Yield of single-strand breaks due to attack on DNA by scavenger-derived radicals.

We have measured the yield of single-strand breaks (SSBs) in plasmid DNA after 137Cs gamma irradiation, in the presence of dimethyl sulfoxide (DMSO). In the presence of oxygen, the formation of SSBs is due to hydroxyl radical attack. As the DMSO concentration is increased from 10(-4) mol dm-3 to 1 mol dm-3, the SSB yield in the presence and absence of oxygen decreases by over 100-fold and less than 10-fold, respectively. From the DMSO and DNA concentration dependencies of the SSB yield in the absence of oxygen, the second-order rate constant for the reaction of the methyl radical (derived from DMSO) and DNA can be estimated as k2 = 8.8 x 10(4) dm3 mol-1 s-1. Several other scavengers were compared with DMSO under anoxia. Radicals derived from isopropyl alcohol and glycerol also caused SSB formation in DNA, while those from 2-deoxyribose, thymine, 1,3-dimethylthymine and 1,3-dimethyluracil did not. In the case of the scavenger tert-butyl alcohol, it is unclear whether the hydrogen atom (H.) or an organic radical is responsible for the higher SSB yield under anoxic conditions.