Solubilization Properties of α-Lactalbumin and β-Lactoglobulin in AOT−Isooctane Reversed Micelles

Solubilization properties of α-lactalbumin and β-lactoglobulin in a sodium bis(ethylhexyl) sulfosuccinate−isooctane reversed micellar phase were measured as a function of aqueous phase pH and ionic strength. Solubilization behavior over a wide range of pH values and ionic strengths indicates that hydrophobic, as well as electrostatic, interactions may play an important role in the solubilization of these proteins. The influence of hydrophobic interactions on the solubility behavior of α-lactalbumin and β-lactoglobulin is reflected in the effect of protein size and net charge on the degree of solubility. Salt concentration also affects the protein solubilization, most likely through its influence on the size of the reversed micellar droplets. This effect of droplet size on protein solubilization can be quantified by defining a “critical micellar radius” above which 50% or more of the protein transfers into the organic phase. The value of this parameter and its constancy over various pH values further supports the hypothesis that α-lactalbumin and β-lactoglobulin solubilize in the reversed micellar interface, stabilized by protein−surfactant hydrophobic interactions. Protein solubility was also found to be sensitive to the type of cation present in the system. Finally, the concentration of protein in the initial aqueous phase was observed to affect the solubilization of both water and protein. A strong linear relationship between water transfer and the amount of α-lactalbumin solubilized within the reversed micellar phase suggests that this protein may act as a cosurfactant, and induce expansion of the surfactant interface by its incorporation in that region.