PHOTOPRODUCTS OF BROMOURACIL‐LABELED DNA AND THE STRUCTURE OF 5‐BROMODEOXYURIDYLYL‐(3' → 5')‐THYMIDINE PHOTOPRODUCT

Abstract

Abstract— –An attempt was made to identify some of the ultraviolet (u.v.) photoproducts of 5‐bromouracil‐labeled DNA (BrU‐DNA). Two synthetic dinucleotides, 5‐bromodeoxyuridylyl‐(3' →5 ')‐thymidine (BrdUpT) and 5‐bromodeoxyuridylyl‐(3' → 5')‐deoxycytidine (BrdUpdC), were prepared. Each gave a single u.v. photoproduct which in turn gave a single acid hydrolysis product. 2‐ 14 C‐BrU‐DNA. prepared from E. coli B3, was irradiated (275–280 nm), hydrolyzed, and paper chromatographed in four systems. Comparison with the two synthetic photoproducts showed that if present at all, BrdUpT and BrdUpdC photoproducts could account for no more than 10 and 3.5 per cent respectively of the total photoproducts. At 55 per cent conversion of BrU into photoproducts, the major 14 C‐photoproduct was uracil (78 per cent); the remaining 22 per cent was made up of at least six products, three of which were reversed by 232 nm irradiation. The debrominated cyclobutane structure proposed by Haug for BrdUpT photoproduct has been shown to be incorrect. It was found to contain one atom of bromine per molecule. On the basis of nuclear magnetic resonance and u.v. spectra, two possible structures are proposed for the photoproduct, each containing an eight‐membered ring.