Interaction of octyl-β-thioglucopyranoside with lipid membranes

Octyl-β-thioglucopyranoside (octyl thioglucoside, OTG) is a nonionic surfactant used for the purification, reconstitution, and crystallization of membrane proteins. The thermodynamic properties of the OTG-membrane partition equilibrium are not known and have been investigated here with high- sensitivity titration calorimetry. The critical concentration for inducing the bilayer ⇆ micelle transition was determined as c(D)/* = 7.3 mM by 90°light scattering. All thermodynamic studies were performed well below this limit. Sonified, unilamellar lipid vesicles composed of 1-palmitoyl-2- oleoyl-sn-glycero-3-phosphocholine (POPC) with and without cholesterol were employed in the titration calorimetry experiments, and the temperature was varied between 28°C and 45°C. Depending on the surfactant concentration in the membrane, the partition enthalpy was found to be exothermic or endothermic, leading to unusual titration patterns. A quantitative interpretation of all titration curves was possible with the following model: 1) The partitioning of OTG into the membrane follows a simple partition law, i.e., X(b) = Kc(D),(f), where X(b) denotes the molar amount of detergent bound per mole of lipid and C(D),(f) is the detergent concentration in bulk solution. 2) The partition enthalpy for the transfer of OTG from the aqueous phase to the membrane depends linearly on the mole fraction R, of detergent in the membrane. All calorimetric OTG titration curves can be characterized quantitatively by using a composition-dependent partition enthalpy of the form ΔH(D)(R) = -0.08 + 1.7 R (kcal/mol) (at 28°C). At low OTG concentrations (R ≤ 0.05) the reaction enthalpy is exothermic; it becomes distinctly endothermic as more and more surfactant is incorporated into the membrane. OTG has a partition constant of 240 M^-1 and is more hydrophobic than its oxygen-containing analog, octyl-β-D-glucopyranoside (OG). Including a third nonionic amphiphile, octa(ethyleneoxide) dodecylether (C₁₂EO₈), an empirical relation can be established between the Gibbs energies of membrane partitioning, ΔG(p), and micelle formation, ΔG(mic), with ΔG(p) =1.398 + 0.647 ΔG(mic) o (kcal/mol). The partition constant of OTG is practically independent of temperature and of the cholesterol content of the membrane. In contrast, the partition enthalpy shows a strong temperature dependence. The molar specific heat capacity of the transfer of OTG from the aqueous phase to the membrane is ΔC(D) = -98 cal/(mol·K). The OTG partition enthalpy is also dependent on the cholesterol content of the membrane. It increases by ~ 1 kcal/mol at 50 mol% cholesterol. As the partition constant remains unchanged, the increase in enthalpy is compensated for by a corresponding increase in entropy, presumably caused by a restructuring of the membrane hydration layer.