Steric stabilization of negatively charged liposomes by cationic graft copolymer

This work characterizes interactions between a cationic graft copolymer, PEO-graft-polyethylenimine (PEO-g-PEI) and negatively charged liposomes composed of 80% (wt) dipalmitoylphosphatidylcholine (DPPC) and 20% (wt) diphosphatidylglycerol. Following addition of the copolymer the effects on the size and surface charge of the liposomes were examined by dynamic light scattering and laser electrophoresis techniques. The study concludes that PEO-g-PEI adsorbs on the surface of the liposomes due to electrostatic interactions with the CL phosphate groups exposed at the outer leaflet of the liposome membrane. This results in neutralization of the liposome surface charge and formation of PEO layer at the liposome surface, which sterically stabilizes the particles. Electroneutral PEO-g-PEI/liposome complexes remain stable in dispersed state (approx. 100 nm) in contrast to the homopolymer PEI/liposome complexes, which aggregate. Binding of PEO-g-PEI did not alter the structure of the lipid bilayer as evaluated by a fluorescence polarization technique, using diphenylhexatriene as a probe, and a microcalorimetry technique.