Interaction of fusogenic synthetic peptide with phospholipid bilayers: orientation of the peptide alpha-helix and binding isotherm.

We studied the binding characteristics of a synthetic 20-residue peptide to supported single planar bilayers of phosphatidylcholine, and the orientation of the peptide by Fourier-transform infrared spectroscopy with an attenuated total reflection method. This peptide, designed to resemble a putative fusion peptide of influenza virus hemagglutinin, assumes an amphiphilic alpha-helix and induces fusion of liposomes in an acidic solution (pH approximately 5). At neutral pH, the peptides were bound to lipid bilayers in the manner of a Langmuir's adsorption isotherm, and their orientation was nearly random or oblique. On the other hand, at acidic pH, the peptides were bound, making their helix axis parallel to the membrane surface, and the binding was cooperative. This cooperativity suggested dimerization of the peptides. These characteristics are expected to be important for the synthetic fusogenic peptide or the fusion peptide in hemagglutinin to induce membrane fusion.