A Proton Magnetic Resonance Study of the Ultraviolet Photoproduct of d(TpT) in Aqueous Solution

The p.m.r. data of d(TpT) and its predominant u.v. photoproduct d(T[p]T) in aqueous solution are compared. The data are consistent with the presence of a cis-syn cyclobutane ring in d(T[p]T). Consideration of the H 1′ chemical shifts leads to the conclusion that changes in the sugar–base torsion angle of both nucleotide fragments of the parent molecule are required to bring the thymine bases into alignment for photodimerization. The coupling constants indicate that the conformation of the 5′-terminal fragment is only slightly affected by the cyclobutane ring formation. Photodimerization brings about a distortion of the sugar pucker in the 3′-terminal fragment. In this fragment both the gauche-gauche and gauche-trans conformers are significantly populated whereas the trans-gauche form is excluded.