Quantifying Block Copolymer Bindings to Lipid Bilayers with Different Composition and Curvature

We have studied the effects of lipid bilayer curvature and composition on the interaction with poly(ethylene oxide)-b-poly(propylene oxide) diblock and triblock copolymers, using unilamellar phospholipid vesicles ranging from 90 to 220 nm in mean diameter. The composition of the bilayer membranes was manipulated by varying the lipid headgroup chemistry and degree of unsaturation in binary lipid mixtures, as well as with the concentration of cholesterol. The polymer-lipid bilayer association was investigated by probing polymer diffusion via pulsed-field-gradient NMR (PFG-NMR). In the presence of lipid vesicles, a fraction of polymer was bound to lipid bilayers, resulting in slower diffusion than that of free polymer. The coexistence of free and bound polymer gave rise to biexponential echo decay curves, which enabled the quantification of polymer binding fraction. Larger molecular weight and higher hydrophobicity of the polymer led to stronger interactions with lipid bilayers. A higher binding fraction was observed when lipid bilayer curvature increased. The polymer-lipid bilayer association was also strongly correlated with the membrane composition. This work demonstrated that PFG-NMR is a powerful tool to quantify polymer binding on the molecular level.