X-ray studies on aqueous dispersions of dipalmitoyl phosphatidylglycerol in the presence of salt

The structure and lamellarity of lipid vesicles composed of dipalmitoyl phosphatidylglycerol in the presence of salt has been studied by small- and wide-angle X-ray diffraction which revealed formation of unilamellar vesicles up to 150 mM NaCl. Analysis of these data indicated that both the bilayer thickness and the distance of the phosphate groups between the two opposite lipid leaflets increased only by a small amount in the presence of salt. Further addition of NaCl did not markedly affect the wide-angle reflections. However, in the small- angle region additional reflections were observed above 300 mM NaCl, originating from the bilayer stacking in multilamellar vesicles. These observations can be explained by the screening of the negatively charged vesicle surface by Na-ions, thereby reducing the electrostatic repulsion between the lipid bilayers and enabling the formation of multilamellar vesicles.