INDUCTION OF SINGLE-STRAND AND DOUBLE-STRAND BREAKS IN PLASMID DNA BYMONOENERGETIC ALPHA-PARTICLES WITH ENERGIES BELOW THE BRAGG-MAXIMUM

The yield of single-strand breaks (ssb) and double-strand breaks (dsb) produced by alpha-particles at the end of their track in DNA-films wa s determined experimentally. Helium nuclei were accelerated to 600 keV in the 400 kV ion accelerator and scattered at a carbon target. The e lastically scattered alpha-particles with energies of 344 keV and 485 keV were used to irradiate supercircular plasmid DNA in vacuo. For the dosimetry of the alpha-particles a surface barrier detector was used and the energy distribution of the alpha-particles determined. The ene rgy loss of the particles in the DNA-layer was calculated. DNA samples were separated into the three conformational isomers using agarose ge l electrophoresis. After fluorochromation the number of ssb and dsb pe r plasmid DNA molecule was established from the band intensities assum ing the validity of Poisson statistics. Linear dose effect correlation s were found for ssb and dsb per plasmid molecule. In the case of 344 keV-alpha-particles the yield of dsb was (8.6 +/- 0.9) x 10(-11) break s/Gy x dalton. The ratio of ssb/dsb was 0.5 +/- 0.2. This is at least a factor of six larger than the ratio found in experiments with higher energy alpha-particles and from model calculations. Similar experimen ts with protons yielded a relative biological effectiveness (rbe) valu e of 2.8 for the induction of double-strand breaks by track end alpha- particles.