Electrostatic Control of Spontaneous Vesicle Aggregation

Aqueous mixtures of cationic cetyltrimethylammonium p-toluenesulfonate (CTAT) and anionic sodium dodecylbenzenesulfonate (SDBS) spontaneously form equilibrium “catanionic” vesicles whose charge depends on the relative amounts of each surfactant. Aggregation of these vesicles was induced by incorporating a small concentration of biotin-lipid in the bilayer, followed by addition of streptavidin as a cross-linking receptor. Freeze−fracture transmission electron microscopy shows that almost no aggregation occurred in solutions with no added electrolyte, in which the Debye length was much larger than the dimensions of the streptavidin biotin linkage, about 2.5 nm. Much higher levels of aggregation (multivesicle aggregates) were observed for solutions with 0.1 M NaCl, in which the Debye length was smaller than the linkage dimensions. At an electrolyte concentration of 0.025 M, in which the Debye length is comparable to the linkage dimensions, significant aggregation, which depended on vesicle concentration, occurred. The short-range nature of the specific recognition interaction makes controlling aggregation via electrostatics possible. These results also suggest that electrostatic interactions are at least somewhat responsible for the stability of the spontaneous vesicles against aggregation. As these catanionic vesicles do not flatten upon aggregation, they can maintain an applied osmotic stress across their bilayer much like phospholipid vesicles.