Cross-strand interactions of fluorinated amino acids in β-hairpin constructs.

We describe herein the design, synthesis, and thermodynamic characterization of fluorinated β-hairpin constructs. Introduction of hexafluoroleucine (Hfl) did not perturb β-hairpin formation, as judged by (1)H NMR structures of four peptides determined to <1 Å backbone RMSDs, allowing direct comparison of thermodynamic stabilities of fluorinated peptides to their hydrocarbon counterparts. Judicious fluorination of peptides often results in increased thermal and chemical stability of the resultant folded structures. However, we found that when cross-strand residue partners were varied, the side-chain interaction energies followed the order Leu-Leu > Hfl-Leu > Hfl-Hfl. All peptides were more structured in 90% MeOH than in aqueous buffers. The peptides with Hfl-Leu or Hfl-Hfl cross-strand partners showed increased interaction energies in this solvent compared to those in water, in contrast to the insignificant effect on Leu-Leu. Our results inform the binding and assembly of peptides containing Hfl in the context of β-sheet structures and may be useful in interpreting binding of fluorinated ligands and peptides to biological targets.