Online monitoring of the aggregation and fusion of DPPC/PA by static and dynamic light scattering

Static and dynamic light scattering were employed to monitor the temperature-induced aggregation and fusion processes of liposomes consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and palmitic acid (PA) in a molar ratio of 1:2. At 25 to 41°C, the size and the relative molecular mass of the liposome increased with increasing temperature indicating the occurrence of aggregation or fusion. Meanwhile, the size showed strong angular dependence, suggesting that the vesicle structure was at least partially destroyed upon aggregation. During the subsequent cooling process, the size and relative molecular mass continued to increase. On the basis of these results, we propose that the aggregation is caused by the adhesion of liposomes in the palmitic acid rich zones. The formed aggregate had a structure between that of bilayers and inverted hexagonal (H_II) packing. Upon aggregation, the ratio of fusion was small. Our results also indicated that the vesicle was kinetically stable. Once the transition from the gel state to semi-inverted hexagonal (H_II) packing was triggered, the aggregation or fusion of the liposome proceeded automatically.