Ion participation in protein adsorption at solid surfaces
Abstract The participation of low-molecular-weight ions in the protein adsorption process is studied by comparing the electrokinetic charges σ ek of the dissolved protein molecule and the uncovered sorbent surface on the one hand and the sorbent/protein complex on the other. For the adsorption of human plasma albumin on haematite a linear relationship has been established between the ion transfer per unit area of sorbent and the degree of coverage of the sorbent by the protein. This suggests a conformation/orientation of the adsorbed albumin molecule that is insensitive to the degree of coverage. The same analysis has been carried out for albumin and ribonuclease at polystyrene surfaces, but only under conditions of plateau adsorption. It appears that ions are transferred between the solution and the adsorbed layer to compensate for the charge contrast between the dissolved protein molecule and the bare sorbent surface. Based on a model analysis for the adsorption of albumin and ribonuclease on negatively-charged polystyrene surfaces, it is predicted that cations are incorporated into the protein—sorbent contact region. Experimental results are in qualitative agreement with this prediction. The reason for the uptake of ions in the compact protein layer is to prevent an accumulation of net charge in the inner region of this low-dielectric layer which, in turn, would lead to energetically unfavourable high electrostatic potentials.