Thermodynamic measurements of bilayer insertion of a single transmembrane helix chaperoned by fluorinated surfactants

Accurate determination of the free energy of transfer of a helical segment from an aqueous into a transmembrane (TM) conformation is essential for understanding and predicting the folding and stability of membrane proteins. Until recently, direct thermodynamically sound measurements of free energy of insertion of hydrophobic TM peptides were impossible due to peptide aggregation outside the lipid bilayer. Here, we overcome this problem by using fluorinated surfactants that are capable of preventing aggregation but, unlike detergents, do not themselves interact with the bilayer. We have applied the fluorescence correlation spectroscopy methodology to study surfactant-chaperoned insertion into preformed POPC (palmitoyloleoylphosphatidylcholine) vesicles of the two well-studied dye-labeled TM peptides of different lengths: WALP23 and WALP27. Extrapolation of the apparent free-energy values measured in the presence of surfactants to a zero surfactant concentration yielded free-energy values of -9.0±0.1 and -10.0±0.1 kcal/mol for insertion of WALP23 and WALP27, respectively. Circular dichroism measurements confirmed helical structure of peptides in lipid bilayer, in the presence of surfactants, and in aqueous mixtures of organic solvents. From a combination of thermodynamic and conformational measurements, we conclude that the partitioning of a four-residue L-A-L-A segment in the context of a continuous helical conformation from an aqueous environment into the hydrocarbon core of the membrane has a favorable free energy of 1 kcal/mol. Our measurements, combined with the predictions of two independent experimental hydrophobicity scales, indicate that the per-residue cost of transfer of the helical backbone from water to the hydrocarbon core of the lipid bilayer is unfavorable and is equal to +2.13±0.17 kcal/mol.