MEASUREMENTS OF ATTRACTIVE FORCES BETWEEN PROTEINS AND END-GRAFTED POLY(ETHYLENE GLYCOL) CHAINS

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 betw een PEG and protein. At low compressive loads, the forces were repulsi ve, but they became attractive when the proteins were pressed into the polymer layer at higher loads, The adhesion was sufficiently robust t hat separation of the streptavidin and PEG uprooted anchored polymer f rom 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 bio logical activity of PEG is not due solely to properties of simple poly mers such as the excluded volume, It is also coupled to the competitiv e interactions between solvent and other materials such as proteins fo r the chain segments and to the ability of this material to adopt high er order intrachain structures.