CUBIC PHASE OF MONOGLYCERIDE-WATER SYSTEMS - ARGUMENTS FOR A STRUCTURE BASED UPON LAMELLAR BILAYER UNITS

A combination of low-angle X-ray diffraction and NMR diffusion techniques may be used for the elucidation of the internal structure of cubic lyotropic liquid crystalline lipid systems. The advantage of the NMR method for this kind of structure determinations lies in the facts that restricted diffusion can be observed and that the lateral diffusion in lamellar systems can be measured directly. The value of the apparent diffusion coefficient depends strongly on the overall structural geometry. Previous X-ray diffraction findings for the cubic phase occurring in aqueous monoglyceride systems have been reevaluated and supplemented and arguments are presented for an internal structure built up of small lamellar bilayer units. The lamellar units are ideally hexagon-shaped disks connected in such a manner that they form a close-packed system of tetrakaidecahedra in which the quadrilateral faces are missing. Thus is obtained an open structure which is optically isotropic and is continuous in three axial directions both with respect to the nonpolar lipid and the polar aqueous regions. This structure concept of two continuous regions where the lipid regions consist of a system of lamellar units obtains support from values for the translational diffusion coefficients of the lipid (and water) measured with the NMR pulsed magnetic field gradient method. This type of cubic phases, which occurs in many monoglyceride systems and which can be mixed with other lipids such as lecithin or cholesterol and also with proteins, may be used as model membrane systems. © 1979, American Chemical Society. All rights reserved.