Polyelectrolyte-lipid vesicle interactions: Solid state deuterium and phosphorus nuclear magnetic resonance studies

Interactions between charged vesicles and polyelectrolytes were investigated by solid state nuclear magnetic resonance (NMR). It was shown with deuterium (2H) NMR that anionic giant unilamellar vesicles (GUVs), containing headgroup-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), fused with cationic small unilamellar vesicles (SUVs) upon mixing. The extent of fusion was quantitatively deduced from the quadrupolar splittings. The interaction of polylysine of different lengths (Lys_n) with phosphatidylserine/POPC multilamellar vesicles (MLVs) was investigated using 2H NMR of headgroup-deuterated POPC. It was discovered that Lys ₃₀ (N = 30) and Lys₁₀₀ (but not Lys₅) added to the MLVs were able to produce two resolvable 2H NMR spectral components consistent with the co-existence of polylysine-bound and polylysine-free domains. At physiological salt concentrations, however, only Lys₁₀₀ yielded 2H NMR resolvable domains. The effect of binding a copolymer of N-isopropylacrylamide, N-[4-(1-pyrenyl)butyl]- N-n-octadecylacrylamide and N-glycydylacrylamide (PNIPAM-PyC₁₈-Gly) on membrane stability in cationic MLVs was also examined using phosphorus (31P) and 2H NMR of headgroup-deuterated POPC. For n-octadecyldiethylene oxide (ODEO) MLVs, PNIPAM-PyC₁₈-Gly induced a complete conversion from a bilayer-type 31P NMR spectrum to one characteristic of lipids undergoing isotropic motional averaging. This response was sustained at high temperatures. For POPC MLVs, only at high levels of cationic lipid and ionic strength did PNIPAM-PyC₁₈-Gly induce even a partial conversion to an isotropic-type 31P NMR spectrum. Raising the temperature eliminated this isotropic spectral component. We conclude that the binding of PNIPAM-PyC₁₈-Gly to POPC MLVs is dominated by electrostatic attraction, while for ODEO MLVs there is the additional contribution of hydrogen bonding. Interactions between POPC-containing cationic GUVs and polystyrene sulfonate (PSSS) (of different molecular weights) were investigated with 2H NMR of headgroup-deuterated POPC. It was discovered that PSSS segregates 1,2-dioleoyl-3-(trimethyl-ammonio) propane (DOTAP) on the GUV membrane, leading to GUV disruption. It was proposed that this disruption occurs following the membrane stress created from PSSS-induced transbilayer flip-flop of DOTAP. When PSSS was complexed with cationic SUVs to form PSSS lipoplexes and added to POPC-containing anionic GUVs, 2H NMR results indicated that PE-containing PSSS lipoplexes bind and fuse with GUVs. When PE is omitted from the PSSS lipoplexes, there is much less fusion observed. What is evident is the importance of the presence of PE in the lipoplexes, which were created to mimic the DNA lipoplexes used in gene transfection. The results of all these studies provide insight into the consequences of vesicle-vesicle and vesicle-polyelectrolyte interactions.