Absolute Cross Sections For Low Energy Electrons Damage To DNA

Low-energy electrons(LEE)can temporarily localize on specific bonds of DNA,thus forming transient negative ions(TNI),which decay into destructive channels leading to DNA damages including single-strand(SSB)and double-strand breaks(DSB).Cross sections(CS)for the interaction of LEE with condensed macromolecules are essential parameters to accurately model radiation induced molecular decomposition and chemical synthesis.The effective CS and other quantitative parameters for LEE interactions are generally measured on nanometer-scale solid films,with thicknesses comparable to electron thermalization distances.The main difficulties involved in the measurement of such CS are:variation of the energy deposition throughout the film and charge accumulation inside the film;the latter shields a proportion of macromolecules from electron irradiation.Such effects complicate the quantitative comparison of the effective CS obtained in films of different thickness and limit the applicability of such measurements.In present study,we measure effective CS for the loss of supercoiled(LS)and formation of SSB,DSB and CL by electron irradiation,at typical energies of 5,6,10 and 15 eV,of lyophilized plasmid DNA films with thicknesses varying between 10 to 20 nm.The attenuation lengths of LEE are calculated to be?12 nm with maximum at 10 eV.The effective CS are shown to depend strongly on the thickness and charging condition of the nanometer-scale films.The penetration factors of different film thicknesses are found to be in the range 0.4-0.7.From these parameters and analysis with the model of Rezaee et al.,the absolute CS of LS,i.e.,the total inelastic CS leading to DNA damages is derived to be 2.0×10-14 cm2 at 10 eV.This value is consistent with previous measurements of the efficiency of LEE to induce DNA damages.The CS to produce SSB is smaller,but similar to that of LS,indicating that SSB is the main conformal damage.The CS to produce DSB and CL are one order of magnitude smaller than that of LS and SSB.In summary,the present comprehensive study provides,for the first time,absolute CS for SSB,DSB and CL,along with that for total DNA damages,in the range 5-15 eV.Such an analysis can be extended to obtain the energy dependence of CS over a wider range(e.g.,2-20eV).The procedure can also be adapted for electron-induced damage/reaction occurring in other molecular solids.