THERMODYNAMIC CHARACTERIZATION OF DILUTE AQUEOUS LIPID DETERGENT MIXTURES OF POPC AND C(12)EO(8) BY MEANS OF ISOTHERMAL TITRATION CALORIMETRY/

The solubilization of POPC lipid bilayers by the nonionic detergent C( 12)EO(8) was studied by isothermal titration calorimetry. The characte ristic transfer enthalpies for the detergent and the lipid between bil ayers and micelles were determined by titration of detergent micelles to lipid membranes and vice versa. For purpose of comparison, the enth alpy and Gibbs free-energy changes for the aggregation of aqueous dete rgent 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 b ecome saturated with detergent and first mixed micelles appear and the mole fraction when the bilayers are completely solubilized and only m ixed micelles are present could be easily determined from the titratio n experiments. The detergent binding to membranes does not follow the mass action law because there are no specific binding sites. An equati on using partitioning of detergent between water and bilayers gave goo d fits to the experimental data. The experiments lead to a consistent set of transfer enthalpies and entropies for the system of monomers, m icelles, and bilayers. Suggestions are made about the thermodynamic na ture of solubilization and partitioning. Finally, besides the limiting compositions of bilayers and micelles, another composition-driven tra nsition was detected within the mixed micellar range. This can be imag ined to correspond to a variation of the micellar shape and size and/o r intermicellar interactions.