Base lesions and AP sites induced in plasmid DNA film irradiated with soft X-rays and low energy electrons

We have investigated the site selective damage induction in plasmid DNA by tuning soft X-ray energy using synchrotron radiation as a light source. We reported that the relative yields of base lesions to single strand breaks are strongly depend on the ionization of nitrogen or oxygen atoms in DNA (Fujii et al., 2009). The specific lesions are thought to be induced by both direct ionization of DNA and indirect impact of Auger or photoelectrons produced by relaxation from the photo-ionization/excitation. The ranges of the secondary electrons are extremely short in living system (< a few hundred nm), and it is generally thought that these electrons additionally ionize or excite atoms proximately located to the first ionizing atom in DNA. In this study, we aim to reveal the role of the low energy electrons in the selective DNA damage. We measure the yields of strand breaks, base lesions, and AP sites produced in thin film of pUC18 plasmid DNA by irradiation of low energy (>500 eV) electrons emitted from an electron generator. The yields of base lesions and AP sites are determined by post-irradiation-treatment of the DNA with enzymatic probes which excise base damage (Nth; pyrimidine base lesion, Fpg; purine base lesion and Nfo; AP site). The obtained data will be compared with the yields obtained by soft X-ray irradiation around oxygen and nitrogen K-edge region.