THERMODYNAMIC STABILITY AND OSMOTIC SENSITIVITY OF SMALL UNILAMELLAR PHOSPHATIDYLCHOLINE VESICLES

Evidence is presented to show that small unilamellar phosphatidylcholi ne vesicles with a diameter of approx. 20 nm are osmotically sensitive . Such vesicles respond to osmotic pressure by swelling or shrinking d epending on the direction of the applied salt gradient. This is true f or small unilamellar vesicles of egg phosphatidylcholine and dimyristo ylphosphatidylcholine below and above their crystal-to-liquid crystal transition temperature. At the transition temperature the vesicles are osmotically insensitive due to the increased bilayer permeability res ulting in rapid dissipation of salt gradients. Positive salt gradients produce shrinking and collapse of spherical phospholipid vesicles to disks. Shrinking of vesicles is associated with H2O and solute efflux, but only limited solute influx. Clustering of lipid molecules in the bilayers of the resulting disks can be detected by EPR spin labeling. Negative salt gradients produce swelling of vesicles which is associat ed with H2O and solute influx. Our experiments are consistent with an osmotically perturbed bilayer. In the presence of osmotic gradients th e influx and efflux of H2O is coupled with the movement of ions and sm all molecules which in the absence of salt gradients or osmotic stress cannot pass the phospholipid bilayer. However, during osmotically ind uced shrinking and swelling of SUV the integrity of the phospholipid b ilayer is maintained to the extent that vesicles do not break, and the refore equilibration between external medium and vesicle cavity does n ot take place.