The partitioning of proteins between water-in-oil microemulsions and conjugate aqueous phases

Abstract

The partitioning of chymotrypsin, pepsin and lysozyme between water-in-oil microemulsion phases stabilised by the anionic surfactant sodium bis-2-ethylhexyl sulphosuccinate (AOT) and conjugate aqueous phases containing NaCl has been measured. For chymotrypsin and pepsin the extent of partitioning as a function of AOT concentration may be described in terms of a partition coefficient between the microemulsion-dispersed water and the bulk water phase. For aqueous phases containing low salt concentrations in equilibrium with microemulsion phases of relatively large droplet sizes, the value of the partition coefficient is sensitive to the overall protein charge. High values of the partition coefficient are observed for positively charged proteins owing to attractive electrostatic interactions with the negatively charged surfactant film. At high salt concentrations, corresponding to small microemulsion droplets, the partitioning is less sensitive to protein charge. Proteins have a higher affinity for the microemulsion-dispersed water than for bulk water even when the overall protein charge is negative. Lysozyme forms a solid-like phase with AOT which dissolves in the oil when sufficient AOT is present for monolayer coverage of the lysozyme surface. Similar partitioning studies with an enzyme substrate provides useful information for the interpretation of enzyme kinetics in microemulsions.