Electron Migration along 5-Bromouracil-Substituted DNA Irradiated in Solution and in Cells

Abstract

Solvated electrons generated in aqueous solution after exposure to ionizing radiation can be scavenged by DNA and then transferred along the DNA molecule. This mechanism of charge transfer provides an opportunity for radiation damage to be targeted to certain regions in the DNA molecule and is a mechanism by which single-strand breaks contribute to locally multiply damaged sites to enhance cell lethality. Experiments were performed in which different amounts of 5-bromouracil (5-BrU) were substituted for thymine in Escherichia coli DNA. The amount of bromide released was assayed after quantitative reaction of radiation-induced solvated electrons with 5-BrU in DNA samples irradiated in solution and irradiated in the cellular environment. By varying the amount of 5-BrU incorporated in the DNA, the average distance between 5-BrU molecules was systematically changed and, because the number of 5-BrU/electron reactions was monitored by the amount of bromine released, the maximum average electron migration distance along the 5-BrU DNA could be estimated. Using this approach, the maximum average electron migration distance in aqueous solutions of 5-BrU DNA was about 6.5 to 10 base distances in nonhybrid 5-BrU DNA (assuming only intrastrand migration). Similar methods revealed charge migration in 5-BrU DNA incorporated into E. coli, and the maximum average migration distance was about 5 to 6 base distances (assuming only intrastrand migration). Only 11-16% of the electrons produced during radiolysis were scavenged by 5-BrU DNA in aqueous solution, and only 1% resulted in the release of bromide from 5-BrU-DNA inside E. coli.