Conversions of Poly(2-aminodeoxyadenylate-5-halodeoxyuridylate) from B to A Forms in High Salt. An NMR and Circular Dichroism Study

Proton one- and two-dimensional nuclear Overhauser enhancement (1D and 2D NOE) spec- troscopy has been used to demonstrate that poly(d2NH₂A-d5IU) and poly(d2NH₂A-d5BrU) are converted from the B to the A conformation in high salt, as found previously for poly(d2NH₂A-dT) [Borah, B., Cohen, J. S., Howard, F. B., & Miles, H. T. (1985) Biochemistry 24, 7456–7462]. The 2D NOE and 1D NOE spectra exhibit strong base proton (H8, H6)-H3‘ cross relaxation, suggesting short interproton distances. These results are indicative of a C3’-endo sugar pucker for both purine and pyrimidine residues in an A or closely related structure. The circular dichroism and UV spectra are consistent with the interpretation of an A conformation in high salt.