Lysozyme in catanionic surfactant mixtures.

We investigate the competition between the associations of oppositely charged protein-surfactant complexes and oppositely charged surfactant complexes. In all systems examined, the most favorable complexation is the one between the two oppositely charged surfactant ions, despite the strong binding known, for example, dodecyl sulfate, DS-, to lysozyme. Thus, the phase behavior of the catanionic system is dominating the features observed also in the presence of protein. The phase behavior of the dilute protein-free dodecyltrimethylammonium chloride-sodium dodecyl sulfate-water system is presented and used as a basis for the discussion on the different solubilization mechanisms. Our results show that the mechanism for resolubilization of a protein-surfactant salt is fundamentally different when it is caused by addition of a second surfactant than when it is accomplished by an excess of the first surfactant. The competition between lysozyme and cationic amphiphiles as hosts for the anionic surfactants was studied experimentally and analyzed quantitatively. Aggregates with C12 cationic surfactants are clearly preferred by the anionic surfactants, while for C10 and particularly C8 a clear excess of cationic surfactant has to be added to completely dissolve the complex salt lysozyme-anionic surfactant.