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) muF/cm(2) with a time constant of approximately 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) = C(0) + betaV(a) (2) where V(a) approximately V(DC) + V(rms) (AC). C(0) and beta depend upon temperature; C(0) decreases linearly with temperature while beta increases linearly. At 20 degrees C, C(0) = 0.559 +/-0.01 (SD) muF/cm(2) and beta = 0.0123 +/-0.0036 (SD) (muF/cm(2))/(mv(2)) and at 34 degrees C, C(0) = 0.472 +/-0.01 and beta = 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.