In Situ Observation of Proton-Induced DNA Fragmentation in the Bragg Peak: Evidence for a Protective Role of Water

Abstract

We report in situ single-molecule measurements of proton-induced double-strand breaks (DSBs) in DNA immersed in water, using real-time fluorescence tracking along the entire proton path, including the Bragg peak region. By chemically suppressing radical-mediated processes, we isolate direct DNA damage mechanisms and determine DSB cross sections as a function of depth. Near the Bragg peak, we observe a tenfold reduction in DSB cross sections in aqueous DNA compared to dry DNA, providing quantitative evidence for the protective role of water. These findings highlight the importance of intermolecular energy dissipation in mitigating radiation-induced damage in condensed biological matter, with implications for radiobiology and proton therapy modeling.

Figures (5)

• Figure 1: Principle of the in situ observation of DSB DNA fragmentation (a) A DNA solution is irradiated with 3 MeV protons up to the Bragg peak region. (b) DNA dynamics at depth z are captured via fluorescence microscopy within a 13 µm imaging depth. (c) Sequential snapshots of DNA after a 400 ms irradiation of 6,000 protons µm$^{-2}$ applied at time t = 0 s. Two T4 DNA molecules are visible, with the upper one undergoing a DSB event resulting in a multiplicity of 2.
• Figure 2: Multiplicity distributions for 1.5-3 MeV proton - T4 DNA collisions in water target within the 25-90 µm depth range. Experimental results are represented by symbols. The high-density shaded areas correspond to fits using the fragmentation model with $\langle\sigma_{DSB}\rangle$=(5.4 $\pm$ 0.8)$\times$10$^{-18}$cm$^2$bp$^{-1}$. The low density shaded area provides prediction for m=1 (no fragmentation) with $\langle\sigma_{DSB}\rangle$=(10.6 $\pm$ 2.0)$\times$10$^{-18}$cm$^2$bp$^{-1}$ as expected for dry DNA.
• Figure 3: Schematic representation of the fragmentation analysis applied to four simulated events, showing: (1) random bond breaking, (2) application of size detection thresholds, and (3) resulting detected fragment multiplicities. The letters w, x, y, and z denote four distinct fragment sizes.
• Figure 4: Depth profile of the raw DSB cross-section over 20 µm depth intervals (blue circle) alongside the stopping power profile of 3 MeV incident protons (magenta line).
• Figure 5: Depth profile of the unfolded DSB cross-sections for DNA in the water target (blue circle). DSB cross-sections for dry (isolated) DNA are shown as red souici2017single and green beaudier2024quantitative circles. Proton energies used to measure these larger cross-sections are converted into depths within the water ziegler2010srim, considering a proton energy of 3 MeV at the entrance face of the target.