Phospholipid crystalline clusters induced by adsorption of novel amphiphilic triblock copolymers to monolayers

Abstract

The molecular arrangement in mixed monolayers resulting from the adsorption of the novel water soluble amphiphilic triblock copolymers of poly(glycerol monomethacrylate) and poly(propylene oxide) (PGMA-b-PPO-b-PGMA) to phospholipid monolayer films has been investigated at the air-water interface. All measurements were performed at an intermediate initial surface pressure (10.2 mN m⁻¹) of the acyl chain perdeuterated phospholipids dipalmitoylphosphatidylcholine (DPPC-d₆₂) or dimyristoylphosphatidylcholine (DMPC-d₅₄), and at a bulk (PGMA₁₄)₂-PPO₃₄ concentration of 8 µM, which is well below its critical micelle concentration. Brewster Angle Microscopy (BAM) images show that in spite of the different physical state of the DPPC-d₆₂ compared to the DMPC-d₅₄ monolayers, at longer equilibration times an optically homogenous mixed monolayer is obtained in both cases. The conformation of the lipid acyl chains was monitored, simultaneously with BAM imaging, through the frequency of the perdeuterated methylene symmetric ν_s(CD₂) stretching band by infrared reflection absorption spectroscopy (IRRAS). It shows that the conformational order brought about by the copolymer adsorption is preserved even after the domains formed initially have completely disappeared and a homogeneous lipid-polymer mixed monolayer has been obtained. Based on these results it is concluded that a mixed monolayer consisting of clusters of lipid molecules, having a substantial conformational order of their lipid acyl chains (i.e., a larger population of trans conformers) and sizes in the mesoscopic to nanoscopic range, surrounded by a network of block copolymer chains is formed.