THE STRUCTURAL CHANGES OF BILAYER LIPID MEMBRANES CAUSED BY POLARIZATION POTENTIAL

Sławomir Kalinowski, Zbigniew Figaszewski

Warsaw University, Białystok Branch, Institute of Chemistry, Al. J. Piłsudskiego 11/4, 15-443 Białystok, Poland

The bilayer lipid membranes made by the Mueller-Rudin method usually contain inside the bilayer remains of solvent used during formation. This solvent increases thickness of the membranes. The bilayers are in equilibrium with the Plateau-Gibbs border, surrounding the bilayers. An external voltage applied to the membrane causes the electrocompression phenomenon. The hydrophobic part of the membranes is compressed. An additional phenomenon is an electrocapillary effect on the border / electrolyte solution interface. Changes of the interfacial tension have influence on the border shape and distribution of the solvent between the bilayer and the border. The both effects cause changes the area and thickness of the bilayer.

During analysis the capacitance-voltage characteristics of the membranes may be observed that the minimum capacitance potential is dependent on the direction of the voltage sweep. A difference between the minimum capacitance potentials of the membrane polarized in the both directions is further called as a hysteresis voltage. The value of this hysteresis depends on the amplitude and the sweep speed of the polarization potential. The composition of the electrolyte solution and its ionic strength have a little influence on this effect. The significant influence on the hysteresis has the lipid composition. The membranes were made from phosphatidylcholine, cholesterol and phosphatidylserine. The presence of phosphatidylserine what is the negative charged lipid increases the hysteresis. The hysteresis of the minimum capacitance potential may be explained by changes of orientation of the polar groups of lipids, evoked by changes of external electric field. The changes of polar group orientation generate changes of the electric field profile inside the hydrophilic and hydrophobic parts of the membranes.