Increase in φX174 DNA Radiation Sensitivity Due to Electric Fields

The application of an external electric field simultaneously with γ irradiation to an aqueous suspension of φX174 DNA (in the RFI form) is shown to increase significantly the number of strand breaks. Tritiated DNA allowed the number of single-strand breaks to be estimated from changes in the scintillation of electrophoretic gel band associated with the fastest mobility moiety. At 400 V ($\approx 2400\ {\rm V}\ {\rm cm}^{-1}$) the corrected increase (corrected for phoresis of DNA on the stainless-steel plates) in the G-value yield is 38%. The increase in damage with field strength appears to follow the increase in reduced dichroism. Dichroism results correspond at 400 V to approximately 10% of the maximum orientation. Our results support the conjecture that this significant increase in DNA-radiation interaction with an electric field is due to field-induced conformation changes in the molecule.