Photochemistry of 5-bromouridine and 5-bromo-2'-deoxyuridine in ice and in 'puddles'

5,5-Diuridinyl and 5,5'-dideoxyuridinyl have been isolated from UV irradiation (254 nm) in a frozen state of 5-bromouridine and 5-bromodeoxyuridine, respectively. These compounds were characterized by UV, IR, NMR,and mass spectra. They are formed in relatively high quantum yields which are comparable to thymidine dimerization and were found to be unaffected upon further irradiation. This photostability suggests that these 5,5'-coupled products, if formed, may accumulate in biological systems. Experiments carried out in 'puddles' indicate, by negative inference, that the main reaction pathway is probably the recombination of two radicals to yield the coupled products in a frozen state. Therefore, the coupled products formation should be explored as a possible mechanism for the increased radiation and photosensitivity of biological systems where thymine in DNA is replaced with 5-halouracil. In addition, these compounds may be synthesized chemically, though in very low yields, approximately 7%. (author)