Pore formation in lipid membranes by alamethicin.

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Abstract
The conformation of the linear peptide antibiotic alamethicin in dipalmitoyl phosphatidylcholine multilayers was investigated in the absence of an electric field by means of IR attenuated total reflection spectroscopy. Alamethicin was incorporated into the lipid membrane in the dry state and in an aqueous environment. Its molecular conformation changed from a helix when dry to an extended chain when aqueous. The extended chain aggregated to di- and multimers spanning the lipid bilayer. The equilibrium concentration of alamethicin in the surrounding water was 90 nM, which is the range of concentrations used in black film experiments. The corresponding molar ratio of lipid of peptide was 80:1. The dipole model for the molecular mechanism of electric field-induced pore formation proposed by several authors is apparently very unlikely because it is based on the assumption that the major part of alamethicin is adsorbed on the membrane surface, from which small amounts flip into the membrane under the influence of an electric field. An alternative mechanism is proposed, based on a field-induced conformational change of the peptide from the extended state to a helix. This transition is favored by the resulting dipole moment of the alamethicin helix.

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