Simple models for adsorption kinetics and their correlation to the adsorption of beta-lactoglobulin A and B

A description of general models for adsorption kinetics is given. Combinations of the models are compared with adsorption data for the self-associating proteins P-lactoglobulin A and B, from Elofsson et al. (U. M. Elofsson, M. A. Paulsson, and T. Arnebrant, Langmuir, submitted). The adsorption onto methylated silica surfaces was measured by these authors by ellipsometry in phosphate buffer (0.01 M, pH 7.0) at five concentrations in the range 0.0003-3 mg/ml. Two models, which agree with the experimental data to a reasonable extent, are presented. These models both contain exchange reactions between adsorbed monomers and dimers from solution. Furthermore, they include three classes of adsorbed molecules, dimers, and two types of monomers. The monomer types differ in the rate with which they could be displaced by dimers from the solution. The difference between the two models was in the description of how the less easily displaced monomer form was obtained. In one model they were obtained by an exchange reaction between monomers, dependent on the surface coverage of dimers. In the other they were obtained by displacement of adsorbed monomers by dimers which dissociated upon adsorption. The models could be used to describe the adsorption kinetics of both the A and B variants of P-lactoglobulin at five different concentrations. However, the rate constants differed with a factor of 7.9 between the two proteins, which is of the same size as the difference in the monomer-dimer equilibrium constant. Other models tested, which were found not to fit the data, include for example simple surface dimerization and pure competitive adsorption. (C) 1997 Academic Press.