| Hydrophobic interactions are a major driving force in the folding of soluble proteins; hence de novo design of such proteins is relatively easy. However, control of structure and function in membrane proteins remains a formidable challenge. Perfluorinated organic molecules are more hydrophobic than their hydrocarbon counterparts and form a distinct immiscible phase. The design, synthesis and structural characterization of peptide systems based on the coiled-coils with highly fluorinated cores are described. Three peptide systems were studied. In one study, all four leucine residues (a position) and three valine residues ( d position) of a model coiled-coil protein were replaced by the unnatural amino acids 5,5,5-trifluoroleucine and 4,4,4-trifluorovaline respectively. The fluorocarbon containing peptide exhibited a melting temperature that was 15°C higher than the control peptide, as well as a larger free energy of unfolding by ∼1.0 kcal/mol. In another study, the peptide systems with hydrophobic cores composed entirely of leucine or hexafluoroleucine showed preference for homodimeric assemblies over the heterodimer as probed by a disulfide exchange assay. The fluorinated assembly is much more stable than the hydrogenated peptide dimer and the heterodimer, as judged by thermal and chemical denaturation studies and is responsible for driving the equilibrium in favor of homodimers. This orthogonal character of highly fluorinated coiled-coil cores was utilized in a new design paradigm for the self-assembly of peptides in the context of nonpolar environments of biological membranes. Transmembrane helix-helix interactions were established in a two step assembly process; first, hydrophobic peptides partitioned into micellar lipids, then subsequent phase separation of simultaneously hydrophobic and lipophobic fluorinated helical surfaces promoted spontaneous self-assembly of higher order oligomers. |
