super(19)F 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 containing biosynthetically incorporated (2S,4S)-5-fluoroleucine ([5-F]-Leu DHFR) and have obtained its super(1)H and super(19)F NMR spectra at 9.4 Tesla. The super(19)F spectrum of [5-F]-Leu DHFR showed 12 fairly sharp peaks (one containing two overlapped signals) for the 13 leucine residues in DHFR, covering a chemical shift range of 15 ppm. The large range of chemical shifts observed could not be explained solely in terms of the electrostatic field effects due to local charge fields and is thought to have a second contribution from side-chain conformational differences ( gamma -gauche effects) between different leucine residues, making super(19)F NMR of aliphatic amino acids in proteins a potentially useful new probe of protein structure.