Giant planar lipid bilayer. III. Maxwell—Wagner impedance dispersion and anesthetic effects upon interfacial capacitance

Abstract

This report describes a facile linear method for estimating the capacitance of the polar head region of planar lipid bilayers from the dependence of the impedance to the applied frequency at an intermediate range. Planar lipid bilayers with a large area (200 mm 2) were formed in 30 m M NiCl 2 aqueous solution from oleylamine. The capacitance and conductance of the total system, including the water, unstirred layer and membrane phases, were measured in the frequency range between 1 and 10 kHz. In this frequency range, a linear relationship exists between the overall (hydrophobic core plus polar head region) capacitance and conductance. By extrapolating the linear plot between the capacitance and conductance to zero conductance, the capacitance of the overall membrane was estimated. The capacitance of the hydrocarbon region of the membrane was estimated by the do transient method. From the difference in the two sets of data, it is possible to estimate the capacitance of the interfacial polar region. According to the increase of the bulk halothane concentrations, the capacitance of the polar region decreased asymptotically, approaching a limiting value. The decrease was monophasic. On the other hand, the core capacitance increased in a biphasic fashion, as reported previously by T. Yoshida, T. Mori, and I. Ueda ( J. Colloid Interface Sci. 96, 39, 1983). The initial increase in the core capacitance was attributed to the decrease in the membrane thickness, whereas the secondary rise was attributed to anesthetic penetration into the membrane core. Because the relative electrical permittivity of anesthetics is intermediate between those of the hydrocarbon core and the interface, the anesthetic adsorption to the polar head decreases the mean relative permittivity of this region and decreases the capacitance, whereas penetration into the lipid core increases the core capacitance. The present result shows that the secondary rise of the core capacitance occurred only after polar region capacitance reached a limiting value. Halothane apparently adsorbs to the polar head region at near clincal concentrations. Penetration into the liquid core did not occur until the anesthetic molecules saturated the interfacial binding sites.