G(dRib)(fr) = approximately 0.11 x G(tot)(fr). Comparing G(dRib)(fr) with total fbr, we found that only about half of the fbr is accounted for by deoxyribose radical intermediates. We conclude that for a hole on cytosine, Cyt(*+), base-to-base hole transfer competes with irreversible trapping by the deoxyribose. In the case of a hole on thymine, Thy(*+), base-to-base hole transfer competes with irreversible trapping by methyl deprotonation. Close proximity of Gua protects the deoxyribose of Cyt but sensitizes the deoxyribose of Thy.