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

Poly(ethylene glycol) (PEG) has long been recognized for its unusual abilit y to resist protein adsorption. This is attributed to the repulsion of prot eins by the polymer segments. Despite its successes, there are several repo rts that PEG does weakly bind proteins. This work tests the hypothesis that the PEG can bind to nonpolar, hydrophobic groups such as the aliphatic sid e chains of amino acids. To do this we measured the force-distance profiles between PEG(5000) brushes and self-assembled alkanethiol monolayers with v arying amounts of nonpolar methyl-terminal groups. The polymer adhesion to these chemically selective surfaces increased with increasing density of su rface methyl groups. The equilibrium thickness of the polymer chains in con tact with the alkanethiol monolayer decreased correspondingly. The brush di d not adhere to lipid bilayers or to bare mica. The results show that PEG w ill 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.