NMR studies of bipyrimidine cyclobutane photodimers

Abstract

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.