Influence of radiation quality on the yield of DNA strand breaks in SV40 DNA irradiated in solution.

Using highly energetic particles to irradiate plasmid DNA in aerobic aqueous solution, we have compiled an extensive database on how yields of DNA single- and double-strand breaks (SSBs and DSBs) vary with radiation quality. This study was performed in a low-scavenging buffer system and covers a wide range of ion species (helium to uranium) and LETs (5 to 16,000 keV/microm). For LETs up to around 40 keV/microm for SSBs and 400 keV/microm for DSBs, the total energy deposition determines cross section. At higher LET, cross sections level off and individual plateaus for particles of different atomic numbers are observed. For each ion species this is more pronounced and occurs at lower LET for SSBs than for DSBs, leading to an increase in the DSB:SSB ratio from 1:70 for X rays to 1:6 at 500 keV/microm. At this LET, the influence of track structure becomes evident, with high local concentrations of ionization events favoring the formation of DSBs and also intratrack recombination reactions. For lower-energy ions, a saturation in production of measurable DSBs is apparent, due to correlated lesion induction within densely ionizing particle tracks. For very heavy low-energy ions, both SSB and DSB cross sections decrease with particle velocity at nearly constant LET, forming individual hooked curves when plotted as a function of LET.