Interaction of octyl-beta-thioglucopyranoside with lipid membranes

Abstract

Octyl-beta-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 cD* = 7.3 mM by 90 degree 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 degrees C and 45 degrees 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., Xb = Kc(D,f), where Xb 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 deltaHD(R) = -0.08 + 1.7 R (kcal/mol) (at 28 degrees C). At low OTG concentrations (R < or = 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-beta-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, deltaGp, and micelle formation, deltaGmic, with deltaGp = 1.398 + 0.647 deltaGmic (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 deltaCp = -98 cal/(mol x K). The OTG partition enthalpy is also dependent on the cholesterol content of the membrane. It increases by approximately 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.

Heat Evolution of the Self-Assembly of Amphiphiles in Aqueous Solutions

“…As an example, octyl thioglucoside (OTG) is a more hydrophobic surfactant than OG. Under these conditions, the apparent reaction enthalpy increased almost linearly with OTG concentration [17]. Thermodynamic analysis of the OTG-POPC system is more complex than analysis of the OG-lipid system because the partition enthalpy for the transfer of OTG from the aqueous phase to the membrane apparently depends on the mole fraction of detergent in the membrane [17].…”

“…In those cases this protocol yielded reliable information [17,18]. Experimentally, the least suitable protocol is the injection of a small volume of the solute into a cell of large volume containing the aggregates.…”

“…Under this assumption, ∆Q for each titration step can be related to ∆H w→b and K: ∆Q ϭ f(∆H w→b , K ) [17,18]. Under this assumption, ∆Q for each titration step can be related to ∆H w→b and K: ∆Q ϭ f(∆H w→b , K ) [17,18].…”

“…Its cmc (9 mM) is lower than that of OG, and its partition coefficient between bilayers and water (240 Downloaded by [University of Auckland, New Zealand] at 19:19 04 June 2016 M Ϫ1 ) is higher than that of OG (120 M Ϫ1 ) [17]. As an example, octyl thioglucoside (OTG) is a more hydrophobic surfactant than OG.…”

The thermodynamics of simple biomembrane mimetic systems

“…The above model assumes that K o and $\mathrm{normal∆}{H}_S^{w\to L}$ are independent of solute and lipid concentration. In many cases, the solute mixes non-ideally with the lipid (see section Non-ideal mixing), therefore these assumptions are not a priori warranted, more refined models allowing for composition-dependent K o and $\mathrm{normal∆}{H}_S^{w\to L}$ have been used,[138–140] but in most cases the experimental data do not justify the introduction of other adjustable parameters (such as a non-ideality parameter). However, it must be noted that a two-parameter model, equation (24), yields good data even if the model assumptions are not strictly fulfilled.…”