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 photoemitted low-energy electrons (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 show that the presence of H2O and changing the atmosphere from N-2 to O-2, while keeping all other experimental parameters constant, increases the formation of double-strand breaks (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 super-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 O-2, H2O, and LEEs effectively contribute synergistically to enhance the formation of DSBs.