Aggregation in a protein-surfactant system.: The interplay between hydrophobic and electrostatic interactions

By precipitating a complex salt Ly(OS)(8) of the positively charged protein lysozyme and the anionic surfactant octyl sulfate OS-, one can generate a true ternary system: water-Ly(OS)(8)-NaOS. Using NMR diffusometry and UV spectroscopy measurements the thermodynamic parameters of the association processes have been determined. The solubility product of the complex salt, K-s, is 10-(28) M-9.(1) On addition of excess surfactant the complex salt is solubilized into micelles that form at a critical association concentration of 74 mM which is nearly a factor of 2 lower than the CMC of 133 mM. At higher protein and surfactant concentrations these micelles first coexist with gel aggregates. The thermodynamically stable gel phase is observed at protein concentrations higher than 7 wt % and has a stoichiometry of around 28 OS- per protein molecule. Thereafter, in the presence of more than ca. 30 OS- per protein, micelles containing a single lysozyme molecule are formed from the gel aggregates. This rich aggregation pattern can be described as caused by a combination of an attractive hydrophobic interaction between hydrophobic patches on the protein surface and the surfactant hydrocarbon chain, and a composition-dependent electrostatic interaction between charged amino acids and the surfactant headgroup. The net force is attractive up to a ratio of surfactant to protein of 8, after which it becomes increasingly repulsive. The gel phase occurs as a compromise between the attractive hydrophobic interaction and the relatively weak electrostatic repulsion.