Abstract
In this study we present for the first time detailed scanning force microscopy (SFM) investigations of carbon ion induced damages in plasmid DNA in order to obtain information about the biological effectiveness of particle radiation. For this purpose, we have combined SFM and gel electrophoresis measurements in a dose range between D = 0 Gy and 5000 Gy. After irradiation with C ions, the percentage of double‐strand breaks (DSBs) increases drastically, i.e. from initially 0% for D = 0 Gy to 38% for D = 5000 Gy. Increasing the dose over the total range is accompanied by a shortening of the average fragment length from 〈L〉 = 1100 nm to 〈L〉 = 575 nm. In addition to our experiments, the average numbers of induced DSBs per irradiated plasmid and per broken plasmid have been calculated from the SFM measurements. The most important among the numerous results is that a significant amount of plasmids has suffered more than two DSBs for all applied doses, indicating multiple DSBs. The number of DSBs per broken plasmid increases from approximately 1.7 after irradiation with a dose of D = 250 Gy to 3.2 after exposure to the highest dose of D = 5000 Gy. The results provide experimental data for the spatially correlated production of DSBs after carbon irradiation, that are relevant to the understanding of its biological effectiveness. Copyright © 2007 John Wiley & Sons, Ltd.