Mechanism of protein extraction from the solid state by water-in-oil microemulsions

The extraction of solid-phase alpha-chymotrypsin, bovine serum albumin (BSA), and lysozyme by water-in-oil microemulsion (w/o-ME) solution containing Aerosol-OT (AOT) was thoroughly examined as a means to maximize protein solubilization in organic solvent media. Protein extraction occurred simultaneously with the adsorption of water and AOT by the solid protein. Water and AOT were desorbed at nearly equal rates, suggesting that both materials were desorbed together as micreomulsions. The solubilization of protein increased linearly with the ratio of solid protein to extractant solution except at a high value of the ratio, where most protein-containing microemulsions were desorbed. Based on our results, a mechanistic model was developed to describe the solid-phase extraction procedure. First, microemulsions are desorbed from solution by the solid protein, resulting in the formation of a solid protein-AOT-water aggregate. Second, when a protein in the solid phase binds to a sufficient number of microemulsions, the resulting aggregate's increased hydrophobicity drives its solubilization into lipophilic solvent. Third, through the exchange of materials between the solubilized precipitate and the remaining microemulsions, protein-containing w/o-MEs are formed. The presence of adsorption is further indicated by an isotherm existing between the water, AOT, and protein content of the resulting solid phase for each protein. The driving force behind adsorption is either AOT-protein interactions or the protein's affinity for microemulsion-encapsulated water, depending on the properties of the protein and the size of the microemulsions, in agreement with the model of P. L. Luisi [Chimia, 44: 270-282 (1990)]. The second step of our model is mass transfer limited for the extraction of solid alpha-chymotrypsin and BSA. The extraction of solid lysozyme was limited by the occurrence of an irreversible precipitation process.