ELECTROSTATIC CONTROL OF SPONTANEOUS VESICLE AGGREGATION

Aqueous mixtures of cationic cetyltrimethylammonium p-toluenesulfonate (CTAT) and anionic sodium dodecylbenzenesulfonate (SDBS) spontaneousl y form equilibrium ''catanionic'' vesicles whose charge depends on the relative amounts of each surfactant. Aggregation of these vesicles wa s induced by incorporating a small concentration of biotin-lipid in th e bilayer, followed by addition of streptavidin as a cross-linking rec eptor. Freeze-fracture transmission electron microscopy shows that alm ost no aggregation occurred in solutions with no added electrolyte, in which the Debye length was much larger than the dimensions of the str eptavidin biotin linkage, about 2.5 nm. Much higher levels of aggregat ion (multivesicle aggregates) were observed for solutions with 0.1 M N aCl, in which the Debye length was smaller than the linkage dimensions . At an electrolyte concentration of 0.025 M, in which the Debye lengt h is comparable to the linkage dimensions, significant aggregation, wh ich depended on vesicle concentration, occurred, The short-range natur e of the specific recognition interaction makes controlling aggregatio n via electrostatics possible. These results also suggest that electro static interactions are at least somewhat responsible for the stabilit y of the spontaneous vesicles against aggregation, As these catanionic vesicles do not flatten upon aggregation, they can maintain an applie d osmotic stress across their bilayer much like phospholipid vesicles.