DNA isolated from the bacteriophage ΦX174 was irradiated with γ-rays in a solution of pH 7.0 buffered with 0.01 M phosphate. At concentrations of a few milligrams per milliliter the yield of inactivation is 1.9 molecules of DNA per 100 eV of absorbed energy. The radio-sensitivity in oxygen and nitrogen does not differ significantly. Bubbling with nitrous oxide increases the yield by a factor of 2. These yields were derived from measurements of the 37% survival dose as a function of DNA concentration. Potassium iodide is strongly protective. Because potassium iodide scavenges OH radicals whereas nitrous oxide converts hydrated electrons into OH radicals, it is concluded that in concentrated DNA solutions at least 95% of the inactivations are due to primary attack by OH radicals, although reducing radicals are not completely inactive. At concentrations of a few micrograms per milliliter the yield of inactivation is lower by an order of magnitude and reducing radicals are relatively more important. From the protection afforded by free nucleotides it is concluded that OH radicals react about five times as fast with free nucleotides as with nucleotides in single-stranded DNA. In the presence of free pyrimidine nucleotides oxygen enhances the radiosensitivity considerably.

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