Modulation of interaction of polycations with negative unilamellar lipid vesicles

Interactions of small unilamellar negative vesicles composed of diphosphatidylglycerol (cardiolipin, CL²⁻), 20 mol%, and phosphatidylcholine (egg yolk lecithin, EL), 80 mol%, with various cationic polymers (CP) derived from poly(4-vinylpyridine) (PVP) were studied in water and water–salt solutions by means of photon correlation spectroscopy, microelectrophoresis, conductometry, and fluorescence techniques. The linear charge density and hydrophilic lipophilic balance of CPs were varied by quaternization of PVP with various amounts of different alkyl bromides (ethyl-(2), heptyl-(7), dodecyl-(12), cetyl-(16)). Substantial differences were observed in the behavior of exhaustingly N-ethylated PVP (CP2) and PVP N-ethylated to 50 mol% (CP2(50)) or 30 mol% (CP2(30)). All of them adsorb to the CL²⁻/EL vesicle membrane, neutralizing the surface negative charge and causing aggregation of the vesicles. However, CP2, a polycation with a maximum linear charge density, strongly enhances transfer of the negative lipid ions from the inner to outer bilayer leaflet, while CP2(50) and CP2(30) do not. Adsorbed CP2 does not disturb integrity of the vesicle membrane and can be completely removed from the surface of aggregated vesicles by adding a simple salt (NaCl) or a negative linear polyelectrolyte (polyacrylic acid (PAA) sodium salt). Such removal is followed by release of the original vesicles. In contrast to that, adsorbed CP2(50) or CP2(30) produce some leak through the lipid bilayer and cannot be completely desorbed either by increasing ionic strength or adding an excess of PAA. The probable reason of these differences is discussed. PVP partially N-alkylated with dodecyl or cetyl bromides (3 mol%) and then completely N-ethylated (CP2,12 and CP2,16), also having a maximum linear charge density, adsorbs to the negative vesicle surface as a result of both electrostatic binding and hydrophobic interaction. Bulky hydrocarbon pendant groups incorporate into the inner bilayer compartment. Similarly to CP2(50) and CP2(30), CP2,12 and CP2,16 cannot be removed from the surface either by adding the simple salt, or an excess of PAA. However, in contrast to CP2(50) and CP2(30), the polycations with the bulky hydrocarbon pendant groups do not cause any leak through the vesicle membrane. Finally, we have succeeded to prepare the ternary vesicles also composed of 20 mol% of CL²⁻, but partially replacing EL for polyoxyethylene 20 cetyl ether (Brij 58) (up to 30 mol%). The CL²⁻/EL/Brij vesicle carries a hydrophilic corona formed by polyoxyethylene chains exposed into water, while hydrophobic cetyl radicals are incorporated in the lipid bilayer. The CL²⁻/EL/Brij vesicles adsorb all studied CPs similar to the binary CL²⁻/EL vesicles. This means that polyoxyethylene corona is permeable for polycationic species restricting neither electrostatic binding nor incorporation of bulky hydrocarbon groups of CP2,16 into the membrane. However, the corona effectively stabilizes the CP-vesicle complexes against aggregation when the membrane surface is neutralized.