Range of the solvation pressure between lipid membranes: dependence on the packing density of solvent molecules

Abstract

Well-ordered multilamellar arrays of liquid-crystalline phosphatidylcholine and equimolar phosphatidylcholine-cholesterol bilayers have been formed in the nonaqueous solvents formamide and 1,3-propranediol. The organization of these bilayers and the interactions between apposing bilayer surfaces have been investigated by X-ray diffraction analysis of liposomes compressed by applied osmotic pressures up to 6 .times. 107 dyn/cm2 (60 atm). The structure of egg phosphatidylcholine (EPC) bilayers in these solvents is quite different than in water, with the bilayer thickness being largest in water, 3 .ANG. narrower in formamide, and 6 .ANG. narrower in 1,3-propanediol. The incorporation of equimolar cholesterol increases the thickness of EPC bilayers immersed in each solvent, by over 10 .ANG. in the case of 1,3-propanediol. The osmotic pressures of various concentrations of the neutral polymer poly(vinylpyrrolidone) dissolved in formamide or 1,3-propanediol have been measured with a custom-built membrane osmometer. These measurements are used to obtain the distance dependence of the repulsive solvation pressure between apposing bilayer surfaces. For each solvent, the solvation pressure decreases exponentially with distance between bilayer surfaces. However, for both EPC and EPC-cholesterol bilayers, the decay length and magnitude of this repulsive pressure strongly depend on the solvent. The decay length for EPC bilayers in water, formamide, and 1,3-propanediol is found to be 1.7, 2.4, and 2.6 .ANG., respectively, whereas the decay length for equimolar EPC-cholesterol bilayers in water, formamide, and 1,3-propanediol is found to be 2.1, 2.9, and 3.1 .ANG., respectively. These data indicate that the decay length is inversely proportional to the cube root of the number solvent molecules per unit volume. Thus, the decay length of the solvation pressure depends on the packing of the solvent molecules in the interbilayer space but is strongly dependent on either the solvent''s dielectric constant or the dipole moment. The magnitude of the solvation pressure, which is largest in water and smallest in 1,3-propanediol, varies with the square of the dipole potential as measured in monolayers in equilibrium with bilayers.