Modeling of ultrasoft X-ray induced DNA damage using structured higher order DNA targets

Abstract

To study the influence of inhomogeneous targets on radiation-induced DNA damage, photon and electron track structure calculations in the biophysical simulation code PARTRAC were carried out taking into account the different mass densities and atomic compositions of DNA, histone proteins and the cell plasma. Particularly interactions of USX rays up to 3keV are affected by the photoionization of carbon, nitrogen, oxygen and phosphorus. Whereas the succeeding relaxation of these low Z elements does not seem to have a strong influence on the production of DNA damage, the differences between the cross-sections of DNA and cell plasma, particularly between the carbon and oxygen K absorption edge (0.29–0.54keV), lead to a significantly local inhomogeneous dose distribution and therefore an increased yield of strand breaks per unit dose, especially when DNA is folded and proteins are attached. Resulting small fragment size distributions reflect the DNA wrapping around the histone octamer and supports an irregular crossed linker model for the chromatin fiber architecture. The evaluation of large fragment size distributions shows deviations from random breakage model, most notably in the case of CK USX irradiation, which should be taken into account, when DSB yields are determined in experiments.