NMR studies of bipyrimidine cyclobutane photodimers

Cyclobutane-type photodimers of dinucleoside monophosphates dCpdT, dTpdC and dTpdT were prepared by ultraviolet irradiation in the presence of acetophenone as photosensitizer. The cytosine-containing derivatives were found to deaminate forming uracil products. Using one- and two-dimensional NMR, the photoproducts were characterized as cis-syn and trans-syn cyclobutane photodimers. On the basis of NOE data the structures of the cis-syn and trans-syn products of dUpdT were determined using distance-geometry and restrained-energy-minimization methods. The cis-syn structures showed (high-ANTI/SYN)/high-ANTI glycosidic linkages while the trans-syn structures were in the SYN-ANTI region. The backbone conformations of both structures were in fair agreement with the coupling-constant-data. The trans-syn structures were found to be very rigid and similar in all three products. For the three cis-syn structures more conformational freedom and more variation among the three structures was observed.