Comparative effects of trifluoromethyl- and methyl-group substitutions in proline

Abstract

Proline is one of a kind. This amino acid exhibits a variety of unique functions in biological contexts, which continue to be discovered and developed. In addition to the reactivity of the primary functional groups, the trans-cis isomerization of the peptidyl-prolyl amide bond and its impact on the protein structure and function are of major interest. A variety of proline ring substitutions occur in nature, and more substitutions have been generated via chemical synthesis. Particularly promising is the 19F-labelling of proline, which offers a relatively new research application area. For example, it circumvents the lack of common NH-NMR reporters in peptidyl-prolyl fragments. Obtaining structural information from selectively fluorine-labelled peptides, proteins, and non-peptidic structures requires the analysis of the physicochemical features of the 19F-carrying proline analogues. To better understand and ultimately predict the potential perturbations (e.g., in protein stability and dynamics) introduced by fluorine labels, we conducted a comprehensive survey of the physicochemical effects of CF3 substitutions at each ring position by comparing the behavior of CF3-substituted residues with the CH3-substituted analogues. The parameters analyzed include the acid-base properties of the main chain functional groups, carbonyl-group interaction around the residue, and the thermodynamics and kinetics of trans-cis isomerization. The results reveal significant factors to consider with the use of CF3-substituted prolines in NMR labeling and other applications. Furthermore, lipophilicity measurements demonstrate that CF3-substituted proline shows comparable hydrophobicity to valine, suggesting the potential application of these residues for enhancing interactions at nonpolar interfaces.