A cyclic peptide-alamethicin forms cation selective channels in lipid bilayers. Anomalous behavior of bilayer capacitance observed during impedance measurements of the planar bilayer containing the peptide is reported. The band of the frequencies used is 30-30000 Hz. The bilayer capacitance measured at a given frequency depends on the value of constant voltage applied to the membrane in series with alternating field. The field dependent capacitance has a negative sign and is equivalent to the inductance so far as phase shift angle between voltage and current is considered. Absolute magnitude of the negative capacitance can exceed several times the geometrical capacitance of the bilayer. In the absence of the applied field the alamethicin molecules lie flat on the membrane surface. The applied field tilts the molecules through the hydrocarbon region of bilayer, so that now the long axis of the peptide monomer is perpendicular to the membrane plane. Lateral diffusion of monomers along the membrane plane results in the formation of oligomers-transmembrane ion channels, and appearance of membrane current. The time period between the moment of tilting of monomers across the lipid bilayer and the moment of the channel formation determines the phase shift between voltage and current seen as a negative capacitance during the impedance measurements.
Electric Conductivity Lipid Bilayers