Radiation-Induced Free Radicals in Oriented Bromouracil-Substituted DNA

Electron paramagnetic resonance has been used to study the free radicals induced by γ 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 K produced two 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 K results in a radical transformation, which may be the 5-bromouracil anion radical transforming into a hydrated uracil-5-yl radical. Above 200 K all radicals decay. As in normal DNA no secondary radicals could be observed from the guanine cation radicals. Replacement of thymine by 5-bro...