Polar Interactions Play an Important Role in the Energetics of the Main Phase Transition of Phosphatidylcholine Membranes

Conformational changes of membrane proteins are accompanied by deformation in the surrounding lipid bilayer. To gain insight into the energetics of membrane deformation, the phase behavior of dimyristoylphosphatidylcholine (DMPC) membranes in the presence of the dipole potential, ψd, modifiers was investigated by differential scanning calorimetry. 7-Ketocholesterol, which weakens ψd and reduces membrane-perpendicular dipole–dipole repulsion, causes a discrete second peak on the high-temperature side of the main transition, whereas 6-ketocholestanol, which strengthens ψd and increases membrane-perpendicular dipole–dipole repulsion, merely produces a shoulder. Measurements on pure DMPC vesicles showed that the observed temperature profile could not be explained by a single endothermic process, that is, breaking of van der Waals forces between hydrocarbon chains alone. Removal of NaCl from the buffer caused an increase in the main transition temperature and the appearance of an obvious shoulder, implicating ...