Measured work of deformation and repulsion of lecithin bilayers.

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Abstract
Three complementary techniques were used to vary the chemical potential of water in lipid/water mixtures. The work of removing water from the multilayer lattice formed in water by the zwitterionic phospholipid egg lecithin was measured. The structural consequences of water removal were observed by x-ray diffraction. There are no discrete classes of bound water in this system; the work of removal is a continuous function of water content and lattice repeat spacing. From 30-3 .ANG. separation between bilayers is an exponential hydration force repulsion with a 2.6 .ANG. decay length. This interaction translates into a very large force to prevent contact between vesicles and planar membranes; it may be important in controlling vesicle-to-cell fusion. As water is removed, bilayers not only move closer but thicken as the lipid polar groups on the same bilayer move closer together. The applied work can be divided into that of direct bilayer repulsion and that of bilayer deformation. A 1st determination of the lateral pressure required to create large increases in bilayer thickness and concomitant decreases in bilayer area was obtained. The lateral pressure reached 25 dyn/cm for a 25% decrease in bilayer area. Systematic measurements of the mechanical properties of bilayers suffering such large deformation will allow critical tests of theories on bilayer stability and phase transition.