Removal of T4 lysozyme from silicon oxide surfaces by sodium dodecyl sulfate: A comparison between wild type protein and a mutant with lower thermal stability

The adsorption and surfactant induced removal of T4 lysozyme was studied by in situ ellipsometry. Two proteins were investigated, wild type T4 lysozyme and a mutant protein where isoleucine 3 had been replaced by tryptophan (tryptophan mutant). The amount adsorbed is higher for the wild type protein than for the tryptophan mutant at the protein concentration employed (0.2 mg/mL). Furthermore, the adsorption kinetics differ between the two proteins, and the tryptophan mutant initially seems to adsorb at a somewhat slower rate than wild type protein, but the adsorbed amount levels off faster. Sodium dodecyl sulfate removes adsorbed proteins of both types from the silicon oxide surface, provided the surfactant concentration is high enough. The surfactant concentration needed to start removal was found to be well below cmc (3-11% of cmc in solution). The effect of ionic strength on this critical concentration follows the trends expected for surfactant aggregation and is not what would be expected for electrostatic interaction between protein and oppositely charged surfactant. This means that the removal starts at a lower surfactant concentration when the ionic strength is increased. Thus, the removal is thought to start at the concentration where the surfactant associates cooperatively to the adsorbed protein. The critical concentration for protein removal is higher, and the rate of protein removal is lower, at discrete surfactant concentration for wild type protein than for tryptophan mutant. This indicates that the onset of cooperative binding occurs at lower concentration for the tryptophan mutant, which is believed to be due to its larger changes in the conformation upon adsorption. These conformational changes might facilitate surfactant binding, for example, by exposing hydrophobic groups.