MECHANISM OF THE INTERACTION OF HYDROPHOBICALLY-MODIFIED POLY-(N-ISOPROPYLACRYLAMIDES) WITH LIPOSOMES

Interactions of hydrophobically-modified poly-(N-isopropylacrylamides) (HM PNIPAM) with phospholipid liposomes were studied as a function of the lipid type, the lipid bilayer fluidity, and the polymer conformat ion. Fluorescence experiments monitoring non-radiative energy transfer (NRET), between naphthalene attached to the HM PNIPAM and 1,6-dipheny l-1,3,5-hexatriene (DPH) incorporated into the lipid bilayer, confirme d the direct penetration of hydrophobic anchor groups linked to the po lymer into the liposome hydrophobic core. Contraction of the polymer b ackbone above the lower critical solution temperature (LCST) resulted in a partial withdrawal. of the anchor groups from the lipid bilayer. Analysis of polymer/lipid mixtures by centrifugation and quasi-elastic light scattering (QELS) revealed the polymer-induced fission of lipos omes in the liquid-crystalline state, resulting in the formation of ve sicles 150-230 nm in diameter. The process is reversible and upon tran sition of the bilayer into the gel state these vesicles are converted into larger aggregates. According to the results of gel-filtration exp eriments the HM PNIPAM is in dynamic exchange between the liquid-cryst alline lipid bilayer and the water solution, while the binding to the bilayer in the gel state is more static in nature. The binding constan t for mixture of KM PNIPAM with DMPC liposomes, evaluated from the cen trifugation experiments, was found to be 120 M-1.