Thermodynamic Studies of the Micellar Properties of a Surfactant Used for Membrane Protein Solubilization and Stabilization

Abstract

The effect of temperature on the micellar properties of the alkylglucoside surfactant n-octyl-β-D-thioglucopyranoside (OTG) used for membrane protein solubilization has been investigated. Critical micelle concentration (CMC), apparent (ϕV) and partial (V¯M) molar volume changes of the studied surfactant, as well as thermodynamic functions (the standard Gibbs free energy (ΔGmico), the standard enthalpy (ΔHmico) and entropy (ΔSmico)) of the OTG micellization process were determined. The above-mentioned parameters were calculated on the basis of the results obtained from measurements of surface tension, density and viscosity of the aqueous solutions of n-octyl-β-D-thioglucopyranoside, as well as pyrene (Py) and 8-anilinonaphthalene-1-sulfonic acid (ANS) fluorescence intensity in aqueous solutions of OTG and dynamic light scattering of aqueous solutions of OTG. Basing on the obtained results it is stated that critical micelle concentration of OTG is much lower compared to that of the earlier studied n-octyl-β-D-glucopyranoside (OGP). The standard Gibbs free energy changes indicate that the tendency of OTG molecules to form aggregates increases with temperature increase. However, this increase is not so evident as in the case of some other sugar-based surfactants. The small temperature effect on the aggregation properties of OTG in water is probably connected with the presence and strength of the hydrogen bonds between the water molecules and sugar units, or the type of linkage between the hydrophobic and hydrophilic parts of the studied surfactant. In addition, the presence of S-linkage in the OTG molecule despite its high enzymatic stability also causes the hydrophobicity increase of the studied surfactant (compared to OGP), which directly influences its micellization process.