RESUMO
NMR and CD studies were carried out on the dinucleotides 5'-methylphospho-N6-dimethyladenylyl-uridine (mpm62-U) and 5'-methylphospho-uridylyl-N6-dimethyladenosine (mpU-m62A) and on the trinucleotide U-m62A-U. A detailed comparison is given of the conformational features of mpm62A-U and mpU-m62A with the corresponding 5'-nonphosphorylated dinucleotides m62A-U and U-m62A, respectively. The behaviour of the trinucleotide U-m62A-U is compared with the properties of the constituent dinucleotides U-m62A and mpm62A-U. Chemical-shift and CD data were used to determine the amount of stacking interactions. For each compound NMR spectra were recorded at two or three sample concentrations in order to separate intermolecular and intramolecular base-base interactions. The coupling constants of the ribose ring are interpreted in terms of the N/S equilibrium, and population distributions along the backbone angles beta, gamma and epsilon are presented. The combined data indicate a strong similarity between mpm62A-U and m62A-U both in degree and in mode of stacking. In contrast, the existence of different types of stacking interactions in mpU-m62A and U-m62A is suggested in order to explain the NMR and CD data. It is concluded that dinucleoside bisphosphates serve better as a model for the behaviour of trinucleotides than dinucleoside monophosphates. The trinucleotide U-m62A-U adopts a regular single-stranded stacked RNA structure with preference for N-type ribose and gamma+ and beta t backbone torsion angles. The difference in behaviour between the U-m62A- part of U-m62A-U and the dimer U-m62A is seen as a typical example of conformational transmission.
