Bilayer Lipid Membranes

Abstract

AbstractThin lipid membranes formed across an aperture in a teflon foil are frequently used as model systems to simulate the hydrophobic diffusion barrier of biological membranes. These membranes have a thickness of about twice the length of a lipid molecule (bilayer membranes) and are prepared either by a spontaneous transition of a thick lamella made from a solution of lipids in a hydrocarbon [1] or by a direct “addition” of two monomolecular lipid layers on the two water surfaces separated by the teflon foil [2]. The aqueous phases on both sides of these planar membranes are easily accessible by electrodes which may serve to study the electrical properties of the membranes. The whole system allows the application of steady state electrical methods as well as that of fast relaxation methods. The latter include the voltage jump‐current relaxation technique [3,6], the charge pulse method [4,5] as well as the temperature jump method [6]. These kinetic techniques have been used throughout recent years to investigate ion transport phenomena across bilayer lipid membranes.Unmodified lipid membranes have been found to represent high energy barriers for the movement of inorganic cations and anions. A simple equivalent electrical circuit of the membrane consists of a parallel arrangement of the membrane resistance RM and the membrane capacitance CM (typical values: RM = 106‐108μcm2, CM = 0.3‐0.8μF/cm2). A refined version composed of three of such elements (one for the hydrocarbonlike membrane interior, the two others for the polar membrane‐water interfaces) has been suggested on the basis of a.c. measurements [7]. These measurements were, however, confined to the rather small band width of 0.1‐100 Hz. A different possibility of testing such a circuit is to make use of the voltage jump method. The induced current relaxation may be measured with a time resolution of at least 1 μs and very high sensitivity by using signal averaging techniques. For the three element circuit described above the relaxation should consist of two exponential terms. At least five different relaxation times were, however, necessary to obtain a sufficiently good fit to the experimental data [8.9]. This finding indicates that even the refined equivalent circuit is insufficient for a phenomenological description of the high frequency behaviour of bilayer lipid membranes. The complex current relaxation may be interpreted on the basis of cooperative dielectric relaxation phenomena associated with the dipoles of the polar head groups of the lipid molecules.