Ultraviolet irradiation of nucleic acids: formation, purification, and solution conformational analyses of oligothymidylates containing cis-syn photodimers.

Acetone-photosensitized UV irradiation of three thymine oligomers, d(TpT), d(TpTpT), and d(TpTpTpT), forms predominantly cis-syn cyclobutyl photodimers. C-18 reverse-phase high-performance liquid chromatography is used to purify the following positional isomers: d(TpT[p]T), d(T[p]TpT), d(TpTpT[p]T), d(TpT[p]TpT), d(T[p]TpTpT), and d(T[p]TpT[p]T), where T[p]T represents the cis-syn photodimer. Conformational properties of the cis-syn dimers and adjacent thymine nucleotides have been investigated in solution by using 1H, 13C, and 31P NMR spectroscopy. These studies show that (1) the photodimer conformation in longer oligothymidylates is similar to that in the dinucleoside monophosphate and (2) the cis-syn dimer induces alterations to a greater degree on the 5' side than on the 3' side of the photodimer. Specifically, the photodimer distorts the exocyclic bonds epsilon(C3'-O3') in Tp- and gamma(C5'-C4') in -pT[p]- on the 5' side and slightly alters the furanose equilibrium of the -pT nucleotide on the 3' side of the dimer.