Summary:
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 pH4.5), were investigated by Differential Scanning Calorimetry (DSC). Two
compositions of PNIPAM-b-PAA with different molar ratios 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
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, which is 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-dependent manner. It is thus
concluded 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 serve as a roadmap in
rationally designing and developing mixed-chimeric drug delivery systems.
Keywords:
DSC, Block copolymer, pH responsive, Temperature responsive, PNIPAM-b-PAA
