Sodium taurocholate-induced lamellar-micellar phase transitions of DPPC - Determined by DSC and X-ray diffraction

The thermotropic transitions of 1,2-dipalmitoylphosphatidytcholine (DPPC) and the structural changes of its lamellar phases have been studied between 0 and 50 degrees C by both DSC and synchrotron small angle X-ray diffraction/scattering as a function of temperature (XRDT) and sodium taurocholate concentration [TC] in the 0-40 mM range ([DPPC]=50 mM) at pH 7.4. The existence of multiple phase transitions (up to 5 peaks within a 5 degrees C interval) in a narrow domain of temperature between 25 and 42 degrees C depending on the [TC]/[lipid] ratio was observed in the DSC curves. XRDT showed that at low ratios they might correspond to transitions between lamellar phases, the structural characteristics of which are given. At higher ratios a lamellar to micellar transition was observed, and the temperature at which it was observed decreased as a function of the TC content. The relationships with DPPC vesicle bilayer permeabilization and solubilization are discussed.