Characterization of lesions induced in linear-formed plasmid DNA by valence ionization and Auger decay at carbon, nitrogen and oxygen

Abstract

Purpose: To study the DNA lesions induced by the Auger decay of carbon, nitrogen, and oxygen using ultrasoft X-rays (USX) that are expected to be important with the DNA repair system of living cells.Materials and methods: pUC19 plasmid DNA dry samples were irradiated with USX photons at 270 and 560 eV and 60Co γ-rays in vacuum at room temperature. The amounts of unaltered base release by the direct radiation effects were quantified using high-performance liquid chromatography. To quantify and characterize the strand break termini the rate at which snake venom phosphodiesterase (SVPD) digested irradiated DNA pretreated with and without calf intestine alkaline phosphatase was measured. Moreover, the piperidine-labile base lesions and abasic sites of the irradiated DNA were estimated using the SVPD method.Results: The yields of unaltered base release for 270, 560 eV photons and 60Co γ-rays were 0.016, 0.014, and 0.018 μmol/J, respectively. The total 3′ termini for the three kinds of photons were around 0.1 μmol/J. The production of 3′ termini with phosphate was found to be predominant with respect to that of 3′OH termini for the three kinds of radiation. The yield of piperidine-labile sites for 270 eV (∼0.1 μmol/J) was slightly larger than that for 560 eV (∼0.07 μmol/J) and also for γ-rays (∼0.082 μmol/J).Conclusion: Although the Auger process in DNA-constituent atoms was expected to induce Auger-specific lesions in the molecule the chemical endpoints would have been covered with a large number of lesions produced from secondary electrons in the surrounding bulk DNA molecules. The present results, however, suggest that a low-energy electron field produced by the USX photons in the bulk DNA is basically not at all specific to DNA damage being produced when compared with the high-energy electron field produced by 60Co γ-rays.