Interaction of n-octyl-β- d-glucopyranoside with globular proteins in aqueous solution

Abstract

The binding of n-octyl glucoside to a range of ten globular proteins in aqueous solution (pH 6.4, ionic strength 0.132 m) at 25°C has been measured by equilibrium dialysis. The binding isotherms which rise steeply at free n-octyl glucoside concentrations below the critical micelle concentration have been fitted to the Hill equation. For all the proteins the Hill coefficients are greater than unity indicating that binding is a positively cooperative process and the Gibbs energies of binding lie in a range from −9.7 kJ mol −1 for bovine serum albumin to −11.4 kJ mol −1 for fibrinogen and Aspergillus niger catalase. The enthalpies of interaction have been measured directly by microcalorimetry and are found to be very small relative to the Gibbs energies of binding consistent with a hydrophobic interaction. No evidence was found for the denaturation of the proteins by octyl glucoside. Binding has been analysed and discussed in terms of a model in which the proteins are incorporated into an octyl glucoside micelle. On this basis saturation binding to ribonuclease, lysozyme and α-chymotrypsin would be satisfactorily described in terms of a prolate ellipsoidal micelle whereas for bovine and Aspergillus niger catalase on oblate ellipsoid would describe the data more satisfactorily.