Irradiation of Plasmid and Phage DNA in Water—alcohol Mixtures: Strand Breaks and Lethal Damage as a Function of Scavenger Concentration

Abstract

We have measured the yields of strand break formation and biological inactivation as a function of OH scavenger concentration for ⁶⁰Co γ-irradiated pBR322 plasmid and M13mp9 RF phage DNA. The yields of single-strand breaks (ssbs), double-strand breaks formed proportionally to dose (αdsbs), and lethal damage (LD) decrease with increasing scavenging capacity σ, their ratios remaining approximately constant up to σ ∼ 10⁸ s⁻¹. On a double-logarithmic plot the yields decrease linearly with σ in parallel lines. At higher scavenging capacities, the yields, while still decreasing, level off to a different extent. Our results for the yields of ssbs and αdsbs confirm those of Krisch et al. (1991) using SV40 DNA.

The data were analysed assuming that DNA damage is brought about by OH radicals, and a non-scavengeable portion arising from the direct radiation effect. Using a model based on non-homogeneous scavenging kinetics, the dependence on scavenging capacity of the ssb yield could be quantitatively accounted for. From the scavenging dependence of the yield of dsbs which are formed quadratically with dose (βdsbs) and which are the result of two independent ssbs within a critical distance h, a value of about 13 basepairs was obtained for h. The parallel decrease in the yield of ssbs and αdsbs with scavenging capacity was rationalized in terms of the Siddiqi—Bothe mechanism (Siddiqi and Bothe 1987). The efficiency of this mechanism was found to be approximately 0·01. From the analysis of the LD yields it was shown that up to σ ∼ 10⁸ s⁻¹, inactivation is predominantly due to single OH radicals which lead to LD with an efficiency of 0·12 per OH-induced ssb. At higher scavenging capacities, a non-scavengeable spur effect similar to the locally multiply damaged sites mechanism of Ward (1988) mainly contributes to LD.