19F nuclear magnetic resonance of 5-fluorouridine-substituted tRNA1Val from Escherichia coli.

The 19F NMR spectrum of Escherichia coli tRNA1Val in which [5-19F]uridine replaces 93% of all uridine and uridine-derived residues has been examined at 93.6 and 235 MHz. The resolution of 11 peaks and visibility of two additional shoulders at either frequency for the 14 FUra residues in the molecule attests to the excellence of 19F as a probe for the structure of tRNA1Val in solution. No significant gain in resolution was attained at the higher frequency. A comparison of the relative areas in the different regions of the 19F spectrum of mixed [FUra]tRNAs with that of [FUra]tRNA1Val suggests that the three single resonances at lowest field in the region 86.5 to 88.5 ppm upfield from trifluoroacetate correspond to the three invariant bases which form tertiary hydrogen bonds in all tRNAs, namely, 8 (U or s4U), 54 (T), and 55 (phi) in unsubstituted tRNAs.