In this paper we report a simple and effective method to build up self-assembled and wellcalibrated layers of plasmid DNA • 1,3-diaminopropane complexes onto Highly Oriented Pyrolitic Graphite (HOPG). The method is based on the self-assembly of the poly-electrolytes onto HOPG in an excess of positively-charged protonated di-amines (Dap 2+) in comparison to the negatively-charged phosphate moieties of the DNA backbone in solution. Although short distortions in the helical parameters of DNA (maximum 12% hypochromicity) are revealed by UV-Vis absorption spectrometry, the native B form of the plasmids is conserved. By fixing the excess of positive charges arising from Dap 2+ cations, it is possible to construct assemblies of a well-defined thickness ranging typically from 1 monolayer (ML) of DNA to 10 ML; 1 ML has a thickness of 2.2 ± 0.5 nm. Adding TRIS-EDTA (TE) buffer lowers considerably the damage rate observed when plasmids are mixed to Dap 2+ in pure water. The thickness of the first dense mono-layer matches well the DNA cross-sectional dimensions indicating that this layer is strongly anchored to the surface; it is insoluble in water. Conversely, thicker layers can be released in aqueous media and the plasmids do not undergo dramatic damage. In presence of TE buffer condensation of the plasmids on the HOPG surface and a further release of the deposits in water yields a loss of supercoiling that ranges typically from 10 to 20% when the layer thickness varies from 22 to 12 nanometers. Such densely-packed and releasable DNA plasmid layers with a very well characterized and constant thickness constitute a substantial progress for biochemical and radiochemical experiments.

It was recently shown that the affinity of doubly charged, 1-3 diaminopropane (Dap 2+ ) for DNA permits the growth on highly ordered pyrolitic graphite (HOPG) substrates, of plasmid DNA films, of known uniform thickness [O. Boulanouar, A. Khatyr, G. Herlem, F. Palmino, L. Sanche, and M. Fromm, J. Phys. Chem. C 115, 21291-21298 (2011)]. Post-irradiation analysis by electrophoresis of such targets confirms that electron impact at 10 eV produces a maximum in the yield of single strand breaks that can be associated with the formation of a DNA − transient anion. Using a welladapted deterministic survival model for the variation of electron damage with fluence and film thickness, we have determined an absolute cross section for strand-break damage by 10 eV electrons and inelastic scattering attenuation length in DNA-Dap complex films.

We measure the desorption of anions stimulated by the impact of 0-20 eV electrons on highly uniform thin films of plasmid DNA-diaminopropane. The results are accurately correlated with film thickness and composition by AFM and XPS measurements, respectively. Resonant structures in the H(-), O(-), and OH(-) yield functions are attributed to the decay of transient anions into the dissociative electron attachment (DEA) channel. The diamine induces ammonium-phosphate bridges along the DNA backbone, which suppresses the DEA O(-) channel and in counter-part increases considerably the desorption of OH(-). The close environment of the phosphate groups may therefore play an important role in modulating the rate and type of DNA damages induced by low energy electrons.