DNA breakage upon K-shell excitation of phosphorus as a model for direct effects in radiation biology

Single-strand breaks (SSBs) and double-strand breaks (DSBs) induced in DNA under phosphorus K-shell resonant absorption have been studied using supercoiled plasmids. The kinetics of the production of SSBs and DSBs exhibits a linear and a quadratic dependence, respectively, on photon fluence. Cross sections and quantum yields have been measured. The resonant photoexcitation of the phosphorus atoms was found to increase the DSB/SSB ratio compared to the off-resonance excitation. This enhancement factor can be related to the measured enhancement of the rate of cellular death and gene mutation in yeast under similar experimental conditions reported previously in the literature. Such resonant excitation of a specific atom belonging to DNA turns out to be an elegant method to investigate pure direct effects.