Monitoring detergent-mediated solubilization and reconstitution of lipid membranes by isothermal titration calorimetry

The solubilization and reconstitution of biological or liposomal membranes by detergents and biomolecules with detergent-like properties play a major role for technical applications (e. g., the isolation of membrane proteins) and biological phenomena (of, e. g., amphiphilic peptides). It is therefore important to know and understand the amounts of a given detergent required for the onset and completion of membrane solubilization and the detergent-lipid interactions in general. Lipid-detergent systems can form a variety of aggregate structures, which can be grouped into two pseudophases (lamellae and micelles) so that solubilization can be approximately described as a phase transition. Here we present a protocol for establishing the phase diagram and a detailed thermodynamic description of a lipid-detergent system based on isothermal titration calorimetry (ITC). The protocol can also be used to detect additive-induced membrane destabilization, permeabilization, domain formation and lipid-dependent transitions between rod-like and spherical micelles. A minimal protocol consisting of all sample preparation procedures and a single solubilization experiment can be accomplished within 2 days; a more extensive series comprising both solubilization and reconstitution experiments requires several days to a few weeks, depending on the number of titrations performed.