Sharma, K. K., Purkayastha, S. and Bernhard, W. A. Unaltered Free Base Release from d(CGCGCG)2 Produced by the Direct Effect of Ionizing Radiation at 4 K and Room Temperature. Radiat. Res. 167, 501–507 (2007).
Unaltered free base release in d(CGCGCG)2 exposed to X rays at 4 K or room temperature was measured by HPLC. Samples were prepared either as films hydrated to a level of Γ = 2.5 mol water/mol nucleotide or as polycrystalline with Γ ∼ 7.5 mol water/mol nucleotide. X irradiation of films at 4 K, followed by annealing to room temperature, resulted in yields for cytosine and guanine of G(Cyt) = 0.036 ± 0.001 μmol/J and G(Gua) = 0.090 ± 0.002 μmol/J. Irradiation of films at room temperature gave similar yields. The yields for polycrystalline d(CGCGCG)2 X-irradiated at room temperature were G(Cyt) = 0.035 ± 0.005 μmol/J and G(Gua) = 0.077 ± 0.023 μmol/J. The total free base release yield, G(fbr), was 0.124 ± 0.008 μmol/J for films and 0.112 ± 0.028 μmol/J for polycrystalline samples. G(fbr) is believed to be a good estimate of total strand break yield. The yields of total free radicals trapped [G(Σfr)] by the d(CGCGCG)2 films at 4 K were measured by EPR. The measured value, G(Σfr) = 0.450 ± 0.005 μmol/J, was used to calculate the yield of trappable sugar radicals, giving Gsugar(fr) = 0.04–0.07 μmol/J. We found that (1) guanine release exceeded cytosine release by more than twofold, (2) Gsugar(fr) cannot account for more than half of the free base release, and (3) G(fbr), G(Cyt) and G(Gua) were independent of the sample temperature during irradiation. Finding (1) suggests that base and or sequence influences sugar damage, and finding (2) is consistent with our working hypothesis that an important pathway to strand break formation entails two one-electron oxidations at the same sugar site.