Mixed Fluorotryptophan Substitutions at the Same Residue Expand the Versatility of F Protein NMR Spectroscopy

The strategy of applying fluorine NMR to characterize ligand binding to a membrane protein prepared with mixtures of tryptophans substituted with F at different positions on the indole ring was tested. The F NMR behavior of 4-, 5-, 6-, and 7-fluorotryptophan were directly compared as a function of both micellar environment and fragment size for two overlapping apelin receptor (AR/APJ) segments; one with a single transmembrane (TM) helix and two tryptophan residues, the other with three TM helices and two additional tryptophan residues. Chemical shifts, peak patterns, and nuclear spin relaxation rates were observed to vary as a function of micellar conditions and F substitution position in the indole ring, with the exposure of a given residue to micelle or solvent being the primary differentiating factor. Titration of the 3-TM AR segment biosynthetically prepared as a mixture of 5- and 7-fluorotryptophan-containing isoforms by two distinct peptide ligands (apelin-36 and apela-32) demonstrated site-specific F peak intensity changes for one ligand but not the other. In contrast, both ligands perturbed H- N HSQC peak patterns to a similar degree. Characterization of multiple fluorotryptophan types for a given set of tryptophan residues, thus, significantly augments the potential to apply F NMR to track otherwise obscure modulation of protein conformation and dynamics without an explicit requirement for mutagenesis or chemical modification.