Mechanisms of Strand Break Formation in Dna Due to the Direct Effect of Ionizing Radiation: The Dependency of Free Base Release on the Length of Alternating CG Oligodeoxynucleotides

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The question of how NA base sequence influences the yield of DNA strand breaks produced by the direct effect of ionizing radiation was investigated in a series of oligodeoxynucleotides of the form (d(CG)n)2 and (d(GC)n)2 The yields of free base release from X-irradiated DNA films containing 2.5 waters/nucleotide were measured by HPLC as a function of oligomer length. For (d(CG)n)2, the ratio of the Gua yield to Cyt yield, R, was relatively constant at 2.4-2.5 for n = 2-4 and it decreased to 1.2 as n increased from 5 to 10. When Gua was moved to the 5′ end, for example going from d(CG)5 to d(GC)5, R dropped from 1.9 ± 0.1 to 1.1 ± 0.1. These effects are poorly described if the chemistry at the oligomer ends is assumed to be independent of the remainder of the oligomer. A mathematical model incorporating charge transfer through the base stack was derived to explain these effects. In addition, EPR was used to measure the yield of trappeddeoxyribose radicals at 4 K following X-irradiation at 4 K. The yield of free base release was substantially greater, by 50-100 nmol/J, than the yield of trapped-deoxyribose radicals. Therefore, a large fraction of free base release stems from a nonradical intermediate. For this intermediate, a deoxyribose carbocation formed by two one-electron oxidations is proposed. This reaction pathway requires that the hole (electron loss site) transfers through the base stack and, upon encountering a deoxyribose hole, oxidizes that site to form a deoxyribose carbocation. This reaction mechanism provides a consistent way of explaining both the absence of trapped radical intermediates and the unusual dependence of free base release on oligomer length.