Radiation-Induced Formation of DNA Double-Strand Breaks in Plasmids and E. coli

Abstract

The chemical reactions induced by high-energy irradiation are in the majority of cases free radical reactions (Henglein et al. 1969). This is due to the fact that the chemical changes are the result of ionizations which produce radical cations and electrons from each other. Electronic excitations contribute negligibly to chemical changes in DNA (Redpath et al. 1981; Morgan et al. 1982). The solvated electrons have a small probability of causing strand break (sb) in DNA (Lafleur et al. 1988). The main source of sb formation are the radical cations. Usually two ways of sb formation are distinguished depending on the location where the ionization has taken place. Either, molecules in the surroundings of the DNA are ionized, e.g. water molecules, proteins, or any other kind of molecules, or the DNA itself is ionized. The ionized molecules convert to neutral radicals by rapid deprotonation, or by reaction with water which also leads to the release of a proton. For instance the water radical cations convert into OH radicals and protons. The OH radicals diffuse to the DNA and react mainly with the bases and to a smaller extent with the sugar (Scholes et al. 1960). The organic radicals can react also with DNA, however, with much smaller rate constants than that of the OH radical. Nevertheless the reactions of organic radicals can lead to inactivation (Jong et al. 1972). These kinds of damage formation of DNA are known as the “indirect effect” of high-energy irradiation. The direct effect originates from the radical cations produced in the bases, sugars or phosphates of the DNA.