Biomedical aspects of the direct measurement of the forces between adsorbed polymers and proteins

Abstract

The forces between mica surfaces covered with an adsorbed layer of a wide range of polymers, polyelectrolytes and proteins are described as a function of the separation between the surfaces. For polymers in a good solvent at full coverages, repulsive forces are always observed, essentially due to osmotic forces. The range of the force increases as the molecular weight of the polymer increases. For adsorbed homopolymers a marked hysteresis on decompression (separation) of the surfaces is noted due to the polymer being forced to the surface and requiring many minutes to relax. For graft copolymers no hysteresis is observed and the force, for a given molecular weight polymer, is much longer range. Polyelectrolytes of opposite charge to mica adsorb flat on the surface in low electrolyte concentrations but extend away from the mica surfaces when adsorbed at high electrolyte concentrations giving rise to a ‘steric’ component to the fores. Forces between adsorbed layers of proteins were also measured. For concanavalin A (a lectin) and cytochrome C long-range forces are dominated by electrostatic and van der Waals forces and are well described by classical colloid science. However, for myelin basic protein a long-range attraction was noted, due possibly to hydrophobic interactions between the adsorbed protein.