Measurement of repulsive forces between charged phospholipid bilayers.

By using an osmotic stress technique (LeNeveu, D. M., et al. (1977) Biophys. J. 18, 209), we have measured the net repulsive force between egg lecithin bilayers containing various amounts of the charged lipids phosphatidylglycerol and phosphatidylinositol. At bilayer separations greater than about 30 A, the repulsion is dominated by electrostatic forces; its variation with both bilayer separation and charge density is well described qualitatively by simple electrostatic double-layer theory. Quantitative agreement requires, however, that only about 50% of the phosphatidylglycerol polar groups be dissociated. At all charge densities, even for pure phosphatidylglycerol, and at bilayer separations less than about 30 A, the repulsion is dominated not by the electrostatic force but by a strong "hydration force" (LeNeveu, D. M., et al. (1977) Biophys. J. 18, 209). We conclude that the hydration force demands more attention than it has enjoyed hitherto in attempts to understand bilayer membrane interaction and fusion.