Formation of bimolecular membranes from lipid monolayers and a study of their electrical properties

Bimolecular membranes are formed from two lipid monolayers at an air-water interface by the apposition of their hydrocarbon chains when an aperture in a Teflon partition separating two aqueous phases is lowered through the interface. Formation of the membrane is monitored by an increase of the electrical capacity, as measured with a voltage clamp. Electrical resistance of the unmodified membrane is analogous to that of conventional planar bilayers (black lipid membranes) prepared in the presence of a hydrocarbon solvent, i.e., 10(6)-10(8) ohm cm(2); the resistance can be lowered to values of 10(3) ohm cm(2) by gramicidin, an antibiotic that modifies the conductance only when the membranes are of biomolecular thickness. In contrast to the resistance, there is a significant difference between the capacity of bilayers made from mono-layers and that of hydrocarbon-containing bilayers made by phase transition; the average values are 0.9 and 0.45 muF cm(-2), respectively. The value of 0.9 muF cm(-2) approximates that of biological membranes. Assuming a dielectric constant of 2.1 for the hydrocarbon region, the dielectric thickness, as calculated from a capacity of 0.9 muF cm(-2), is 22 A. This value is 6-10 A smaller than the actual thickness of the hydrocarbon region of bilayers and cell membranes, as determined by x-ray diffraction. The difference may be due to a limited penetration of water into the hydrocarbon region near the ester groups that would lower the electrical resistance of this region and reduce the dielectric thickness. Asymmetric membranes have been formed by adjoining two lipid monolayers of different chemical composition.