Conformational analysis of oligoarabinonucleotides. An NMR and CD study

A 500 and 300 MHz proton NMR study of the series of oligoarabinonucleotides 5'aAMP, 3'aAMP, aA-aA, (aA-)2aA and (aA-)3aA is presented. In addition, circular dichroism is used to study the stacking behaviour of aA-aA. The complete 1H-NMR spectral assignment of the compounds (except the tetramer) is given. Proton-proton and proton-phosphorus coupling constants, obtained by computer simulation of the high-field region of the spectra, yield information on the conformation of the arabinose rings (N- or S-type) and on the intramolecular stacking properties of the dimer and the trimer. The monomers 5'aAMP and 3'aAMP exhibit a preference for N- and S-type sugar conformation, respectively. It is shown that the dimer aA-aA at low temperature prefers a mixed stacked state of the type aA(S)-aA(N). In the trimer the aA(2)-aA(3) fragment exhibits a conformation similar to that found in the dimer, whereas the aA(1) residue prefers to adopt S-type sugar and has some tendency to stack upon residue aA(2).