An Investigation into the Mechanisms of DNA Strand Breakage by Direct Ionization of Variably Hydrated Plasmid DNA

The mechanisms by which ionizing radiation directly causes strand breaks in DNA were investigated by comparing the chemical yield of DNA-trapped free radicals to the chemical yield of DNA single strand break (ssb) and double strand break (dsb), as a function of hydration (Γ). Solid-state films of plasmid pUC18, hydrated to 2.5 < Γ< 22.5 mol, were X-irradiated at 4 K, warmed to room temperature, and dissolved in water. Free radical yields were determined by EPR at 4 K. With use of the same samples, Gel electrophoresis was used to measure the chemical yield of total strand breaks, which includes prompt plus heat labile ssb; G‘total(ssb) decreased from 0.092 ± 0.016 μmol/J at Γ= 2.5 to 0.066 ± 0.008 μmol/J at Γ= 22.5. Most provocative is that at Γ= 2.5 the yield of total ssb exceeds the yield of trapped deoxyribose radicals:  G‘total(ssb) − G‘sugar(fr) = 0.06 ± 0.02 μmol/J. Nearly 2/3 of the strand breaks are derived from precursors other than radicals trapped on the deoxyribose moiety. To account for these ...