UC Irvine

1.1. Thin lipid films are generally assumed to be homogeneous equilibrium structures with a definite chemical stoichiometry. However, occasional reports in the literature suggest that these assumptions may not be valid in all cases because of microlenses of solvent trapped in the films. We have studied in detail the specific capacitance of thin films formed from a chromatographically pure synthetic phospholipid in order to examine the validity of these assumption.2.2. The method of White ((1970) Biophys. J. 10, 1127–1148) for measuring the specific capacitance (Cm) of planar lipid bilayers has been improved to allow Cm to be determined with a precision of ±0.3% and accuracy of ±3.0%. Bilayer area (Am) is ascertained from photographs using a weight-area method. It is shown that calculations of Am based on measurements of film diameter using a microscope reticle are subject to a number of uncertainties which can greatly limit the precision and accuracy of area determinations.3.3. The total capacitance (CT), area, and specific capacitance of thin lipid films formed from 1,2-bisdihydrosterculoyl-3-sn-glycerophosphorylcholine in n-decane were measured as a function of time and applied voltage (VA). CT, Am, and Cm generally varied with time and were non-reproducible. Cm typically varied by 20% from film to film. A possible cause of these variations is microlenses of solvent trapped in the films and equations are derived which describe their effects on Cm. It is concluded that the bilayer films studied must have a non-reproducible stoichiometry and a non-uniform thickness. The variations with time are probably a result of a disproportionation of n-decane (Andrews, D. M. and Haydon, D. A. (1968) J. Mol. Biol. 32, 149–150).4.4. CT of films in approximate equilibrium increases in the presence of an applied voltage (VA) due to an increase in both Cm and Am. The dependence of Cm on VA is accurately described by the equation Cm=C0+ßVA2. A similar, but approximate, equation is derived assuming the bilayer to be an elastic system of constant density which can be deformed by the force generated by the electric field.