Structural Features of an Exocyclic Adduct Positioned opposite an Abasic Site in a DNA Duplex

Structural studies have been extended to dual lesions where an exocyclic adduct is positioned opposite an abasic site in the center of a DNA oligomer duplex. NMR and energy minimization studies were performed on the 1 N²-propanodeoxyguanosine exocyclic adduct (X) positioned opposite a tetrahydrofuran abasic site (F) with the dual lesions located in the center of the (C1-A2-T3-G4-X5-G6-T7-A8-C9) (G10-T11-A12-C13-F14-C15-A16-T17-G18) X•F 9-mer duplex. Two-dimensional NMR experiments establish that the X•F 9-mer helix is right-handed with Watson-Crick A•T and G•C base pairing on either side of the lesion site. NOEs are detected from the methylene protons of the exocyclic ring of X5 to the imino protons of G4•C15 and G6•C13 which flank the lesion site, as well as to the H1′ and H1″ protons of the cross strand F14 tetrahydrofuran moiety. These NMR results establish that the exocyclic adduct X5 is positioned between flanking G4•C15 and G6•C13 base pairs and directed toward the abasic lesion F14 on the partner strand. These studies establish that the exocyclic ring of the 1 N²-propanodeoxyguanosine adduct fits into the cavity generated by the abasic site.