Ultrathin layers (<20 nm) of pBR322 plasmid DNA were deposited onto 2.5 μm thick polyester films and exposed to proton Bragg-peak energies (90-3000 keV) at various fluences. A quantitative analysis of radio-induced DNA damage is reported here in terms of single- and double-strand breaks (SSB and DSB, respectively). The corresponding yields as well as G-values and the cross sections exhibit fairly good agreement with the rare available data, stemming from close experimental conditions, namely, based on α particle irradiation. SSB/DSB rates appear to be linear when plotted against linear energy transfer (LET) in the whole energy range studied. All the data present a maximum in the 150-200 keV energy range; as for LET, it peaks at 90 keV. We also show that fragmentation starts to be significant for proton fluences greater than 1 × 10 cm at the Bragg-peak energies. Finally, we determine the average proton track radial extension, r, corresponding to an occupation probability of 100% DSB in the Bragg-peak region. The r values determined are in excellent agreement with the radial extensions of proton tracks determined by simulation approaches in water. When plotted as a function of LET, both SSB and DSB cross sections bend back at high LETs.

In this study, we used a bench-top cold-cathode ultra-soft X-ray (USX) generator to expose aqueous DNA plasmid solutions to low-LET radiation under various scavenging conditions. Single- and double-strand breaks were assessed using classic gel electrophoresis quantification of linear, circular and supercoiled plasmid DNA topologies. With their very low penetration range in water, USX can only interact with matter up to short distances, of the order of 50 μm. We validated a stirring procedure which makes it possible to expose 100 µL of aqueous samples (2 mm thick). The scavenging of OH radicals by Tris buffer was studied at ambient temperature under aerobic conditions and compared to data gathered in the literature. A very good agreement was found with the rare data dealing with DNA plasmid exposed to Al Kα photons at low temperature (T ≤ 277 K), which therefore validated the experimental procedure. The yields for DNA single-strand breaks determined during this study enabled the ratio of indirect to direct effects to be determined at 96.2%, in good agreement with the value of 97.7% stemming from a study based on γ-ray irradiation of frozen solutions of plasmid DNA. Then, arginine was used both to create a "biological-like" chemical environment around the DNA plasmids and as an OH radical scavenger, in vitro. Although arginine has a greater scavenging (protecting) power than Tris, surprisingly, it led to higher rates of strand breakage. Based on the specific binding modes of arginine to DNA, we suggest that the side effects observed are due to the presence of arginine near to, but also inside, the DNA double helix.