Do low surfactants concentrations change lysozyme colloid properties?

Multi-component aqueous/octane systems, which contain low-molecular-weight ionic surfactants (sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB)) and lysozyme, were investigated using the scintillation phase method and the pendant drop technique. Experiments were carried out in the solutions of high ionic strength. Interfacial tension measurements reveal the appearance of local extremums for the SDS–lysozyme mixture that were attributed to changes in the nature of the SDS–lysozyme complex, which was confirmed by a radiochemical approach (determination of the distribution ratios and values of the surface excess of each component on the background of the other component). The formation of lysozyme–ionic surfactant complexes was observed for both cationic and anionic surfactants. Low surfactant concentrations of both SDS and DTAB result in the same changes of the colloidal behavior of protein. We proposed two possible mechanisms of interaction of lysozyme with cationic and anionic surfactants at low concentrations. The interactive process associated with the formation of lysozyme–ionic surfactant complexes by means of protein binding sites following by co-adsorption with SDS and by step-by-step hydrophilization and displacement from the adsorption layer by DTAB. The increase in the detergent concentration results in the usual substitution of protein from the adsorption layer that was observed by interfacial tension measurements and the radiochemical method.