Thermodynamic Characterization of Dilute Aqueous Lipid/Detergent Mixtures of POPC and C₁₂EO₈ by Means of Isothermal Titration Calorimetry

Abstract

The solubilization of POPC lipid bilayers by the nonionic detergent C12EO8 was studied by isothermal titration calorimetry. The characteristic transfer enthalpies for the detergent and the lipid between bilayers and micelles were determined by titration of detergent micelles to lipid membranes and vice versa. For purpose of comparison, the enthalpy and Gibbs free-energy changes for the aggregation of aqueous detergent monomers to micelles as well as for the partitioning into lipid bilayers were analyzed. The phase boundaries between pure bilayers and pure micelles, i.e., the detergent mole fraction, when the bilayers become saturated with detergent and first mixed micelles appear and the mole fraction when the bilayers are completely solubilized and only mixed micelles are present could be easily determined from the titration experiments. The detergent binding to membranes does not follow the mass action law because there are no specific binding sites. An equation using partitioning of detergent between water and bilayers gave good fits to the experimental data. The experiments lead to a consistent set of transfer enthalpies and entropies for the system of monomers, micelles, and bilayers. Suggestions are made about the thermodynamic nature of solubilization and partitioning. Finally, besides the limiting compositions of bilayers and micelles, another composition-driven transition was detected within the mixed micellar range. This can be imagined to correspond to a variation of the micellar shape and size and/or intermicellar interactions.

Detergent-phospholipid mixed micelles with a crystalline phospholipid core

“…The only calorimetric response to be expected here is the heat of dilution of the detergent͞phospholipid mixture into the detergent solution. It has been shown previously that this enthalpy is smaller than that of the chain-melting transition at least by an order of magnitude (18). Titration of a C 12 E 8 ͞POPC mixture (1:2 mol͞mol) at 30°C into a 10 mM solution of C 12 E 8 yielded a negligible enthalpy, indicating that the POPC dilution does not involve a chain-melting transition (Fig.…”

Leakage and lysis of lipid membranes induced by the lipopeptide surfactin

“…A fit of C S sol (C L ) yields R m sol , the minimum local surfactin-tolipid mole ratio within mixed micelles, the corresponding aqueous concentration C S w,sol , and K for mixed micelles. Figure 1a shows the raw data of an ITC solubilization experiment (Heerklotz et al 1996;Heerklotz and Seelig 2000b) titrating a 1 mM POPC vesicle suspension with a 1 mM micellar solution of surfactin in 6 lL injections. Initially, the surfactin concentration in the cell is small; the injected surfactin micelles dissolve and most of the surfactin is inserted into the lipid vesicles.…”

“…Weak detergents are active only in very high concentrations, which are unrealistic for a biological system. Potent synthetic detergents such as Triton X-100 (Heerklotz 2002;Paternostre et al 1988) or C 12 EO 8 (Edwards and Almgren 1991;Heerklotz et al 1996) solubilize membranes at about R b sat % 0.6 or even less, depending on lipid species and temperature.…”

Detergent-dependent Kinetics of Truncated Plasmodium falciparumDihydroorotate Dehydrogenase

“…We have adapted the partitioning model to describe partitioning of lipid-like CoQ analogues into detergent micelles. Since the models have been derived previously (40,41), we present only a brief explanation relevant to our adaptation of the models. Partitioning equilibrium can be described using a partition coefficient (K) to relate the concentration of CoQ free in solution (C Q,f ) to the CoQ-to-detergent mixed micelle ratio (X b ) by X b ϭ K⅐C Q,f .…”