Nuclear magnetic resonance studies on transfer ribonucleic acid: assignment of AU tertiary resonances.

The hydrogen-bonded ring NH nuclear magnetic resonance (NMR) spectra of several transfer ribonucleic acid (RNA) species have been examined with particular emphasis on the extreme low-field portion. Betwen --13.8 and --15 ppm there are two extra resonances which are not derived from cloverleaf base pairs. A combined approach involving undermodified tRNAs, chemical modification, and hairpin fragment studies has assigned the T54--A58 resonance at --14.3 ppm in yeast tRNAPhe and Escherichia coli tRNA1 Val., the U8--A14 resonance has been assigned at --14.3 ppm, and the s4U8--A14 resonance in bacterial tRNAs has been assigned at --14.9 ppm. The T54--A58 resonance shifts between --14.3. and --13.8 ppm depending on the surrounding nucleotide sequence in the ribothymidine loop.