Affinity-based reversed micellar protein extraction: II. effect of cosurfactant tail length

The selectivity of protein extraction by reversed micellar solutions can be improved by the addition of affinity cosurfactants bearing ligands which bind strongly to the target protein. The interactions between cosurfactant and protein, as well as the interfacial activity of both the free cosurfactant and the protein-cosurfactant complex, were accounted for in a model of the affinity-partitioning process. The aqueous phase dissociation constant was used to describe the protein-ligand interactions. The interfacial partition coefficient for several cosurfactant families varied with tail length according to the well-established hydrophobic effect. Control studies with alkylated chymotrypsin showed that when longer hydrophobic tails are irreversibly attached to the protein, the protein partitions more strongly to the reversed micellar phase. In contrast, for reversible protein-cosurfactant binding, the model predicts a maximum in protein uptake when the cosurfactant tail length is varied; the decrease at longer tail lengths is due to the lowered aqueous phase concentration of affinity cosurfactant, resulting in the formation of fewer protein-cosurfactant complexes. This behavior was confirmed experimentally.