Radiation-Induced Free Radicals in Oriented Bromouracil-Substituted DNA

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Abstract
EPR was used to study the free radicals induced by .gamma.-irradiation in 5-bromouracil-substituted DNA from Bacillus subtilis. The DNA samples were oriented by wet spinning and contained 30% humidity. As in normal DNA, irradiation at 77.degree. K produced 2 major radical components. One is assigned to guanine cation radicals, similar to those in normal DNA but produced in concentrations 20-50% higher than in normal DNA. The other component is assigned to 5-bromouracil anion radicals, apparently replacing the thymine anion radicals in normal DNA. The G value for total radical formation is about 2.0, not significantly higher than the G value of 1.5 determined for normal DNA. Warming the sample above 77.degree. K results in a radical transformation, which may be the 5-bromouracil anion radical transforming into a hydrated uracil-5-yl radical. Above 200.degree. K all radicals decay. As in normal DNA no secondary radicals could be observed from the guanine cation radicals. Replacement of thymine by 5-bromouracil thus results in a modest increase in the guanine cation radical yield. The quantitative results of this study do not directly explain the up to 3-fold sensitization observed in biological systems unless small differences in initial products are enhanced under in vivo conditions. The further reactions of the new 5-bromouracil anion species may also contribute to the sensitization by 5-bromouracil substitution in DNA.

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