Contribution of hydrogen bonding to the association of liposomes and an anionic hydrophobically modified poly(N-isopropylacrylamide)

yThe interactions between a random copolymer of N-isopropylacrylamide (NIPA M), N-[4-(1-pyrenyl)-butyl]-N-n-octadeclacrylamide, and N-glycylacrylamide (Gly) (PNIPAM-Py-Gly) with a series of vesicles have been examined by fluor escence spectroscopy, centrifugation assays, and capillary electrophoresis. The copolymer exhibits pH-sensitivity due to the carboxylic acid group of the glycine residues (15 mol %), thermosensitivity due to the NIPAM I resid ues (84 mol %), and amphiphilic characteristics due to the pyrenyl octadecy l moieties (1 mol Sc). In the presence of salts, complex formation was dete cted between the copolymer and cationic or anionic nonphospholipid liposome s based on the nonionic surfactant n-octadecyldiethylene oxide ((EO)(2)C18H 37) and cationic phospholipid liposomes. Weak interaction was detected betw een the copolymer and neutral phospholipid liposomes under the same conditi ons. The effects of the charge density of the liposomes and the lipid conce ntration on the copolymer/liposome interactions were examined as a function of pH and temperature. The binding of the copolymer to the liposomes is sh own to be controlled primarily through hydrogen bond formation between the hydroxyl groups of the (EO)(2)C18H37 and the amide groups of the NIPAM resi dues, through electrostatic interactions between the charged surfactants of the bilayer and the glycine residues, and, a lesser extent, by hydrophobic forces.