Chain breakage accompanying the photolysis of IdUrd-DNA.

Abstract

Abstract DNA isolated from Escherichia coli and substituted with various amounts of iododeoxyuridine (12‐95%) was irradiated at either 265. 300 or 313 nm and the frequency of chain breakage measured by sedimentation in either neutral or alkaline sucrose gradients. The wavelength dependence of the photochemical cross section for chain breaks paralleled that for iodine loss, and the average frequency of chain breakage per halogen loss was 0.50 ± 0.07. Approximately 80‐90% of the breaks observed in alkali are alkali‐labile bonds and are not observed under neutral conditions. The presence of ethanol during irradiation reduced the frequency of chain breakage by more than an order of magnitude. These results are interpreted in terms of a model in which photo‐induced deiodination leads to the formation of a uracilyl radical which then abstracts a hydrogen atom from either a nearby sugar moiety or from another hydrogen donor such as ethanol. The resulting modified sugar can then rearrange to form either a clean chain break or an alkali‐labile bond.