milk from western cattle (16). The bovine b-lactoglobulins A description of general models for adsorption kinetics is given. are small globular proteins, and the primary structure con-Combinations of the models are compared with adsorption data sists of 162 amino acids. The A and B variants differ only for the self-associating proteins b-lactoglobulin A and B, from at two positions, 64 and 118, which are Asp and Val for b-Elofsson et al. (U. M. Elofsson, M. A. Paulsson, and T. Arnebrant, lactoglobulin A and Gly and Ala for b-lactoglobulin B (17).
Langmuir, submitted). The adsorption onto methylated silica sur-The molecular masses are 18 364 and 18 278 Da, respecfaces was measured by these authors by ellipsometry in phosphate tively, as calculated from their amino acid composition. A buffer (0.01 M, pH 7.0) at five concentrations in the range 0.0003complicated self-association behavior is found in solution, 3 mg/ml. Two models, which agree with the experimental data to and depending on concentration, pH, temperature, etc., difa reasonable extent, are presented. These models both contain exchange reactions between adsorbed monomers and dimers from ferent oligomeric forms are dominating (18-22). At neutral solution. Furthermore, they include three classes of adsorbed mole-pH the proteins exist in a monomer-dimer equilibrium. cules, dimers, and two types of monomers. The monomer types The experimental data used in this work for modeling bdiffer in the rate with which they could be displaced by dimers lactoglobulin adsorption are from Elofsson et al. (1) who from the solution. The difference between the two models was in have shown that the adsorption behavior of the A and B the description of how the less easily displaced monomer form variants of b-lactoglobulin, both on methylated and hydrowas obtained. In one model they were obtained by an exchange philic silica surfaces, was highly related to the degree of reaction between monomers, dependent on the surface coverage self-association in solution. The adsorption isotherms, as of dimers. In the other they were obtained by displacement of measured by ellipsometry, of the two variants have a similar adsorbed monomers by dimers which dissociated upon adsorption.
shape but are shifted so that at the same total protein concen-
The models could be used to describe the adsorption kinetics of tration there are lower amounts adsorbed for variant A than both the A and B variants of b-lactoglobulin at five different concentrations. However, the rate constants differed with a factor of for B. This shift was suggested to be related to the shift in 7.9 between the two proteins, which is of the same size as the the equilibrium constant for dimer-monomer dissociation.
difference in the monomer-dimer equilibrium constant. Other
The aim of this work has been to find kinetic models that models tested, which were found not to fit the data, include for could describe the adsorption of b-lactoglobulin to hyexample simple surface dimerization and pure competitive adsorpdrophobic surfaces and by doing so gain a better understandtion.