Complexes between anionic liposomes and spherical polycationic brushes. An assembly of assemblies.

This paper has at its objective the assembling of liposomal assemblies onto nanoparticles. In this manner, one generates nanoparticles with a high loading capacity. Thus, spherical spherical polycationic "brushes" (SPBs) were synthesized by graft polymerizing a cationic monomer, (trimethylammonium)ethylmethacrylate chloride, onto the surface of monodisperse polystyrene particles, ca. 100 nm in diameter. These particles were complexed with small unilamellar anionic liposomes, 40-60 nm in diameter, composed of egg lecithin (EL) and anionic phosphatidylserine (PS(1-)) in PS(1-)/EL ratios from 0.10 to 0.54, a key parameter designated as ν. These complexes were then characterized according to electrophoretic mobility, dynamic light scattering, conductivity, fluorescence, and cryogenic transmission electron microscopy, with the following main conclusions: (a) All added liposomes are totally associated with SPBs up to a certain saturation concentration (specific for each ν value). (b) The number of liposomes per SPB particle varies from 40 (ν = 0.1) to 14 (ν = 0.5). (c) At sufficiently high liposome concentrations, the SPBs experience an overall change from positive to negative charge. (d) SPB complexes tend to aggregate when their initial positive charge has been precisely neutralized by the anionic liposomes. Aggregation is impeded by either positive charge at lower lipid concentrations, or negative charge at higher lipid concentrations. (e) The liposomes remain intact (i.e., do not leak) when associated with SPBs, at ν ≤ 0.5. (f) Complete SPB/liposome dissociation occurs at external [NaCl] = 0.3 M for ν = 0.1 and at 0.6 M for ν = 0.5. Liposomes with ν = 0.54 do not dissociate from the SPBs even in NaCl solutions up to 1.0 M. (g) Complexation of the PS(1-)/EL liposomes to the SPBs induces flip-flop of PS(1-) from the inner leaflet to the outer leaflet. (h) The differences in the ability of PS(1-) (a cylindrical lipid) and CL(2-) (a conical lipid) to create membranes defects are attributed to geometric factors.