The determination of the microscopic dose-damage relationship for DNA in an
aqueous environment is of a fundamental interest for dosimetry and
applications in radiation therapy and protection. We combine geant4 particle-
scattering simulations in water with calculations concerning the movement of
biomolecules to obtain the energy deposit in the biologically relevant
nanoscopic volume. We juxtaposition these results to the experimentally
determined damage to obtain the dose-damage relationship at a molecular level.
This approach is tested for an experimentally challenging system concerning
the direct irradiation of plasmid DNA (pUC19) in water with electrons as
primary particles. Here a microscopic target model for the plasmid DNA based
on the relation of lineal energy and radiation quality is used to calculate
the effective target volume. It was found that on average fewer than two
ionizations within a 7.5-nm radius around the sugar-phosphate backbone are
sufficient to cause a single strand break, with a corresponding median lethal
energy deposit being E1/2=6±4 eV. The presented method is applicable for
ionizing radiation (e.g., γ rays, x rays, and electrons) and a variety of
targets, such as DNA, proteins, or cells
