Aggregation of dipalmitoylphosphatidylcholine vesicles.

Quasi-elastic and 90 degrees light scattering were used to study the aggregation of dipalmitoylphosphatidylcholine vesicles at temperatures below the gel-liquid-crystalline phase transition as a function of concentration, temperature, and size. Increased vesicle concentration did not appreciably change aggregate size but did change the total number of aggregates in a manner consistent with a bimolecular collisional mechanism for the conversion of aggregates to fused vesicles. Increased temperature decreased aggregation, indicating that the disaggregation rate constant increased faster than the aggregation rate constant. As a function of size, aggregation decreased slightly from small to 700 A diameter vesicles and increased considerably for 950 A diameter vesicles. A model of the interaction of small vesicles below the gel-liquid-crystalline phase transition is presented in which aggregation precedes fusion and collision between aggregates triggers fusion.