F-19 NUCLEAR-MAGNETIC-RESONANCE CHEMICAL-SHIFTS OF FLUORINE-CONTAINING ALIPHATIC AMINO-ACIDS IN PROTEINS - STUDIES ON LACTOBACILLUS-CASEI DIHYDROFOLATE-REDUCTASE CONTAINING (2S,4S)-5-FLUOROLEUCINE

We have prepared Lactobacillus casei dihydrofolate reductase containin g biosynthetically incorporated (2S,4S)-5-fluoroleucine ([5-F]-Leu DHF R) and have obtained its H-1 and F-19 NMR spectra at 9.4 Tesla. The F- 19 spectrum of [5-F]-Leu DHFR showed 12 fairly sharp peaks (one contai ning two overlapped signals) for the 13 leucine residues in DHFR, cove ring a chemical shift range of 15 ppm. The large range of chemical shi fts observed could not be explained solely in terms of the electrostat ic field effects due to local charge fields and is thought to have a s econd contribution from side-chain conformational differences (gamma-g auche effects) between different leucine residues, making F-19 NMR of aliphatic amino acids in proteins a potentially useful new probe of pr otein structure.