Yields of strand breaks and base lesions induced by soft X-rays in plasmid DNA.

The yields of soft-X-ray-induced DNA damages have been measured by using closed-circular plasmid DNA. Several DNA solutions with three kinds of radical scavenger capacity and also fully hydrated DNA samples were irradiated to compare the contribution by indirect reaction of diffusible water radicals, such as OH*, with those by direct action of secondary electrons. The yields of prompt single- (SSBs) and double-strand breaks (DSBs) decrease with increasing scavenging capacity. The SSB yields for soft X-rays are approximately midway those between gamma-ray and ultrasoft X-ray data previously reported. Heat labile sites are observed only in the low scavenger condition. The yields of the base lesions revealed by post irradiation treatment with base excision repair enzymes showed a similar value for Nth and Fpg protein except in the hydrated sample. These results indicate that the direct effect of soft X-rays induces the damages with different efficiency from those by indirect effect.