Variation of Single-Strand Break Yield with Scavenger Concentration for Plasmid DNA Irradiated in Aqueous Solution

We have measured the yield of single-strand breaks (SSBs), induced by¹³⁷ Cs γ radiation as assayed by agarose gel electrophoresis, for three plasmids and SV40 DNA irradiated in aerobic aqueous solution. DNA SSBs are caused mainly by the hydroxyl radical under these conditions. To characterize the reactivity of DNA with the hydroxyl radical, we investigated the variation of the G value for SSBs [G(SSB)] with the concentration of added hydroxyl radical scavengers. We find that simple competition kinetics does not describe our results, but that a nonhomogeneous kinetics model is in good agreement. At a DNA concentration of 50 μg cm^-3, G(SSB) for the direct effect is about$1\times 10^{-5}\ \mu {\rm mol}\ {\rm J}^{-1}$ for the DNA substrates studied. This is equivalent to 2× 10^-10 SSB${\rm Gy}^{-1}\ {\rm Da}^{-1}$. Estimates of the efficiency of SSB induction per${\rm OH}^{\bullet}$ radical interaction with DNA (0.32-0.44) reveal that all plasmids are essentially equal in reactivity.