Genetically Encoded Pentafluorophenylalanine Enables Quantitative Probing of Local Protein Malleability by 19F NMR.

Aromatic ring flips in proteins provide a direct probe of local structural fluctuations, yet their rates are typically too fast for quantitative measurement by NMR spectroscopy. Here we show that site-specific incorporation of 2,3,4,5,6-pentafluoro-l-phenylalanine (F5Phe) reshapes the torsional energy landscape of aromatic side chains, slowing ring flips by over 2 orders of magnitude and shifting them into the slow-exchange regime accessible by 19F NMR. F5Phe can be genetically encoded with high fidelity and minimal structural perturbation, as confirmed by high-resolution X-ray crystallography across multiple proteins. The resulting 19F NMR spectra enable direct, quantitative measurements of ring-flip kinetics without the need for isotope labeling or complex multidimensional experiments. Application to a diverse set of proteins demonstrates that ring-flip rates vary widely even within the same hydrophobic cluster, revealing highly localized conformational fluctuations rather than global unfolding events. Pressure-dependent measurements yield small activation volumes, indicating that the structural rearrangements enabling ring flips are spatially confined. Ligand binding and protein-protein interactions modulate ring-flip rates in a site-specific manner, providing a sensitive readout of allosteric effects on local protein malleability. These results establish fluorinated aromatic amino acids as a general chemical strategy to engineer dynamic observables in proteins, transforming aromatic ring flips into a broadly applicable probe of local conformational dynamics and allostery.