Destabilization of cationic lipid vesicles by an anionic hydrophobically modified poly(N-isopropylacrylamide) copolymer:: a solid-state 31P NMR and 2H NMR study

The effect of binding PNIPAM-Py-Gly, a copolymer of N-isopropylacrylamide, N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide and N-glycydyl-acrylamide, on membrane stability in cationic multilamellar vesicles (MLVs) was examined using solid-state phosphorus ((31)p) and deuterium (H-2) nuclear magnetic resonance (NMR) spectroscopy. For MLVs of composition n-octadecyldiethylene oxide (ODEO)+cholesterol (CHOL)+1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)+dimethyldioctadecylammonium bromide (DODAB) (molar Patios 75:10.5:10.5:4), PNIPAM-Py-Gly induced a complete conversion from a bilayer-type P-31 NMR spectrum to one characteristic of lipids undergoing isotropic motional averaging, indicating the existence of regions of high local membrane curvature. This response was sustained even at elevated temperatures. For MLVs of composition POPC+1,2-dioleoyloxy-3-(trimethylammonio)-propane (DOTAP), only at high levels of DOTAP and ionic strength did PNIPAM-Py-Gly induce even a partial conversion to an isotropic-type P-31 NMR spectrum. At lower pH this effect was diminished. Raising the temperature eliminated the isotropic P-31 NMR spectral component, and this effect was not reversible upon returning to room temperature. H-2 NMR spectroscopy of headgroup-deuterated DOTAP and POPC confirmed the P-31 NMR results, but did not provide specific surface electrostatic information. We conclude that the binding of PNIPAM-Py-Gly to phospholipid-based vesicles is dominated by electrostatic attraction between cationic lipids and the polymer's glycine residues. At high binding levels, the polymer assumes a collapsed conformation at the surface, resulting in regions of high local curvature of the lipid assembly. For ODEO-based liposomes, these effects are magnified by the additional contribution of hydrogen bending to the strength of polymer binding. (C) 1998 Elsevier Science B.V. All rights reserved.