Probing Peptidylprolyl Bond cis/trans Status Using Distal ¹⁹ F NMR Reporters

A method for measuring peptidylprolyl bond cis-trans conformational status in peptide models is described, using 4-fluorophenylalanine (4FPhe) as a distal reporter for ¹⁹ F NMR. The %cis-Pro population was measured for peptides of the general structure Ac-X-Pro-Z-Ala-Ala-4FPhe (X and Z are proteinogenic amino acids) at pH 7.4, and provided conformational populations consistent with literature values obtained by more complex methods. This approach was applied to probe the prolyl bond status in pentapeptide models of the intrinsically disordered C-terminal region of α-synuclein, which mirrored the preferences in the Ac-X-Pro-Z-Ala-4FPhe models. Advantageously, the ¹⁹ F reporter group does not need to be adjacent to or attached to proline to provide quantifiable signals and distal 4-fluorophenylalanines can be placed so as not to influence prolyl bond conformation. Finally, we demonstrated that the prolyl bond status is not significantly affected by pH when there are ionisable amino acid residues at the carboxyl side of proline, which makes ¹⁹ F NMR an invaluable tool with which to study proline isomerism at a range of pHs and in different solvents and buffers.