Effect of decane or sodium decanoate on the thermodynamics of globular protein solutions

Abstract

The elucidation of the effect of decane or sodium decanoate (models of the nonpolar or polar lipids) on the molecular parameters and thermodynamic properties of proteins has been carried out in binary (protein-solvent) and ternary (protein-protein-solvent) aqueous solutions. Ovalbumin and 11S globulin, globular proteins of well-known structure, were considered. Light-scattering data manifest protein association in aqueous medium under the influence of both decane and sodium decanoate. Association of protein molecules in aqueous medium is higher under the influence of the nonpolar decane molecules. The thermodynamic parameters for different types of pair interactions in the systems ( the second virial coefficients) were estimated from static light-scattering data in the binary and ternary aqueous solutions of biopolymers without and with decane or sodium decanoate. The values of the second virial coefficients obtainedfrom binary solutions exhibit high thermodynamic affinity of protein associates formed under the influence of decane or sodium decanoate for aqueous medium. Maximal thermodynamic affinity of the protein associates for the solvent was observed in the case of the polar sodium decanoate added to the proteins. The values of the cross second virial coefficients obtained from ternary solutions indicate that the addition of the nonpolar decane molecules to the proteins increases the strength of the thermodynamically unfavorable interactions between protein molecules that are different in nature. On the other hand, the addition of polar sodium decanoate to the proteins leads to a decrease in the thermodynamically unfavorable protein-protein pair interactions in aqueous medium. The influence of the decane or sodium decanoate on the conformational state of the proteins in aqueous medium was characterized by differential scanning microcalorimetry. The data obtained manifest significant changing of the conformational stability of the protein globule under the influence of decane or sodium decanoate. The character of this influence strongly depends on the nature of both the low-molecular-weight organic compounds and the proteins studied.