Characterization of the size distribution of unilamellar vesicles by gel filtration, quasi-elastic light scattering and electron microscopy

The size and size distribution of unilamellar phospholipid vesicles present in unsonicated phosphatidic acid and mixed phosphatidic acid/phosphatidylcholine dispersions were determined by gel filtration, quasi-elastic light scattering and freeze-fracture electron microscopy. The vesiculation in these dispersions was induced by a transient increase in pH as described previously (Hauser, H. and Gains, N. (1982) Proc. Natl. Acad. Sci. USA 79, 1683–1687). The resulting phospholipid dispersions are heterogeneous consisting of small unilamellar vesicles (average radius r < 50 nm) and large unilamellar vesicles (average r ranging from about 50 to 500 nm). The smallest vesicles with r = 11 ± 2 nm are observed with dispersions of pure phosphatidic acid, the population of these vesicles amounting to about 80% of the total lipid. With increasing phosphatidylcholine content the radius of the small unilamellar vesicles increases and at the same time the population of small unilamellar vesicles decreases. The average radius of small unilamellar vesicles present in phosphatidic acid/phosphatidylcholine dispersions (mole ratio, 1:1) is 17.5 ± 2 nm, the population of these vesicles amounting to about 70% of the total lipid. By a combination of gel filtration, quasi-elastic light scattering and freeze-fracture electron microscopy it was possible to characterize the large unilamellar vesicles. This population is heterogeneous with its mean radius also increasing with increasing phosphatidylcholine content. After separating the large unilamellar vesicles from small unilamellar vesicles on Sepharose 4B it can be shown by quasi-elastic light scattering that in pure phosphatidic acid dispersions 80–90% of the large unilamellar vesicle population consist of vesicles with a mean radius of 170 nm. In mixed phosphatidic acid/phosphatidylcholine dispersions this radius increases to about 265 nm as the phosphatidylcholine content is raised to 90 mol%.