Salt-induced phase separation in the liquid crystalline phase of phosphatidylcholines

The effects of alkali chlorides on multilamellar vesicles of various phosphatidylcholines in the lamellar liquid crystalline L-alpha-phase were investigated by using small-angle X-ray scattering. At alkali chloride concentrations above 70 mM (LiCl) a phase separation in the liquid crystalline phase of POPC is induced. The splitting of the first and second order diffraction peaks into two major discrete components indicates a separation into different lamellar liquid crystalline (smectic A) phases. Detailed data-analysis applying the modified Caille theory proves that the phases mainly differ in the interbilayer water thickness by about two hydration layers. The lipid bilayer profile itself remains essentially the same in all liquid crystalline phases. A comparison of differently prepared samples and rapid mixing experiments in combination with simultaneous time-resolved X-ray diffraction suggest that the phase separation is osmotically driven. (C) 2001 Elsevier Science B.V. All rights reserved.