Through-Space Scalar ¹⁹F–¹⁹F Couplings between Fluorinated Noncanonical Amino Acids for the Detection of Specific Contacts in Proteins

Abstract

Fluorine atoms are known to display scalar ¹⁹F-¹⁹F couplings in nuclear magnetic resonance (NMR) spectra when they are sufficiently close in space for nonbonding orbitals to overlap. We show that fluorinated noncanonical amino acids positioned in the hydrophobic core or on the surface of a protein can be linked by scalar through-space ¹⁹F-¹⁹F (^TS<i>J</i>_FF) couplings even if the ¹⁹F spins are in the time average separated by more than the van der Waals distance. Using two different aromatic amino acids featuring CF₃ groups, <i>O</i>-trifluoromethyl-tyrosine and 4-trifluoromethyl-phenylalanine, we show that ¹⁹F-¹⁹F TOCSY experiments are sufficiently sensitive to detect ^TS<i>J</i>_FF couplings between 2.5 and 5 Hz in the 19 kDa protein PpiB measured on a two-channel 400 MHz NMR spectrometer with a regular room temperature probe. A quantitative <i>J</i> evolution experiment enables the measurement of ^TS<i>J</i>_FF coupling constants that are up to five times smaller than the ¹⁹F NMR line width. In addition, a new aminoacyl-tRNA synthetase was identified for genetic encoding of <i>N</i>⁶-(trifluoroacetyl)-l-lysine (TFA-Lys) and ¹⁹F-¹⁹F TOCSY peaks were observed between two TFA-Lys residues incorporated into the proteins AncCDT-1 and mRFP despite high solvent exposure and flexibility of the TFA-Lys side chains. With the ready availability of systems for site-specific incorporation of fluorinated amino acids into proteins by genetic encoding, ¹⁹F-¹⁹F interactions offer a straightforward way to probe the spatial proximity of selected sites without any assignments of ¹H NMR resonances.