Interactions between proteins and synthetic polypeptides adsorbed at solid-liquid interfaces

Abstract

The forces acting between two atomically smooth mica surfaces immersed in aqueous electrolyte have been measured, in the range of surface separations D, 0 ∼ 1000 nm, both in the absence and in the presence of macromolecules adsorbed onto the surfaces from solution. In this way the interactions between the adsorbed macromolecular layers themselves were determined. Interactions between the bare mica surfaces were short-range and in good accord with DLVO (electrostatic-double layer) theory for both electrolyte concentrations (10 −1 and 10 −3 M KNO 3). Two macromolecular systems were used as adsorbing species; (i) soluble calf-skin collagen and (ii) synthetic poly-L-lysine (MW = 77,000). The forces between adsorbed layers of collagen were monotonically repulsive, with interactions starting at D ∼ 650 nm. This indicates an adsorbed layer extending some 300 nm from each surface, and suggests the rod-like collagen monomers (of length 300 nm and diameter 1.5 nm) are adsorbed end-on normal to the mica substrate. This finding is in accord with recent independent viscometric and radiometric studies of monomeric collagen adsorbed onto glass substrates. For the case of the adsorbed polylysine layers, the forces are again monotonically repulsive, and onset of interactions occurs at D ∼ 120–140 nm. As the surfaces approach, irreversible changes in the surface structure of the adsorbed polyelectrolyte take place and the final quasi-equilibrium adsorption configuration is that of a dense, flat surface layer. A close analysis of the force—distance profiles reveals that the interaction is a combination of both reversible, repulsive field-type (DLVO) forces, and a repulsive steric component which is largely irreversible.