The cyclobutane dimers of 2'-deoxyuridine, 2'-deoxycytidine, 5-methyl-2'-deoxycytidine and 5-bromo-2'-deoxyuridine.

Irradiation of DNA and RNA pyrimidine nucleosides with UV light in frozen aqueous solution or in solution with acetone often results in the formation of cyclobutane dimers (CBDs). Many of these photodimers have not been characterized. We present here the results of work designed to achieve the isolation, spectroscopic characterization and determination of the stereochemical nature of a number of little studied or previously unstudied CBDs of four 2′-deoxyribonuclesides. These nucleosides are 2′-deoxyuridine (dUrd), 2′-deoxycytidine (dCyd), 5-methyl-2′-deoxycytidine (5-MedCyd) and 5-bromo-2′-deoxyuridine (5-BrdUrd). In particular, we have isolated and characterized six dUrd CBDs, five dCyd CBDs, five 5-MedCyd CBDs and four 5-BrdUrd CBDs. Photoproducts were studied by UV spectroscopy, mass spectrometry, proton NMR spectroscopy and via chemical approaches. Also presented are results from less definitive studies of a number of (6-4) (or 5-4) photoadducts of these nucleosides. In addition, results from exploratory photochemical studies of other 2′-deoxyribonucleosides in frozen solution, as well as some mixtures of two nucleosides, are given. The latter results indicate that 5-iodo-2′-deoxyuridine (5-IdUrd), 5-bromo-2′-deoxycytidine and 5-iodo-2′-deoxycytidine each form putative CBDs and that 5-BrdUrd is capable of forming putative mixed CBDs and (6-4) and/or (5-4) adducts with thymidine (Thd); 5-IdUrd similarly forms a (6-4) (or (5-4)) adduct with Thd.