Strand breaks in plasmid DNA, natural and brominated, by low-energy X-rays

Single- and double-strand breaks were measured in Col E1 plasmid DNA, natural and partially brominated, irradiated with monoenergetic X-rays (from a synchrotron radiation source) on both sides of the K-absorption edge for Br. The fraction of the undamaged supercoiled form decreased exponentially with the photon dose; its yield in the brominated DNA did not exhibit any energy-dependence. This result is consistent with the calculated relative contributions of photoelectric interactions with individual component atoms which show that an electron flux originating in light atoms outweighs that from bromine. However, X-rays of energy above the Br K-edge appear slightly more efficient in producing double-strand breaks. This result, as well as those reported in the literature on the Auger enhancement of different biological endpoints in cells containing brominated DNA, seem to suggest that the positive charge and submolecular effects associated with the photo-absorption in bromine play some role in damaging processes, besides the initial distribution of deposited electron energy.