(2 S ,4 S )-5-Fluoroleucine, (2 S ,4 R )-5-Fluoroleucine, and 5,5'-Difluoroleucine in Escherichia coli PpiB: Protein Production, 19 F NMR, and Ligand Sensing Enhanced by the γ-Gauche Effect.

Global substitution of leucine for analogues containing CH 2 F instead of methyl groups delivers proteins with multiple sites for monitoring by 19 F nuclear magnetic resonance (NMR) spectroscopy. The 19 kDa Escherichia coli peptidyl-prolyl cis-trans isomerase B (PpiB) was prepared with uniform high-level substitution of leucine by (2 S ,4 S )-5-fluoroleucine, (2 S ,4 R )-5-fluoroleucine, or 5,5'-difluoroleucine. The stability of the samples toward thermal denaturation was little altered compared to the wild-type protein. 19 F nuclear magnetic resonance (NMR) spectra showed large chemical shift dispersions between 6 and 17 ppm. The 19 F chemical shifts correlate with the three-bond 1 H- 19 F couplings ( 3 J HF ), providing the first experimental verification of the γ-gauche effect predicted by [Feeney, J. J. Am. Chem. Soc. 1996, 118, 8700-8706] and establishing the effect as the predominant determinant of the 19 F chemical shifts of CH 2 F groups. Individual CH 2 F groups can be confined to single rotameric states by the protein environment, but most CH 2 F groups exchange between different rotamers at a rate that is fast on the NMR chemical shift scale. Interactions between fluorine atoms in 5,5'-difluoroleucine bias the CH 2 F rotamers in agreement with results obtained previously for 1,3-difluoropropane. The sensitivity of the 19 F chemical shift to the rotameric state of the CH 2 F groups potentially renders them particularly sensitive for detecting allosteric effects.