Thermodynamics of the α-helix-coil transition of amphipathic peptides in a membrane environment: implications for the peptide-membrane binding equilibrium

Abstract

Amphipathic α-helices are the membrane binding motif in many proteins. The corresponding peptides are often random coil in solution but are folded into an α-helix upon interaction with the membrane. The energetics of this ubiquitous folding process are still a matter of conjecture. Here, we present a new method to quantitatively analyze the thermodynamics of peptide folding at the membrane interface. We have systematically varied the helix content of a given amphipathic peptide when bound to the membrane and have correlated the thermodynamic binding parameters determined by isothermal titration calorimetry with the α-helix content obtained by circular dichroism spectroscopy. The peptides investigated were the antibiotic magainin 2 amide and three analogs in which two adjacent amino acid residues were substituted by their d-enantiomers. The thermodynamic parameters controlling the α-helix formation were found to be linearly related to the helicity of the membrane-bound peptides. Helix formation at the membrane surface is characterized by an enthalpy change of ΔHhelix≈−0.7 kcal/mol per residue, an entropy change of ΔShelix≈−1.9 cal/molK residue and a free energy change of ΔGhelix=−0.14 kcal/mol residue. Helix formation is a strong driving force of peptide insertion into the membrane and accounts for about 50 % of the free energy of binding. An increase in temperature entails an unfolding of the membrane-bound helix. The temperature dependence can be described with the Zimm-Bragg theory and the enthalpy of unfolding agrees with that deduced from isothermal titration calorimetry.