Oxygen Effect on 0-30 eV Electron Damage to DNA Under Different Hydration Levels: Base and Clustered Lesions, Strand Breaks and Crosslinks.

Studies on radiosensitization of biological damage by O began about a century ago and it remains one of the most significant subjects in radiobiology. It has been related to increased production of oxygen radicals and other reactive metabolites, but only recently to the action of the numerous low-energy electrons (LEEs: 0-30 eV) produced by ionizing radiation. We provide the first complete set of G-values (yields of specific products per energy deposited) for all conformational damages induced to plasmid DNA by LEEs (G (O)) and 1.5 keV X-rays (G(O)) under oxygen at atmospheric pressure. The experiments are performed in a chamber, under humidity levels ranging from 2.5 to 33 water molecules/base. Photoelectrons from 0 to 30 eV are produced by X-rays incident on a tantalum substrate covered with DNA. Damage yields are measured by electrophoresis as a function of X-ray fluence. The oxygen enhancement ratio G(O)/G(N), which lies around 2 for potentially lethal cluster lesions, is similar to that found with cells. The average ratio, G(O)/G(O), of 12 for cluster lesions and crosslinks strongly suggest that DNA damages that harm cells are much more likely to be created by LEEs than any other initial species generated by X-rays in the presence of O.