A coiled coil with a fluorous core

The design, synthesis, and structural characterization of a highly fluorinated peptide system based on the coiled coil region of the yeast transcription factor GCN4 is described. All four leucine residues (a position) and three valine residues (d position) were replaced by the unnatural amino acids 5,5,5-trifluoroleucine and 4,4,4-trifluorovaline, respectively. The peptide is highly alpha -helical at low micromolar concentrations as judged by circular dichroism spectra, sediments as a dimeric species in the 5-30 muM concentration range, and exhibits a dimer melting temperature that is 15 degreesC higher than a control peptide with a hydrocarbon core. Furthermore, the apparent free energy of unfolding as calculated from guanidinium hydrochloride denaturation experiments is larger by 1.0 kcal/mol for the fluorinated peptide than its hydrocarbon counterpart. We conclude that additional stability is derived from sequestering the more hydrophobic trifluoromethyl groups from aqueous solvent. These studies introduce a new paradigm in the design of molecular self-assembling systems, one based on orthogonal solubility properties of liquid phases.