Elasticity and Phase Behavior of DPPC Membrane Modulated by Cholesterol, Ergosterol, and Ethanol

Giant vesicles formed of 1,2-dipalmitoylphosphatidylcholine (DPPC) and sterols (cholesterol or ergosterol) in water and water/ethanol solutions have been used to examine the effect of sterol composition and ethanol concentration on the area compressibility modulus (Ka), overall mechanical behavior, vesicle morphology, and induction of lipid alkyl chain interdigitation. Our results from micropipette aspiration suggest that cholesterol and ergosterol impact the order and microstructure of the gel (Lβ′) phase DPPC membrane. At low concentration (10–15 mol%) these sterols disrupt the long-range lateral order and fluidize the membrane (Ka ∼ 300 mN/m). Then at 18 mol%, these sterols participate in the formation of a continuous cohesive liquid-ordered (Lo) phase with a sterol-dependent membrane density (Ka ∼ 750 for DPPC/ergosterol and Ka ∼ 1100 mN/m for DPPC/cholesterol). Finally at ∼40 mol% both cholesterol and ergosterol impart similar condensation to the membrane (Ka ∼ 1200 mN/m). Introduction of ethanol (5–25 vol.) results in drops in the magnitude of Ka, which can be substantial, and sometimes individual vesicles with lowered Ka reveal two slopes of tension versus apparent area strain. We postulate that this behavior represents disruption of lipid-sterol intermolecular interactions and therefore the membrane becomes interdigitation prone. We find that for DPPC vesicles with sterol concentrations of 20–25 mol%, significantly more ethanol is required to induce interdigitation compared to pure DPPC vesicles; ∼7 vol. more for ergosterol and ∼10 vol. more for cholesterol. For lower sterol concentrations (10–15 mol%), interdigitation is offset, but by <5 vol.. These data support the idea that ergosterol and cholesterol do enhance survivability for cells exposed to high concentrations of ethanol and provide evidence that the appearance of the interdigitated (LβI) phase bilayer is a major factor in the disruption of cellular activity, which typically occurs between ∼12 and ∼16 vol. ethanol in yeast fermentations. We summarize our findings by producing, for the first time, “elasticity/phase diagrams” over a wide range of sterol (cholesterol and ergosterol) and ethanol concentrations.