Influence of 2-aminoadenosine, A', on the conformational behaviour of d(T-A'-T-A'). A one-dimensional proton NMR study at 300 MHz and 500 MHz

Proton NMR studies at 300 MHz and 500 MHz in aqueous solution were carried out on the modified self-complementary tetranucleoside trisphosphate d(T-A'-T-A'), in which d(A') represents 2-aminodeoxyadenosine. The NMR spectra were observed at two sample concentrations over the temperature range 2-70 degrees C. Assignments, based on homonuclear decoupling and nuclear Overhauser enhancement (NOE) experiments, are given. The concentration dependence of the chemical shift vs temperature profiles was used to extract information concerning duplex formation. The proton-proton and proton-phosphorus coupling constants were obtained at four temperatures and yielded accurate conformational data on the sugar ring and on the back-bone angles beta, gamma and delta. From the observed line broadening, shift profiles, NOEs, and from the observation of imino proton resonances, it is concluded that the compound exists as a miniduplex at low temperature. Comparison of these observations with similar observations on the parent compound d(T-A-T-A) indicate that substitution of dA by dA' increases the tendency towards duplex formation. At low temperature the compound adopts a stacked B-DNA type structure: destacking occurs on raising the temperature. The sequence-dependent sugar ring geometry [Mellema, J.-R., Pieters, J. M. L., van der Marel, G. A., van Boom, J. H., Haasnoot, C. A. G. & Altona, C. (1984) Eur. J. Biochem. 143, 285-301] is present in this molecule. The conformational parameters of d(T-A'-T-A') and d(T-A-T-A) are quite similar, thus substitution of dA by dA' has no measurable influence on the geometry of the sugar ring or on the backbone angles beta, gamma and delta.