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

To study the effect of hydration level and plasmid packing on strand break induction in DNA by ultrasoft X-ray.

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 Electromagnétique synchrotron facility (Orsay, France). Single and double strand breaks (SSB and DSB) were quantified by gel electrophoresis.

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)x10(-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.

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 more often absorbed in the hydration shell and yield longer range electrons than 250 and 380 eV photons, induce more DSB per Gray and per Dalton, especially for the looser plasmid (pSP).