Strand breaks induced in plasmid DNA by ultrasoft X-rays: Influence of hydration and packing

Purpose: To study the effect of hydration level and plasmid packing on strand break induction in DNA by ultrasoft X-ray.Materials and methods: Bluescript (pBS, tight packing) and pSP189 (pSP, loose packing) plasmids were irradiated by 250, 380, and 760 eV ultrasoft X-rays at the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique synchrotron facility (Orsay, France). Single and double strand breaks (SSB and DSB) were quantified by gel electrophoresis.Results: The number of DSB per Gray and per Dalton in pBS plasmids were (5.6 ± 0.1), (6.3 ± 0.1) and (8.5 ± 0.4)×10−12 at 250, 380 and 760 eV, respectively. They were respectively 1.4 ± 0.1, 1.1 ± 0.1 and 1.9 ± 0.2 times larger for pSP plasmids. SSB/DSB ratios varied between 4.4 and 6.4.Conclusion: The observed dependency of strand break induction by ultrasoft X-rays on the hydration level of DNA in plasmids films may be associated with: (i) Damage transfer from the water shell to the DNA and/or (ii) change in packing. 760 eV photons which are mor...