Interactions of Ethylene Oxide-Propylene Oxide Block Copolymers with Lipid Bilayers

Abstract

Nonionic poly(propylene oxide)-b-poly(ethylene oxide) (PPO-PEO) block copolymers, known as Pluronics or poloxamers, have been widely studied for their great potential in cell membrane stabilization and permeabilization due to their amphiphilicity and biocompatibility. A hydrophilic dominant commercial poloxamer P188 shows a reasonable stabilization effect in vivo on dystrophic muscle cell membranes. Despite recent advances made in this field, the structure-function relationships and the underlying mechanism of polymer protection are not fully understood. The goals of this dissertation are to understand the fundamental mechanism of polymer-membrane association and to further design effective polymers to improve the therapeutic approach of cell membrane stabilization for Duchenne muscular dystrophy. Herein, we developed a simple yet powerful method that enables the quantification of the relatively weak interactions between the copolymers and vesicular lipid bilayers based on distinct diffusivity of free and bound polymers, using pulsed-field-gradient NMR (PFG-NMR). This is the first quantitative study that systematically investigated polymer binding to lipid membranes in the literature to our knowledge, which provides direct evidence regarding how polymer structure and membrane composition and curvature dictate their interactions. With lab-synthesized PPO-PEO diblock analogs as complements to the commercial triblock poloxamers, we found that polymers with larger molecular weight and higher hydrophobicity result in stronger polymer-membrane association. Also, the lipid bilayer composition plays a critical role. Notably, polymer binding drops 10-fold in a universal fashion as cholesterol concentration in the bilayer increases from 0 to 30 mol.%. Switching the lipid headgroup from choline to glycerol significantly enhanced polymer binding. Additionally, polymer protection efficacy on liposomes against induced lipid peroxidation was assessed and compared with their stabilization efficacy in vitro and in vivo.