An NMR and conformational investigation of the trans-syn cyclobutane photodimers of dUpdT

Both trans-syn cyclobutane-type photodimers of 2'-deoxyuridylyl (3'-5') thy midine (dUpdT) were formed by deamination of the corresponding trans-syn cy clobutane photodimers of 2'-deoxycytidylyl (3'-5') thymidine (dCpdT) and we re examined by H-1-, C-13-, and P-31-nmr spec troscopy. One- and two-dimens ional nmr experiments provided a nearly complete assignment of the H-1, C-1 3, and P-31 resonances. Scalar and nuclear Overhauser effect contacts were used to determine the conformation of the deoxyribose rings, exocyclic bond s, cyclobutane rings, and glycosidic linkages. Isomer I (S-type class; CB-; SYN-ANTI) and isomer II (N-type class; CB+; ANTI-SYN) exhibit markedly dif ferent conformational features. P-31 chemical shifts show that the relative flexibility is dUpdT > isomer II > isomer I. The conformations of these sp ecies are very similar to those of other previously examined trans-syn phot odimers. Among bipyrimidine photodimers of a given diastereomeric form (i.e ., trans-syn I or II), the nmr-derived conformational parameters are nearly invariant, regardless of base substitution pattern. This contrasts with th e substituent-dependent variation of cyclobutane ring conformation observed by Kim et al. (Biopolymers, 1993, Vol. 33, pp. 713-721)for an analogous se ries of cis-syn photodimers. Steric crowding of cyclobutane ring substituen ts is offered as an explanation for the difference in substituent effects b etween the families of cis-syn and trans-syn photodimers. (C) 1999 John Wil ey & Sons, Inc.