Conformational analysis of an RNA triplet in aqueous solution: m62A‐U‐m62A studied by two‐dimensional nuclear magnetic resonance at 500 MHz

The complete assignment of the 18 ribose proton NMR signals of the trinucleoside diphosphate m62A(1)‐U(2)‐m62A(3) is given. The analysis was carried out by means of spin‐echo‐correlated spectroscopy and two‐dimensional J‐resolved spectroscopy at 500 MHz. Analysis of the chemical shifts and coupling constants demonstrates that the trimer does not form a classical right‐handed helix. Instead, the interior U(2) fragment appears to be “bulged‐out”, concomitant with a next‐nearest‐neighbour interaction between the terminal purine bases. An analogy with the behaviour of the first three residues in the tetramer m62A(1)‐U(2)‐m62A(3)‐U(4) is noted. An analysis of the coupling constants in terms of the N/S equilibrium and population distribution around the torsional degrees of freedom along the sugar‐phosphate backbone is presented.