Interactions of Hydrophobically Modified Poly(N-Isopropylacrylamides) with Liposomes

Abstract

The interactions of Dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) small unilamellar vesicles and hydrophobically modified poly-(N-isopropylacrylamides) have been examined in water at 25°C with mixtures of random copolymers of N-isopropylacrylamide and N-[4-(1-pyrenyl)butyl]-N-n-octadecylacrylamide (PNIPAM-C18Py/200, and NIPAM and N-[2-(1-naphthyl)ethyl]-N-n-octadecyl-acrylamide (PNIPAM - C18Na/200). These polymers form interpolymeric micelles in water. From fluorescence experiments using non-radiative energy transfer between excited naphthalene and pyrene, it was established that the polymeric micelles are disrupted irreversibly upon contact with the liposomes. The polymer chains reorganize on the surface of the liposome in a slow process which results in the spatial separation of the octadecyl groups previously held in close contact within the polymeric micelles. Upon saturation coverage of the liposome surface, coated liposomes and free polymeric micelles coexist in solution. Analogous experiments with labeled derivatives of the NIPAM homopolymer reveal that a different mechanism is operative with polymers that do not carry octadecyl substituents.