A dual-stimuli-responsive polymer into phospholipid membranes

Abstract

In this study, we investigate the thermotropic effects of diblock copolymer poly(N-isopropylacrylamide)-block-poly(acrylic acid) (PNIPAM-b-PAA) on fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers and its ability to alter the membranes’ organization, fluidity and phase behavior. The composition of the diblock copolymer and the nature of dispersion medium (pH and ionic strength) were also examined. For these purposes, pure DPPC lipid and polymer–lipid mixed systems, hydrated in three different dispersion media (i.e., HPLC-grade water, phosphate buffer saline and hydrochloric acid solution of pH 4.5), were investigated by differential scanning calorimetry. Two compositions of PNIPAM-b-PAA with different molar ratio of the polymeric blocks were used. PNIPAM-b-PAA presents great scientific interest due to the combination of the special characteristics of its homopolymer components; it is dual responsive both in temperature and in pH changes. The incorporation of the PNIPAM-b-PAA into the DPPC bilayers causes particularly significant perturbations in their thermotropic behavior, slightly different in each dispersion medium. The results indicated the ordering of the polymer guest near the polar head group surface probably by its PAA block and, on the other hand, the penetration of the PNIPAM block into the hydrophobic bilayer core, causing membrane disruption in a temperature-depended manner. We can conclude that the lipid–polymer interactions seem to be affected by the pH and the ionic strength of the hydration medium, as well as the polymer content incorporated in the DPPC bilayer. These studies could be a roadmap in order to rationally design and develop chimeric liposomes.