Novel Properties of DDAB: Matrix Effect and Interaction with Oleate.

We report an investigation on the interaction of the positively charged surfactant DDAB (didodecyldimethylammoniumbromide) with preformed vesicles from POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine).Addition of 1.9 mM DDAB to preformed POPC vesicles of different concentrations results in mixed vesicles,owing to the avid uptake of DDAB by POPC. This leads to the so-called matrix effect: when preformedPOPC liposomes with a narrow size distribution are present in the aqueous solution, there is a fast formationprocess of mixed vesicles and the final size distribution closely resembles that of the preformed POPCliposomes. The final mixed vesicle system is stable in size and size distribution. The effect is independent ofthe initial size of the POPC vesicles and requires low relative concentrations of POPC with respect to DDAB(up to 1:4). Aside this, we report about the time stability of DDAB vesicles as a function of the aqueousbuffer ionic strength and we investigate the interaction between negatively charged surfactant oleate andDDAB. The formation of mixed vesicles is observed, whose size distribution is strongly dependent on themixing ratio of the two surfactants. Interestingly, the mixing of DDAB and oleate vesicles for values ofDDAB molar fraction close to 0.4 leads to a very narrowly distributed vesicular species centered around 100nm.