SODIUM TAUROCHOLATE-INDUCED LAMELLAR-MICELLAR PHASE-TRANSITIONS OF DPPC - DETERMINED BY DSC AND X-RAY-DIFFRACTION

The thermotropic transitions of 1,2-dipalmitoylphosphatidytcholine (DP PC) and the structural changes of its lamellar phases have been studie d between 0 and 50 degrees C by both DSC and synchrotron small angle X -ray diffraction/scattering as a function of temperature (XRDT) and so dium taurocholate concentration [TC] in the 0-40 mM range ([DPPC]=50 m M) at pH 7.4. The existence of multiple phase transitions (up to 5 pea ks within a 5 degrees C interval) in a narrow domain of temperature be tween 25 and 42 degrees C depending on the [TC]/[lipid] ratio was obse rved in the DSC curves. XRDT showed that at low ratios they might corr espond to transitions between lamellar phases, the structural characte ristics of which are given. At higher ratios a lamellar to micellar tr ansition was observed, and the temperature at which it was observed de creased as a function of the TC content. The relationships with DPPC v esicle bilayer permeabilization and solubilization are discussed.