APPLICATION OF ISOTHERMAL TITRATION CALORIMETRY FOR DETECTING LIPID-MEMBRANE SOLUBILIZATION

Isothermal titration calorimetry is presented to be an excellent metho d of detecting lipid membrane solubilization upon the addition of dete rgents. As an example, the titration of the non-ionic detergent C(12)E O(6) to the lipid POPC is studied. The main contribution to the measur ed titration heat is the detergent's enthalpy difference upon transfer between micelles and membranes. The detergent added as micelles is tr ansferred into membranes up to the saturating detergent per lipid mol/ mol ratio, R(sat) = 1.45 +/- 0.07. Beyond R(sat) the membranes are sol ubilized to micelles. Subsequently, the titration heat changes its sig n from endothermic to exothermic. After complete membrane dissolution at R(sol) = 5.0 +/- 0.4 the titration heat vanishes nearly completely.