Surface changes induced by osmotic stress and its influence on the glycerol permeability in lipid bilayers

The penetration rate of glycerol across lipid bilayers can be assayed dispersing liposomes filled with a 0.1 M glucose solution in an isotonic or a hypertonic solution of glycerol. The kinetic of glycerol permeation is found to be different in each of those cases. Liposomes dispersed above the phase transition temperature in hypertonic solutions show an increase in the surface polarization as measured by means of merocyanine 540. Under this condition, the permeation of glycerol shows a two-step kinetic which is indicative of a non-fickean diffusion process. In contrast, liposomes dispersed in isotonic solutions of the permeant show a fickean behavior. The changes in polarization of the membrane interface are ascribed to variations in the surface potential due to the osmotic collapse and the glycerol concentration in contact with the outer surface. The permeability of polar molecules can, in consequence, be considered as a function of the surface potential of the liposome which is congruent with previous data in literature reporting that water permeability increases as a function of the zeta potential of liposomes shrunken in hypertonic solutions.