Environmentally Ultrasensitive Fluorine Probe to Resolve Protein Conformational Ensembles by (19)F NMR and Cryo-EM

[Image: see text] Limited chemical shift dispersion represents a significant barrier to studying multistate equilibria of large membrane proteins by (19)F NMR. We describe a novel monofluoroethyl (19)F probe that dramatically increases the chemical shift dispersion. The improved conformational sensitivity and line shape enable the detection of previously unresolved states in one-dimensional (1D) (19)F NMR spectra of a 134 kDa membrane transporter. Changes in the populations of these states in response to ligand binding, mutations, and temperature correlate with population changes of distinct conformations in structural ensembles determined by single-particle cryo-electron microscopy (cryo-EM). Thus, (19)F NMR can guide sample preparation to discover and visualize novel conformational states and facilitate image analysis and three-dimensional (3D) classification.