What is the initial chemical precursor of DNA strand breaks generated by direct-type effects?

The present study tests the hypothesis that the majority of DNA strand breaks produced by direct-type effects are due to sugar free radical precursors and that these radicals are produced by direct ionization of the sugar-phosphate backbone or by hole transfer to the sugar from tightly bound water. Well-defined crystalline DNA samples of d(CGCG)(2), d(CGCACG:GCGTGC), d(GTGCGCAC)(2), and d((GCACGCGTGC)(2) were irradiated at 4 K, and their free radical dose response determined from 0 to 1800 kGy. A model is proposed that effectively describes the dose response curves. It includes the following parameters: the free radical concentration at saturation C(max), the free radical yields G(b) and G(s), and the destruction constants k(b) and k(s). The subscripts b and s refer to base-centered and sugar-centered radicals, respectively. In each of these systems, the free radical concentration exhibits a remarkable resistance to dose saturation up to at least 1500 kGy. As predicted, G(b) > G(s), the G(b)/G(s) ratio varying between 4 and 12. Likewise, k(b) > k(s), the k(b)/k(s) ratio varying between 28 and 81. The lower cross-section for destruction of the sugar-centered radicals is consistent with the expectation that they are relatively radiation resistant. G(b)/G is between 0.81 and 0.92, indicating that at low doses the bases trap out 80-90% of the total free radical population. The remaining 10-20% are located on the sugar. At high dose, a larger fraction of the radicals are trapped on the backbone as seen from the ratio C(mxS)/C(mxB), which ranges from 3.5 to 8. This unusually late onset of dose saturation closely parallels that observed for strand break products in earlier studies. There is, therefore, a good correlation between the dose response profiles of sugar-trapped radicals and strand breaks. These observations strongly support the hypothesis that sugar radicals are precursors to the majority of strand breaks produced by the direct-type effect in DNA.