Forces between model polypeptides and proteins adsorbed on mica surfaces

Abstract

The forces of interaction between proteins and model polypeptides adsorbed onto mica have been measured as a function of the distance of separation between the mica surfaces in aqueous solution. Results were obtained for the basic polypeptide, poly-l-lysine, of molecular weight 4,000 and 75,000, cytochrome c, concanavalin A and myelin basic protein. For cytochrome c no interaction between the adsorbed protein layers was noted until the surfaces were separated by 12.5 nm whereupon an attraction was measured, indicating that the negatively charged mica has been neutralised by adsorption of the positively charged cytochrome c. For all the other proteins, and the poly-l-lysine the force at long range was repulsive and could be explained simply in terms of electrical double layer theory. At short surface separations attractions were noted in all cases (although for concanavalin A calcium and manganese ions needed to be present). These attractions can be accounted for by van der Waals forces for all the systems, with the exception of myelin basic protein for which an attraction ten times larger than that predicted by van der Waals forces was measured. We propose that this additional attraction may be due to hydrophobic interactions between the adsorbed myelin basic protein layers.