Biophysical Journal 10: 1127-1148 (1970)
© 1970 the Biophysical Society

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A Study of Lipid Bilayer Membrane Stability Using Precise Measurements of Specific Capacitance

ABSTRACT

A method is described for measuring the specific capacitance (C_m) of lipid bilayer membranes with an estimated experimental error of only 1%. The gross capacitance was measured with an AC Wheatstone bridge and a photographic technique was used to determine the area of thin membrane. The results of measurements on oxidized cholesterol-decane membranes formed in 1 x 10^-2 M KCl show that C_m depends upon temperature, voltage, time, and the age of the bulk membrane solutions. For a freshly thinned membrane (from 5 week old solution), C_m increases exponentially from an initial value of 0.432 ±0.021 (SD) µF/cm² with a time constant of 15 min. A 100 mv potential applied across the membrane for 10-20 min prior to making measurements eliminated this time dependence and produced final-state membranes. C_m of final-state membranes depends upon applied voltage (V_a) and obeys the equation C_m = C₀ + ßV_a² where V_a V_DC + V^rms_AC. C₀ and ß depend upon temperature; C₀ decreases linearly with temperature while ß increases linearly. At 20°C, C₀ = 0.559 ±0.01 (SD) µF/cm² and ß = 0.0123 ±0.0036 (SD) (µF/cm²)/(mv²) and at 34°C, C₀ = 0.472 ±0.01 and ß = 0.0382 ±0.0039. These variations in C_m are interpreted as resulting from thickness changes. The possibility that they result from diffuse layer and/or membrane dielectric phenomena is discussed and found to be unlikely. The results are discussed in terms of membrane stability by constructing hypothetical potential energy vs. thickness curves.

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