A molecular dynamics simulation of DNA damage induction by ionizing radiation

We present a multi-scale simulation of the early stage of DNA damages by the indirect action of hydroxyl ((OH)-O-center dot) free radicals generated by electrons and protons. The computational method comprises of interfacing the Geant4-DNA Monte Carlo with ReaxFF molecular dynamics software. A clustering method was employed to map the coordinates of (OH)-O-center dot-radicals extracted from the ionization-track-structures onto nano-meter simulation voxels filled with DNA and water molecules. The molecular dynamics simulation provides the time-evolution and chemical reactions in individual simulation voxels as well as the energy-landscape accounted for the DNA-(OH)-O-center dot chemical reaction that is essential for the first-principle enumeration of hydrogen abstractions, chemical bond breaks, and DNA-lesions induced by collection of ions in clusters less than the critical dimension which is approximately 2-3 angstrom. We show that the formation of broken bonds leads to DNA-base and backbone damages that collectively propagate to DNA single and double-strand breaks. For illustration of the methodology, we focused on particles with an initial energy of 1 MeV. Our studies reveal a qualitative difference in DNA damage induced by low energy electrons and protons. Electrons mainly generate small pockets of (OH)-O-center dot-radicals, randomly dispersed in the cell volume. In contrast, protons generate larger clusters along a straight-line parallel to the direction of the particle. The ratio of the total DNA double-strand breaks induced by a single proton and electron track is determined to be approximate to 4 in the linear scaling limit. In summary, we have developed a multi-scale computational model based on first-principles to study the interaction of ionizing radiation with DNA molecules. The main advantage of our hybrid Monte Carlo approach using Geant4-DNA and ReaxFF is the multi-scale simulation of the cascade of both physical and chemical events which result in the formation of biological damage. The tool developed in this work can be used in the future to investigate the relative biological effectiveness of light and heavy ions that are used in radiotherapy.