DNA damage induced by subexcitation electrons

Full text: The passage of ionizing radiation (β-, X-, or γ rays) through a living cell produces about 40 x 103 electrons/MeV, with more than 50% having energies well below the excitation threshold for water (M. Michaud et al, Physical Review, 44(9), 5623-5627, (1991)). We have previously shown that 5-20 eV electrons cause DNA strand breaks via a resonant process with a maximum at 10 eV (B. Boudaiffa et al, Science 287, 1658-1660, (2000)). The present results demonstrate that very low energy electrons in the range of 1 to 5 eV cause single strand breaks (SSB) in DNA. Plasmid DNA [pGEM 3zf(-), 3199pb] is extracted from the host bacteria [ E.coli, JM109 ] purified and resuspended in distilled and deionised water. It is deposited on a clean chemically tantalum substrate, lyophilised and placed in an UHV chamber for 24 hours before irradiation. After irradiation, plasmid DNA is retrieved from the UHV chamber and the samples are dissolved in buffer (Tris-EDTA, 10mM/1mM, pH 8). The different topological forms of DNA resulting from single strand break formation are separated by electrophoresis gel, stained by SYBR Green 1 (Molecular Probes), scanned by laser (Storm, Molecular Dynamics) and quantified using the imageQuant program (Molecular Dynamics). The quantification protocol has been optimized to maximize both sensitivity and linearity. The effective cross section for inducing SSB by the impact of 2 eV electrons is estimated to be 2.3 x 10-13 cm2