Measurements of attractive forces between proteins and end-grafted poly(ethylene glycol) chains

Abstract

The surface force apparatus was used to measure directly the molecular forces between streptavidin and lipid bilayers displaying grafted M r 2,000 poly(ethylene glycol) (PEG). These measurements provide direct evidence for the formation of relatively strong attractive forces between PEG and protein. At low compressive loads, the forces were repulsive, but they became attractive when the proteins were pressed into the polymer layer at higher loads. The adhesion was sufficiently robust that separation of the streptavidin and PEG uprooted anchored polymer from the supporting membrane. These interactions altered the properties of the grafted chains. After the onset of the attraction, the polymer continued to bind protein for several hours. The changes were not due to protein denaturation. These data demonstrate directly that the biological activity of PEG is not due solely to properties of simple polymers such as the excluded volume. It is also coupled to the competitive interactions between solvent and other materials such as proteins for the chain segments and to the ability of this material to adopt higher order intrachain structures.Poly(ethylene glycol) (PEG) is used extensively to improve the biocompatibility of foreign materials for both in vivo and ex vivo applications (1-3). Its prevalent use is due largely to its low toxicity and low immunogenicity (1). In addition, due to its protein resistance, it is widely used as a stabilizing surface coating in biological environments (3-7). For example, PEG-functionalization of liposomes increased their blood circulation times by nearly an order of magnitude (4). In the clinic, ethylene oxide surface grafts are used to reduce protein adsorption onto the surfaces of biomedical polymers (1-3, 5-7). This is important for controlling the biological responses to the latter, in part, because protein adsorption is a well established first step in the humoral response against foreign materials (2,8,9). Thus, by preventing the unwanted adsorption of bioactive agents onto the surfaces of medical polymers, the surface grafting of hydrophilic polymers is one of the more effective, general strategies used to manipulate the biological activity of medical materials (3,(9)(10)(11)(12)(13)(14).The unusual efficacy of PEG as an apparently biologically passivating surface coating is linked to both the presumed biological inertness of the polymer backbone and to its solvated configuration (1, 3, 5, 7). In most cases, the proposed mechanisms for the protein resistance of PEG borrow from current theories for structureless polymers in isotropic liquids (5,(15)(16)(17)(18)(19). For example, PEG's ability to repel proteins is attributed to its large excluded volume (3,5,(15)(16)(17)(18)(19), its configurational entropy (20, 21), surface coverage by grafted chains (3,15,18), and the thickness of grafted layers (1,3,9,17,18). These latter attributes are all well described theoretically for both terminally anchored and soluble simple chains in simple solvents (22)(23)(24)(25). Based on ...

Control of protein crystal nucleation around the metastable liquid–liquid phase boundary

“…3, corresponding to PEG-induced repulsion between protein molecules. The emerging complex picture of the interactions in a PEG-protein system agrees with recent direct force measurements (35). They show strong, nonentropic attraction at high polymer-protein separations, corresponding to low PEG concentrations, and strong repulsion at short separations.…”

“…Such a theory may address the issue raised by another set of recent results: can the L-L phase transition be viewed as an extension of crystal nucleation when the critical size tends toward zero (20)? If polymer additives are considered, concentration-dependent polymer solution behavior and specific interactions between polymer and protein molecules should be addressed (34)(35)(36) …”

Hydrogel Microparticles as Sensors for Specific Adhesion: Case Studies on Antibody Detection and Soil Release Polymers

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Poly(ethylene glycol) doubly and singly cationized by different alkali metal ions: Relative cation affinities and cation-dependent resolution in a quadrupole ion trap mass spectrometer

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