Formation of Bimolecular Membranes from Lipid Monolayers and a Study of Their Electrical Properties

TLDR

Electrical resistance of these biomolecular membranes is comparable to conventional planar bilayers (10⁶–10⁸ Ω cm²) but can be reduced to ~10³ Ω cm² by gramicidin, likely due to limited water penetration near ester groups that lowers resistance and dielectric thickness. Bimolecular membranes are formed by bringing two lipid monolayers together at an air‑water interface through a Teflon aperture, and their formation is tracked by a rise in capacitance; assuming a dielectric constant of 2.1, the measured 0.9 µF cm⁻² corresponds to a 22 Å hydrocarbon thickness, and asymmetric membranes can be produced by pairing monolayers of different composition. The capacitance of bilayers formed from monolayers (0.9 µF cm⁻²) is roughly twice that of hydrocarbon‑containing bilayers (0.45 µF cm⁻²) and matches biological membranes, yet the inferred dielectric thickness (22 Å) is 6–10 Å thinner than the true hydrocarbon region measured by X‑ray diffraction, suggesting limited water penetration.

Abstract

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 μF cm -2 , respectively. The value of 0.9 μF 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 μF cm -2 , is 22 Å. This value is 6-10 Å 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.