Protein adsorption in relation to bulk phase properties. Beta-lactoglobulins in solution and at the solid/liquid interface
Abstract: The adsorption at solid/liquid interfaces of b-lactoglobulin (b-lg) A and B have been studied by in situ ellipsometry and related to the properties of these proteins in solution. At low temperatures (4-25°C) the adsorption of the b-lgs was found to be highly related to their self-association behaviour in solution. The adsorption isotherms at 25°C, both on methylated and hydrophilic silica, showed a step character with two different levels of adsorbed amounts at high and low concentrations, respectively, the higher level being in the range of a close packed monolayer. Evidence for preferential adsorption of higher oligomers, was found at methylated silica surfaces. The results indicated that dimers adsorbs via exchange reactions at the surface and that the thermal stability of the proteins in solution could be related to the degree of conformational changes of monomers at methylated silica surfaces. Both indications were further confirmed by applying simple adsorption models. A specific interaction for b-lg B with phosphate ions was found to take place at higher ionic strength, which presumably gives rise to increased dissociation, manifested by a decrease in adsorbed amount. Indications were found that the interaction may take place for the A variant as well, though, not influencing the adsorption. Heat induced aggregation in solution was studied by dynamic and static light scattering. A lag phase, dependent on heating temperature, pH and concentration, was found both for the individual variants of b-lg and for a purified commercial sample of mixed variants. A difference in aggregation rate between b-lg A and B could be related to a difference in the cooperativity in the thermal unfolding behaviour in agreement with the smallest unit in the aggregates being monomeric for the A variant and dimeric for the B variant. Both variants formed aggregates in presence of N-ethylmaleimide (NEM), although it was indicated that these aggregates had a different structure (linear in shape). Adsorption chromium surfaces at elevated temperatures was studied under the same conditions as for the aggregation in solution. A similar lag phase behaviour as was observed for the bulk phase aggregation was found for the thickness of the adsorbed layer. During the lag phase a highly close packed monolayer of proteins was formed. The build-up of multilayers after the lag period was found to not be limited by transport from the bulk phase and the densities in these layers were significantly lower than in the initial monolayer. The effect of NEM on the high temperature adsorption confirmed the importance of disulphide interchange reactions in the build-up of heavier deposits from b-lg. In conclusion, the oligomeric state of the proteins in solution was found to influence the adsorption to methylated and hydrophilic silica, both in terms of adsorbed amounts and adsorption kinetics. The thermal stability in solution could, at surface coverage corresponding to less than a close packed monolayer, be related to the amounts adsorbed. At high temperatures, the adsorption of b-lg A and B to chromium surfaces showed behaviour analogous to their aggregation properties in solution.
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