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

F 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.