Experimental and Monte Carlo simulation studies of the thermodynamics of polyethyleneglycol chains grafted to lipid bilayers

Experimental measurements of the affinity of binding of fluorescent acylate d polyethyleneglycol (PEG) conjugates to bilayers containing varying levels of phosphatidylethanolamine-PEGs (PE-PEGs) have been combined with Monte C arlo simulations to investigate the properties of the polymer chains at a P EG-grafted lipid interface. The affinity of binding of such conjugates to l arge unilamellar phosphatidylcholine/phosphatidylethanolamine (9:1) vesicle s decreases 27-fold as the size of the coupled PEG chain increases from 1 t o 114 monomer units. Incorporation of increasing amounts of PE-PEG2000 or P E-PEG5000 into the vesicles progressively reduces the affinity of binding o f acylpeptide-PEG2000 or -PEG5000 conjugates. Monte Carlo simulations of su rfaces with grafted PEG chains revealed no significant dependence of severa l characteristic properties of the polymer chains, including the average in ternal energy per polymer and the radii of gyration, on the grafting densit y in the range examined experimentally. The average conformation of a surfa ce-grafted PEG2000 or PEG5000 chain was calculated to be fairly extended ev en at low grafting densities, and the projected cross-sectional areas of th e grafted PEG chains are considerably smaller than those predicted on the b asis of the estimated Flory radius. The experimental variation of the bindi ng affinity of acylated conjugates for bilayers containing varying mole fra ctions of PE-PEG2000 or -PEG5000 is well explained by expressions treating the surface-grafted PEG polymers either as a van der Waals gas or as a syst em of rigid discs described by scaled particle theory. From the combined re sults of our experimental and simulation studies we conclude that the graft ed PEG chains exist in a "mushroom" regime throughout the range of polymer densities examined experimentally and that the diminished affinity of bindi ng of acylated-PEG conjugates to bilayers containing PE-PEGs results from o cclusion of the surface area accessible for conjugate binding by the mobile PE-PEG polymer chains.