Energetics of the Mixing of Phospholipids in Bilayers Determined Using Vesicle Solubilization

Here, we report an experimental approach for determining the change in the free energy and the enthalpy that accompanies the mixing of the anionic phosphatidylglycerol and the zwitterionic phosphatidylcholine. The enthalpy change originates in the thermal changes of disrupting lipid bilayer vesicles titrated into a surfactant micelle solution and is monitored using isothermal titration calorimetry. The difference in the solubilization enthalpies between pure and mixed lipid vesicles yields the lipid mixing enthalpy. The Gibbs free energy changes are estimated by determining the thermodynamic equilibrium constants of forming a molecular complex between phospholipids and methyl-β-cyclodextrin. We provide direct experimental evidence that mixing of the anionic lipid and the zwitterionic lipid is explained well by the entropic term of the electrostatic free energy of a charged surface in the Gouy–Chapman model. The present strategy enables us to determine the precise energetics of lipid–lipid interactions in near-native environments such as liposomes without any chemical modification to lipid molecules.