The Role of Humidity and Oxygen Level on Damage to DNA Induced by Soft X-rays and Low-Energy Electrons.

Single- and double-strand breaks induced by soft X-rays (1.5 keV) and photo-emitted LEEs (0-30 eV) were measured in dry and humid thin films of plasmid DNA irradiated under different controllable levels of oxygen at standard ambient temperature and pressure (SATP). G values derived from these experiments shows that the presence of H2O and changing the atmosphere from N2 to O2, while keeping all other experimental parameters constant, increases the formation of DSBs by factors of 4.5 and 11.8 for X-rays and LEEs, respectively. Under an oxygenated environment in humid DNA films, the additional LEE-induced damage resulting from the combination of water and oxygen exhibits a supper-additive effect, which leads to the formation of DSBs with a G value almost 7 times higher than that obtained by X-ray photons. These results indicate that O2, H2O and LEEs effectively contribute synergistically to enhance the formation of DSBs.