Radiation Damage to DNA: The Indirect Effect of Low-Energy Electrons

We report the effect of the DNA hydration level on damage yields induced by soft X-rays and photoemitted low-energy electrons (LEEs) in thin films of plasmid DNA irradiated in N-2 at atmospheric pressure under different humidity levels. Contrary to a dilute solution of DNA, the number of H2O molecules per nucleotide (Gamma) in these films can be varied from Gamma = 2.5 to similar to 33, where Gamma <= 20 corresponds to layers of hydration and Gamma = 33 to an additional bulk-like water layer. Our results indicate that DNA damage induced by LEEs does not increase significantly until the second hydration shell is formed. However, this damage increases dramatically as DNA coverage approaches bulk-like hydration conditions. A number of phenomena are invoked to account for these behaviors, including dissociative electron transfer from water-interface electron traps to DNA bases, quenching of dissociative electron attachment to DNA, and quenching of dissociative electronically excited states of H2O in contact with DNA.