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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

Purpose : To measure action spectra for the induction of single-strand breaks (SSB) and double-strand breaks (DSB) in plasmid DNA by low-energy photons and provide estimates for the energy dependence of strand-break formation important for track-structure simulations of DNA damage. Materials and methods : Plasmid pMSG-CAT was irradiated as a monolayer, under vacuum, with 7-150 eV photons produced by a synchrotron source. Yields of SSB and DSB were determined by the separation of the three plasmid forms by gel electrophoresis. Results : The yields of SSB per incident photon increased from 1.4 x 10 -15 SSB per plasmid per photon/cm 2 at 7 eV to 7.5 x 10 -14 SSB per plasmid per photon/cm 2 at 150 eV. Direct induction of DSB was also detected increasing from 3.4 x 10 -17 DSB per plasmid per photon/cm 2 at 7 eV to 4.1 x 10 -15 DSB per plasmid per photon/cm 2 at 150 eV. When the absorption cross-section of the DNA was considered, the quantum efficiency for break formation increased over the energy range studied. Over the entire energy range, the ratio of SSB to DSB remained constant. Conclusions : These studies provide evidence for the ability of photons as low as 7 eV to induce both SSB and DSB. The common action spectrum for both lesions suggests that they derive from the same initial photoproducts under conditions where the DNA is irradiated in vacuum and a predominantly direct effect is being observed. The spectral and dose-effect behaviour indicates that DSB are induced predominantly by single-event processes in the energy range covered.