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

International audienceD oTostudytheeffectofhydrationlevel andplasmidpackingonstrandbreakinductioninDNAbyultrasoftX-ray. Materialsandmethods: Bluescript(pBS, tightpacking)andpSP189(pSP, loosepacking)plasmidswereirradiatedby250, 380, and760eVultrasoft X-rays at theLaboratoirepour l'UtilisationduRayonnement Electromagne ́tiquesynchrotron facility(Orsay, France). Singleanddoublestrandbreaks(SSBandDSB)werequantifiedbygel 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)610712at 250, 380and760eV, respectively. Theywererespectively1.4+0.1, 1.1+0.1and1.9+0.2times largerforpSPplasmids. SSB/DSBratiosvariedbetween4.4and6.4. Conclusion: Theobserveddependencyof strandbreakinductionbyultrasoft X-rays onthehydrationlevel of DNAin plasmidsfilmsmaybeassociatedwith: (i)Damagetransferfromthewatershell totheDNAand/or(ii)changeinpacking. 760eVphotonswhicharemoreoftenabsorbedinthehydrationshell andyieldlongerrangeelectronsthan250and380eV photons, inducemoreDSBperGrayandperDalton, especiallyforthelooserplasmid(pSP