Photoaddition of 5‑Bromouracil to Uracil in Oligonucleotides Leading to 5,5′-Bipyrimidine-Type Adducts: Mechanism of the Photoreaction

5-Bromouracil (^BrU) modified di- and hexanucleotides having ^BrU flanked on the 5′ or the 3′ side by uracil (U) have been synthesized, and their photochemical reactivity was examined under the conditions of irradiation with near UV light. The results indicate that the primary photochemical process in all of these compounds involves the formation of an intermediate cyclobutane phodoadduct composed of ^BrU and U, which undergoes further photochemically and thermally induced transformations to 5,5′-bipyrimidine type adducts