We have measured the yield of single-strand breaks (SSBs) in plasmid DNA after137 Cs γ 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$10^{-4}\ {\rm mol}\ {\rm dm}^{-3}$ to$1\ {\rm mol}\ {\rm dm}^{-3}$, 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$k_{2}=8.8\times 10^{4}\ {\rm dm}^{3}\ {\rm mol}^{-1}\ {\rm s}^{-1}$. 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 (${\rm H}^{\bullet}$) or an organic radical is responsible for the higher SSB yield under anoxic conditions.

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