Retrograde dissociation of protein–surfactant complexes induced by mixed micelle formation

Abstract

The binding of sodium n-dodecyl sulfate (SDS) to bovine insulin in the presence of constant concentrations of the less hydrophobic surfactants, sodium n-decyl sulfate (SDeS), sodium n-octyl sulfate (SOS) and n-octyl-β-D-gly- copyranoside (OBG) has been measured by equilibrium dialysis by using radiolabelled [14C]SDS. The binding isotherms for all the binary surfactant systems (SDS–SDeS, SDS–SOS and SDS–OBG) exhibit maxima and the binding levels at the maxima decrease with increase in the constant concentration of the less hydrophobic surfactant. The maxima are explained in terms of the greater stability (lower critical micelle concentration) of mixed micelles formed by the binary mixture of the surfactants relative to the stability of the protein–surfactant complexes. By the use of ideal-solution theory for mixed micelle formation between the anionic surfactant mixtures and regular solution theory for anionic–non-ionic surfactant mixtures it is shown that the protein–surfactant complexes dissociate as the mixed micelles become enriched in the more hydrophobic surfactant. It is suggested that the phenomenon might be termed ‘retrograde dissociation’.