Interactions of poly(ethylene oxide) brushes with chemically selective surfaces

Poly(ethylene glycol) (PEG) has long been recognized for its unusual ability to resist protein adsorption. This is attributed to the repulsion of proteins by the polymer segments. Despite its successes, there are several reports that PEG does weakly bind proteins. This work tests the hypothesis that the PEG can bind to nonpolar, hydrophobic groups such as the aliphatic side chains of amino acids. To do this we measured the force-distance profiles between PEG(5000) brushes and self-assembled alkanethiol monolayers with varying amounts of nonpolar methyl-terminal groups. The polymer adhesion to these chemically selective surfaces increased with increasing density of surface methyl groups. The equilibrium thickness of the polymer chains in contact with the alkanethiol monolayer decreased correspondingly. The brush did not adhere to lipid bilayers or to bare mica. The results show that PEG will adsorb to nonpolar, hydrophobic surfaces. These findings may provide a possible explanation for previous direct force measurements of protein-PEG adhesion, and reports of PEG complexation with partially folded proteins.