Lalpha-phase separation in phosphatidylcholine-water systems induced by alkali chlorides.

The effects of alkali chlorides on phosphatidylcholine-water bilayer systems in the Lalpha-phase were investigated by using small- and wide-angle X-ray scattering. The ternary system LiCl-POPC-H2O under isothermal conditions has shown that above Li+/POPC molar ratios of 0.1 and a lipid concentration above 5% (w/w), a splitting of the lamellar Bragg diffraction peaks into discrete components indicates a phase separation into different lamellar liquid crystalline (smectic A) phases. It is also shown that in saturated distearoyl phosphatidylcholine and in egg phosphatidylcholine, alkali chlorides induce Lalpha-phase separation. The number and repeat distance of the coexisting lamellar phases depend on the nature and concentration of the alkali chloride, the concentration of the phosphatidylcholine, and the degree of the acyl chain unsaturation.

Characterization of the sub-main-transition in distearoylphosphatidylcholine studied by simultaneous small- and wide-angle X-ray diffraction.

Simultaneous small- and wide-angle X-ray diffraction was used to investigate the structural conversions in the so-called sub-main-transition of fully hydrated multilamellar vesicles of distearoyl phosphatidylcholine (DSPC). The small-angle diffraction patterns show a modification in the supramolecular structure of the corrugated lipid bilayer and reveal that the sub-main-transition does not abolish the general features of the ripple phase (P(beta')). Concomitantly in the wide-angle region the diffraction patterns exhibit a rearrangement of the hydrocarbon chain packing and an onset of chain melting. The presence of KCl show an enhancement of the effects of the sub-main-transition. Moreover, in the ripple phase (P(beta')) below the sub-main-transition the hydrocarbon chains are positioned on an orthorhombic lattice in the presence of KCl.