Application of isothermal titration calorimetry for detecting lipid membrane solubilization

Isothermal titration calorimetry is presented to be an excellent method of detecting lipid membrane solubilization upon the addition of detergents. As an example, the titration of the non-ionic detergent C12EO6 to the lipid POPC is studied. The main contribution to the measured titration heat is the detergentʹs enthalpy difference upon transfer between micelles and membranes. The detergent added as micelles is transferred into membranes up to the saturating detergent per lipip mol/mol ratio, Rsat = 1.45 ± 0.07. Beyond Rsat the membranes are solubilized to micelles. Subsequently, the titration heat changes its sign from endothermic to exothermic. After complete membrane dissolution at Rsol = 5.0 ± 0.4 the titration heat vanishes nearly completely.