A quantitative determination of free radicals and strand breaks in pure and radiosensitized DNA.

The yields of free radicals and of single strand breaks were studied quantitatively after gamma-irradiation at 77 K of solid calf thymus DNA without and with incorporated misonidazole. At 77 K the rate of formation of the free radicals (the nature of which had been previously elucidated in oriented DNA samples) was found to be 1.7 x 10 (-12) and 2.4 x 10 (-12) per rad per dalton for DNA without and with misonidazole, respectively. The corresponding yield of single strand breaks in samples thawed and dissolved before alkaline sucrose gradient ultracentrifugation was found to be 1.2 x 10(-13) per rad per dalton, not significantly (within 30%) different for DNA without and with misonidazole. The significant quantitative difference between induction of free radicals and single strand breaks shows that there is no simple relation between the appearance of the two types of lesions.