Non-homogeneous Kinetics in the Competition of Single-stranded Calf-thymus DNA and Low-molecular Weight Scavengers for OH Radicals: A Comparison of Experimental Data and Theoretical Models

Summary

The yield of single-strand breaks induced by ⁶⁰Co γ-radiation in single-stranded calf-thymus DNA has been measured in aqueous, N₂O/O₂ saturated DNA solutions as a function of the concentration of added OH scavengers. Single-strand break yields have also been determined, relative to the yield of OH radicals, using pulse radiolysis. Essentially the same dependence on scavenging capacity was obtained under both irradiation conditions. At the highest scavenging capacity used (≈ 10⁹ s⁻¹), about eight times more strand breaks are formed than would be predicted from homogeneous competition kinetics. The experimental data are compared with values obtained from theoretical models which are based on non-homogeneous kinetics, and include the effects of spurs and of the direct action of radiation. The comparison shows that the observed dependence of the single-strand break yield on the scavenging capacity can be quantitatively accounted for without the need of empirical adjustment of parameters using a DNA model where the macromolecules are approximated by structureless cylinders of radius R^c = 0·80 nm which on their surface react with OH radicals at a diffusion-controlled rate. The factors determining the scavenger dependence of the single-strand break yield are discussed. The increase on going to higher scavenging capacities in the apparent rate constant of the reaction of OH with DNA is rationalized in terms of the mean diffusion length of OH radicals in solution. Estimates are given for the fractions of strand breaks due to randomized OH radicals, OH radicals from spurs, and the direct radiation effect.