Unique concentration dependence on the fusion of anionic liposomes induced by polyethyleneimine

We investigated the interaction of branched polyethyleneimine (PEI) with anionic vesicles formed with 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS) by spectroscopic measurements and microscopic observations. PEI induced the fusion of the vesicles only over a specific concentration range, which was found to be below the transition of vesicular zeta-potential from negative to positive value. Above the concentration window of membrane fusion, charge inversion of DOPS vesicles due to the further adsorption of PEI onto the surface of vesicle, inhibited the membrane fusion events. The PEI with low molecular weight induced the membrane fusion in a wider range of concentration than high molecular weight PEIs. Our results suggest that the mechanism of PEI interaction with the lipids depends on the molecular weight and stoichiometry of the polycation to negatively charged lipids.