Liquid Crystalline Structures in the Lecithin‐Cholate‐Water System Studied by NMR and X‐Ray Diffraction Methods

Although bile salts have several properties typical for a surfactant substance they show features that differ from those of ordinary amphiphiles. For instance, they do not aggregate in the cooperative manner and they never form lyotropic liquid crystalline phases. However, they have a large capacity to break lecithin bilayers by forming mixed micellar aggregates. The phase diagram for egg yolk lecithin-sodium cholate-water has been determined by Small and coworkers [1]. The extension and appearance of the different phases are similar to those in the phase diagrams for ordinary soap-alcohol-water systems. Concerning the structures on the molecular level of the different phases, suggestions have been given by Small, but no unambiguous experimental evidence for most of these structures have been reported. Here we present some different NMR studies in order to inquire into the molecular packing and order in the aggregates. Complementary x-ray studies are also performed. 1H-NMR shows that, in contrast to what is found for ordinary soap systems, spectra for lecithin-cholate systems are essentially the same for the so called hexagonal (E) and lamellar phases at a given lecithin/cholate ratio. The order parameter calculated from 2H quadrupole splittings of the-methyl groups of dilauroyl lecithin vary continuously between the E and lamellar phases, contrary to data obtained for soap systems. All these findings show that the molecular ordering is very similar in the two phases. It is also found that the molecular order decreases strongly with increasing cholate concentration. The x-ray diffraction picture for the E-phase shows reflexions that do not unambiguously fit the hexagonal structure. 2H water quadrupole splittings of a between glass plates macroscopically oriented E-phase sample showed that it orients uniaxially with at least a three-fold symmetry around the symmetry axis. Lipid diffusion coefficients can be conveniently obtained with pulsed NMR for both cubic [2] and lamellar [3] phases. The diffusion coefficients obtained are of the same order of magnitude, leading to the conclusion that the molecular diffusion occur over macroscopical distances in the cubic phase [2,3]. Thus the cubic phase consists of continuous lipid aggregates. Preliminary diffusion data obtained for the E-phase indicate that also this phase consists of continuous amphiphile aggregates. This is not compatible with the structure previously proposed, where micelles are stacked to form long rods.