Solution conformation of an RNA hairpin loop

The hairpin conformation adopted by the RNA sequence 5'GCGAUUUCUGACCGCC3' has been studied by one- and two-dimensional NMR spectroscopy. Exchangeable imino spectra in 60 mM Na+ indicate that the hairpin has a stem of six base pairs (indicated by boldface type) and a loop of three nucleotides. NOESY spectra of nonexchangeable protons confirm the formation of the stem region. The duplex has an A-conformation and contains an A.C apposition; a G.U base pair closes the loop region. The stem nucleotides have C3'-endo sugar conformations, as expected of an A-form duplex, whereas the three loop nucleotides adopt C2'-endo sugar puckers. Stacking within the loop, C8 upon the sugar of U7, stabilizes the structure. The pH dependence of both the exchangeable and nonexchangeable NMR spectra is consistent with the formation of an A+.C base pair, protonated at the N1 position of adenine. The stability of the hairpin was probed by using absorbance melting curves. The hairpin structure with the A+.C base pair is about +2 kcal/mol less stable in free energy at 37 degrees C than the hairpin formed with an A.U pair replacing the A+.C pair.