Binding of antibacterial magainin peptides to electrically neutral membranes: thermodynamics and structure

Magainins are positively charged amphiphatic peptides which permeabilize cell membranes and display antimicrobial activity. They are usually thought to bind specifically to anionic lipids, and binding studies have been performed almost exclusively with negatively charged membranes. Here we demonstrate that binding of magainins to neutral membranes, a reaction which is difficult to assess with spectroscopic means, can be followed with high accuracy using isothermal titration calorimetry. The binding mechanism can be described by a surface partition equilibrium after correcting for electrostatic repulsion by means of the Gouy-Chapman theory. Unusual thermodynamic parameters are observed for the binding process. (i) The three magainin analogues that were investigated bind to neutral membranes with large exothermic reaction enthalpies DeltaH of -15 to -18 kcal/mol (at 30 degrees C). (ii) The reaction enthalpies increase with increasing temperature, leading to a large positive heat capacity DeltaC(p) of approximately 130 cal mol(-)(1) K(-)(1) (at 25 degrees C). (iii) The Gibbs free energies of binding DeltaG are between -6.4 and -8.6 kcal/mol, resulting in a large negative binding entropy DeltaS. The binding of magainin to small unilamellar vesicles is hence an enthalpy-driven reaction. The negative DeltaH and DeltaS and the large positive DeltaC(p) contradict the conventional understanding of the hydrophobic effect. CD experiments reveal that the membrane-bound fraction of magainin is approximately 80% helical at 8 degrees C, decreasing to approximately 60% at 45 degrees C. Since the random coil --> alpha-helix transition in aqueous solution is known to be an exothermic process, the same process occurring at the membrane surface is shown to account for up to 65% of the measured reaction enthalpy. In addition to membrane-facilitated helix formation, the second main driving force for membrane binding is the insertion of the nonpolar amino acid side chains into the lipid bilayer. It also contributes a negative DeltaH and follows the pattern for the nonclassical hydrophobic effect. Addition of cholesterol drastically reduces the extent of peptide binding and reveals an enthalpy-entropy compensation mechanism. Membrane permeability was measured with a dye assay and correlated with the extent of peptide binding. The level of dye efflux is linearly related to the amount of surface-bound peptide and can be traced back to a membrane perturbation effect.