Lipid Bilayer Characterization - Electrical

Electrical

Electrical measurements are the most straightforward way to characterize one of the more important functions of a bilayer, namely its ability to segregate and prevent the flow of ions in solution. Accordingly, electrical characterization was one of the first tools used to study the properties of model systems such as black membranes. It was already known that the cell membrane was capable of supporting an ionic gradient and that this gradient is responsible for the ability of neurons to send signals via an action potential. Demonstrating that similar phenomena could be replicated in vitro was an important verification of the utility of model systems.

Fundamentally, all electrical measurements of bilayers involve the placement of an electrode on either side of the membrane. By applying a bias across these electrodes and measuring the resulting current, it is possible to determine the resistance of the bilayer. This resistance is typically quite high for intact bilayers, often exceeding 100 GΩ since the hydrophobic core is impermeable to charged hydrated species. Because this resistance is so large, the presence of even a few nanometer-scale holes results in a dramatic increase in current and can be easily determined. The sensitivity of this system is such that even the activity of single ion channels can be resolved. In such DC measurements, it is necessary to use electrochemically active electrodes to provide the necessary positive charges on one side and negative charges on the other. The most common system is the silver/silver chloride electrode since this reaction is stable, reversible, involves a single electron transfer and can produce large currents. In addition to simple DC current measurements it is also possible to perform AC electrical characterization to extract information about the capacitance and complex impedance of a bilayer. Because capacitance is inversely proportional to thickness and bilayers are very thin they typically have a very large capacitance, on the order of 2µF/cm2. Capacitance measurements are particularly useful when dealing with black lipid membranes, as they can be used to determine when the solvent/lipid plug thins down to a single bilayer.