Equilibrium process in bile salt-phospholipid systems as studied by combined DSC and X-ray diffraction

The structures formed by sodium taurocholate (TC) and 1,2-dipalmitoylphosphatidylcholine (DPPC) mixtures and their evolution as a function of time, were analyzed by synchrotron X-ray diffraction/scattering as a function of temperature (XRDT) coupled with differential calorimetry. TC/DPPC mixtures in molar ratios (r) ranging from 0.3 to 4.5 were examined after equilibration times from 4 days to 10 months. Our results clearly show that the supramolecular arrangements of the TC/DPPC mixtures are governed by very slow kinetics. The initial TC/DPPC micellar solution progressively evolves at 4degreesC towards liquid-crystalline structures with or without coexistence with micelles. The small-angle X-ray scattering (SAXS) analysis shows that an unstable hexagonal phase can be formed after 15 days. Long equilibration times lead to lamellar structures. For r = 1 and 3, a lamellae-micelle transition, induced by increasing temperature, is shown by SAXS in relation to the melting of the DPPC chains, as evidenced by wide-angle X-ray scattering (WAXS) and DSC.