A comparative high-field NMR study on tri-, tetra-, penta-, and hepta-mer ribonucleotides modelling the branch site of Group II and nuclear messenger RNA spliced lariat introns

Conformational analysis of the branched RNA systems (1)–(4) using variable-temperature 600 MHz proton and 223 MHz phosphorus-31 n.m.r. spectroscopy revealed that (i), generation of a lariat structure, typical for Group II and nuclear messenger RNA splicing, can occur with preservation of the A-geometry of the RNA strand; (ii), mimicry of the lariat branch site requires the presence of at least one nucleotide linked to the 5'-site of the branch point [as in tetramer (2) and heptamer (4)], since the absence of this nucleotide [as in trimer (1) and pentamer (3)] results in an unnatural molecular conformation which is largely dominated by adenine 2'–5' guanine base stacking.