Characterization of the reaction rate coefficient of DNA with the hydroxyl radical

Using agarose gel electrophoresis, we have measured the yield of single-strand breaks (SSBs) induced by 137Cs gamma irradiation in a variety of plasmid DNA substrates ranging in size from 2.7 kb to 38 kb irradiated in aerobic aqueous solution in the presence of the hydroxyl radical scavenger dimethyl sulfoxide (DMSO). Under these conditions DNA SSBs are caused mainly by the hydroxyl radical. Using the competition between DMSO and DNA for the hydroxyl radical, we have estimated the rate coefficient for the reaction of the hydroxyl radical with DNA. The results cannot be characterized by conventional steady-state competition kinetics. However, it is possible to describe the second-order rate constant for the reaction as a function of the scavenging capacity of the solution. The second-order rate constant increases with increasing scavenging capacity, rising from about 5 x 10(8) dm3 mol-1 s-1 at 10(5) s-1 to about 10(10) dm3 mol-1 s-1 at 10(10) s-1. This dependence of the second-order rate constant on the scavenging capacity appears to be more pronounced for larger plasmids.