Critical energies for SSB and DSB induction in plasmid DNA by low-energy photons: action spectra for strand-break induction in plasmid DNA irradiated in vacuum

ations deposit a wide range of energies within the Purpose: To measure action spectra for the induction of single- critical target (i.e. a small section of the DNA helix) strand breaks (SSB) and double-strand breaks (DSB) in plasmid (Goodhead 1989, Goodhead and Nikjoo 1989, Nikjoo DNA by low-energy photons and provide estimates for the et al. 1997, Ottolenghi et al. 1997). These will be from energy dependence of strand-break formation important for zero energy up to several hundred electron volts, track-structure simulations of DNA damage. Materials and methods: Plasmid pMSG-CAT was irradiated as a although densely ionizing radiation will produce a monolayer, under vacuum, with 7‐150eV photons produced greater fraction of higher energy depositions than by a synchrotron source. Yields of SSB and DSBwere determined sparsely ionizing radiation. Many modelling studies by the separation of the three plasmid forms by gel have equated this distribution of energy events with electrophoresis. the likely yields and spectra of lesions that will be Results: The yields of SSB per incident photon increased from 1.4O 10O 15 SSB per plasmid per photon/cm 2 at 7eV to produced in DNA by radiations of diU ering LET