The Control of Membrane Properties by Synthetic Polymers

This review describes the effects caused by synthetic polymers on model lipid membranes. Among a large variety of synthetic polymers, we focused on two quite different groups. One of them consists of polyelectrolytes, whose adsorption on lipid membranes is driven by Coulomb forces, and the other group consisting of Pluronics and polyethoxylated surfactants that have amphiphilic structure and interact with lipid bilayers by hydrophobic forces. This choice was determined by the continuously increasing interest to the application of these polymers in biomedical research. It was shown that interactions of cationic polymers with oppositely charged lipid bilayers result in the formation of lipid domains in the membranes, influence the lateral diffusion of lipid molecules and proteins incorporated into the bilayer, modulate functional activity of membrane proteins and in some cases even favor asymmetrical distribution of lipids. Incorptions of amphiphilic copolymers into lipid bilayers may result in changes in the rate of flip-flop and membranes microviscosity. Polymer-membrane interactions in both cases cause changes in the membrane premeability, obviously resulting from formation of disturbances in the membrane packing. In case of Pluronics, non-covalent interactions between the membrane-bound polymer and the permeant may also contribute to the transportation of the latter through lipid bilayer. Special attention in this review is paid to the relatioship between the polymer structure and its ability to modulate membrane properties. A qualitative thermodynamic treatment of the polymer-membrane interactions allowed to explain the polymer effects on the bilayer structure and led to a list of properties that the polymer structure must possess in order to affect membrane permeability.