Mechanisms of direct radiation damage to DNA: the effect of base sequence on base end products

It has been generally assumed that product formation in DNA damaged by ionizing radiation is relatively independent of base sequence, i.e., that the yield of a given product depends primarily on the chemical properties of each DNA constituent and not on its base sequence context. We examined this assumption by comparing direct-type end products produced in films of d(CTCTCGAGAG)(2) with those produced in films of d(GCACGCGTGC)(2). Here we report the product yields in d(CTCTCGAGAG)(2) hydrated to Γ = 2.5 and 15, where Γ is the hydration level given in moles of H(2)O/mole of nucleotide. Of the 17 products monitored by GC/MS, seven exhibited statistically significant yields: 8-oxoGua, 8-oxoAde, 5-OHMeUra, 5,6-diHUra, 5,6-diHThy, 5-OHCyt, and 5-OHUra. These yields at Γ = 2.5 are compared with the yields from our previously reported study of d(GCACGCGTGC)(2) (after projecting the yields to a CG/AT ratio of 1). The ratio of projected yields, d(CTCTCGAGAG)(2) divided by d(GCACGCGTGC)(2), are 1.3 ± 0.9, 1.8 ± 0.3, 1.6 ± 0.6, 11.4 ± 4.7, 0.2 ± 0.1, >28, and 0.8 ± 1.1, respectively. Considering just d(CTCTCGAGAG)(2), the ratios of yields at Γ = 2.5 divided by yields at Γ = 15 are 0.7 ± 0.2, 0.5 ± 0.1, 2.3 ± 4.0, 3.4 ± 1.2, 3.5 ± 3.3, 1.2 ± 0.2, and 0.4 ± 0.2, respectively. The effects of sequence and hydration on base product yields are explained by a working model emphasizing the difference between two distinctly different types of reaction: (i) radical reactions that progress to nonradical intermediates and product prior to dissolution and (ii) reactions that stem from radicals trapped in the solid state at room temperature that go on to yield nonradical product after sample dissolution. Based on these findings, insights into rates of hole and excess electron-transfer relative to rates of proton transfer are discussed.