Influence of Cholesterol and Bilayer Curvature on the Interaction of PPO–PEO Block Copolymers with Liposomes

Abstract

Interactions of nonionic poly(ethylene oxide)-b-poly(propylene oxide) (PEO-PPO) block copolymers, known as Pluronics or poloxamers, with cell membranes have been widely studied for a host of biomedical applications. Herein, we report how cholesterol within phosphatidylcholine (POPC) lipid bilayer liposomes and bilayer curvature affects the binding of several PPO-PEO-PPO triblocks with varying PPO content and a tPPO-PEO diblock, where t refers to a tert-butyl end group. Pulsed-field-gradient NMR was employed to quantify the extent of copolymer associated with liposomes prepared with cholesterol concentrations ranging from 0 to 30 mol % relative to the total content of POPC and cholesterol and vesicle extrusion radii of 25, 50, or 100 nm. The fraction of polymer bound to the liposomes was extracted from NMR data on the basis of the very different mobilities of the bound and free polymers in aqueous solution. Cholesterol concentration was manipulated by varying the molar percentage of this sterol in the POPC bilayer preparation. The membrane curvature was varied by adjusting the liposome size through a conventional pore extrusion technique. Although the PPO content significantly influences the overall amount of block copolymer adsorbed to the liposome, we found that polymer binding decreases with increasing cholesterol concentration in a universal fashion, with the fraction of bound polymer dropping 10-fold between 0 and 30 mol % cholesterol relative to the total content

Spatial Distribution of PEO–PPO–PEO Block Copolymer and PEO Homopolymer in Lipid Bilayers

“…Another possible reason is that cell membrane constituents other than the lipid molecules (e.g., cholesterol) might interfere with PEO8.4K binding to the bilayer. A recent study by our group using POPC liposomes revealed a dramatic reduction in the binding of various Poloxamers as the concentration of cholesterol was increased beyond 20 mol % relative to lipid . These two effects may operate in concert at therapeutic doses in living tissue.…”

“…A recent study by our group using POPC liposomes revealed a dramatic reduction in the binding of various Poloxamers as the concentration of cholesterol was increased beyond 20 mol % relative to lipid. 57 These two effects may operate in concert at therapeutic doses in living tissue.…”

Influence of the Headgroup on the Interaction of Poly(ethylene oxide)-Poly(propylene oxide) Block Copolymers with Lipid Bilayers

“…For triblock Pluronics, N EO represents the number of repeat units of EO in one PEO block. Triblock N PO and N EO values differ slightly from those provided by the supplier, as reported in refs and .…”

“…The weight fraction of PEO ( w PEO ) was determined from the molar ratios of PPO to PEO measured by 1 H NMR spectroscopy. Details of the characterization procedures and the corresponding results were reported in our previous studies; , data relevant to the current study are summarized in Table .…”

“…Polymers involved in this study include commercial Pluronics and laboratory-synthesized diblock analogs, and their chemical structures are shown in Scheme a,b, respectively. Polymer binding to liposomes was determined by pulsed-field gradient nuclear magnetic resonance (PFG-NMR) based on the distinctly different diffusivities of free and bound polymers. , Lipid headgroup composition was manipulated by mixing 1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine (POPC, Scheme c) with 1-palmitoyl-2-oleoyl- sn -glycero-3-phospho-(1′-rac-glycerol) sodium salt (POPG; Scheme d) at various molar ratios to compare the effect of glycerol headgroups with choline headgroups. Pure POPC liposomes were used as controls because phosphatidylcholine is one of the major components found in eukaryotic cell membranes and is widely used as the basic constituent in model membranes. − POPG, on the other hand, is a relatively minor component in eukaryotic cell membranes and is mostly found in bacterial cell membranes .…”