Receptor mediated binding of avidin to polymer covered liposomes

Fluoresence technique involving a receptor-mediated fluorescence increase of bodipy-labeled avidin upon binding to biotinylated lipids has been used to investigate the steric barrier effect of submicellar concentrations of poly(ethylene glycol)-phospholipids (PE-PEG(2000) and PE-PEG(5000)) incorporated into pure DPPC liposomes as well as PE-PEG(5000) incorporated into DPPC liposomes containing 20 mol% cholesterol. It is found that the incorporation of PE-PEG lipopolymers into DPPC lipid bilayers lowers the receptor-mediated adhesion of avidin to the biotinylated liposomes. The most pronounced screening effect is observed at surface densities corresponding to the mushroom conformation of the polymer. Furthermore, the results show that the steric baric effect induced by the surface-grafted polymers becomes stronger when the length of the polymer chain increases. In addition it is found that cholesterol improves the barrier effect of PE-PEG(5000) at low lipopolymer concentrations while no effect is observed at higher concentrations. The results reveal that both the surface density and the polymer length of the PE-PEG lipopolymers play a major role for the accessibility of avidin to biotin surface receptors. However, none of the lipopolymers were capable of completely preventing avidin from reaching the surface bound ligands. Cholesterol only affected the barrier effect at lipopolymer concentrations below the mushroom to brush transition. Consequently, from a steric stabilization viewpoint there is no rationale for incorporating cholesterol into liposomes when the PE-PEG lipopolymer concentration exceeds the mushroom to brush transition. The results presented in this study are of importance in relation to a deeper understanding of the interaction of liposome degrading enzymes and proteins with polymer covered liposomes as well as for the receptor-based targeting and interaction of liposomes with cell surface receptors.