High-energy electron irradiation of proteins and nucleic acids: collisional stopping power and average energy loss.
Inactivation of proteins due to the direct action of ionizing radiation and the electron energy loss spectra of organic materials indicate that an average of 60-66 eV of energy is lost from high energy electrons in each inelastic collision with target molecules. The average energy loss per inelastic collision with high energy electrons in solid, carbon-based materials, proteins and nucleic acids is calculated from mass collisional stopping powers and empirical total inelastic cross-sections. Bragg's Additivity Law is used for the calculation of the mean excitation energy of molecules. For simple organic compounds, the calculated average energy loss is close to that obtained by direct observation of the energy loss suffered by electrons as they pass through thin films of organic material. The density effect correction for the rate of energy loss, important in the more complex case of proteins irradiated with 10 MeV electrons, is determined using the comparable mass collisional stopping power of water and proteins. In this manner, a value is obtained for the average energy per inelastic collision of high energy electrons with proteins, which is similar to the average energy per inactivating event of proteins. Analogous calculations for nucleic acids are also presented.