Conformational analysis of the single-stranded ribonucleic acid A-A-C-C. A one-dimensional and two-dimensional proton NMR study at 500 MHz.

Proton NMR studies at 300 MHz and 500 MHz are reported on the ribotetranucleotide A-A-C-C. The complete 1H-NMR spectral assignment at 20 degrees C is given. Two-dimensional NMR was used to elucidate spin multiplets in 'crowded' regions. Nuclear Overhauser enhancement (NOE) experiments made an unambiguous spectral assignment possible and yielded information on interproton distances. The N/S equilibrium of the riboses and the rotamer populations around some backbone torsion angles are presented. A large preference for N-type ribose and gamma+ and beta t backbone torsions is observed, in particular in the central A-C unit of A-A-C-C. Information on distances between protons of different nucleotide units, obtained from NOE experiments, constitutes a probe of base-base stacking. It is concluded that A-A-C-C offers a good model for RNA single-strand conformation.