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

Amphipathic alpha-helices are the membrane binding motif in many proteins.The corresponding peptides are often random coil in solution but are folded into an alpha-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 a-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 a-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 Delta H-helix approximate to - 0.7 kcal/mol per residue, an entropy change of Delta S-helix approximate to - 1.9 cal/molK residue and a free energy change of Delta G(helix) approximate to - 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. (C) 1999 Academic Press.