NMR SOLUTION STRUCTURE OF AN OLIGODEOXYNUCLEOTIDE DUPLEX CONTAINING THE EXOCYCLIC LESION 3,N-4-ETHENO-2'-DEOXYCYTIDINE OPPOSITE THYMIDINE -COMPARISON WITH THE DUPLEX CONTAINING DEOXYADENOSINE OPPOSITE THE ADDUCT

The exocyclic 3,N-4-etheno-2'-deoxycytidine adduct was incorporated at the center of the oligodeoxynucleotide duplex d(C-G-T-A-C-epsilon C-C -A-T-G-C). d(G-C-A-T-G-T-G-T-A-C-G), and its solution structure was an alyzed using high-resolution proton NMR spectroscopy and molecular dyn amics simulations. The experimental data indicate that the oligodeoxyn ucleotide duplex adopts a right-handed helical structure with sugar pu ckers in the C2'-endo/C3'-exo range and Watson-Crick hydrogen bond ali gnments for all base pairs. NOE connectivities established a syn orien tation for the glycosidic torsion angle of the exocyclic adduct. Restr ained molecular dynamics simulations, using the full relaxation matrix approach, produced a three-dimensional model in agreement with the ex perimental data. The structure shows only minor perturbations in the s ugar-phosphate backbone and a 27 degrees bend of the helical axis at t he lesion site. On the refined model a well-formed hydrogen bond betwe en T(N3H) and epsilon C(N4) stabilizes the epsilon C(syn). T(anti) bas e pair alignment, reflecting the preference of the adduct for the syn orientation. Furthermore, the epsilon C(syn). T(anti) base pair stacks with flanking base pairs. We discuss a correlation between the mutage nic properties of the adduct and the three-dimensional structure of th e epsilon C . dA and epsilon C . T duplexes.