Destabilization of cationic lipid vesicles by an anionic hydrophobically modified poly(N-isopropylacrylamide) copolymer: a solid-state P-31 NMR and H-2 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, o n 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-phosphoc holine (POPC)+dimethyldioctadecylammonium bromide (DODAB) (molar Patios 75: 10.5:10.5:4), PNIPAM-Py-Gly induced a complete conversion from a bilayer-ty pe P-31 NMR spectrum to one characteristic of lipids undergoing isotropic m otional averaging, indicating the existence of regions of high local membra ne curvature. This response was sustained even at elevated temperatures. Fo r MLVs of composition POPC+1,2-dioleoyloxy-3-(trimethylammonio)-propane (DO TAP), only at high levels of DOTAP and ionic strength did PNIPAM-Py-Gly ind uce even a partial conversion to an isotropic-type P-31 NMR spectrum. At lo wer pH this effect was diminished. Raising the temperature eliminated the i sotropic P-31 NMR spectral component, and this effect was not reversible up on returning to room temperature. H-2 NMR spectroscopy of headgroup-deutera ted DOTAP and POPC confirmed the P-31 NMR results, but did not provide spec ific surface electrostatic information. We conclude that the binding of PNI PAM-Py-Gly to phospholipid-based vesicles is dominated by electrostatic att raction between cationic lipids and the polymer's glycine residues. At high binding levels, the polymer assumes a collapsed conformation at the surfac e, resulting in regions of high local curvature of the lipid assembly. For ODEO-based liposomes, these effects are magnified by the additional contrib ution of hydrogen bending to the strength of polymer binding. (C) 1998 Else vier Science B.V. All rights reserved.