The interactions of hydrophobically-modified poly-(N-isopropylacrylamides) with liposomes

The interactions between a random copolymer of N-isopropylacrylamide (NIPAM), N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide and N-glycylacrylamide (Gly) (PNIPAM-Py-Gly) with a series of vesicles have been examined by fluorescence spectroscopy. The copolymer exhibits pH-sensitivity due to the carboxylic acid groups of the glycine residues ( 15 mol %), thermosensitivity due to the NIPAM residues (84 mol %), and amphiphilic characteristics due to the pyrenyl octadecyl moieties (1 mol %). In the presence of salts, complex formation was detected between the copolymer and cationic or anionic non-phospholipid liposomes based on the non-ionic surfactant, n-octadecyl-diethylene oxide ((EO)(2)C18H37), and cationic phospholipid liposomes. Weak interaction was detected between the copolymer and neutral phospholipid liposomes under the same conditions. The binding of the copolymer to the liposomes is shown to be controlled primarily through hydrogen bond formation between the hydroxyl groups of the (EO)(2)C18H37 and the amide groups of the NIPAM residues, electrostatic interactions between the charged surfactants of the bilayer and the glycine residues and to, a lesser extent, by hydrophobic forces.