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Inverse cubic liquid-crystalline phases of phospholipids and related lyotropic systems

The authors have previously found that saturated phospholipids such as phosphatidyl-ethanolamines can, in certain cases, adopt as many as three different inverse bicontinuous cubic phases in water, of probable space groups Ia3d (No 230), Im3m (No 229) and Pn3m (No 224). They found that these cubic phases could be induced to appear by reducing the chain length or by increasing the hydrophilicity of the headgroup of the phospholipid molecule. All of these cubic phases are located in the phase diagrams between the lamellar and the inverse hexagonal (HII) phases. They now report the observation of a novel inverse face-centred cubic phase, of probable space group Fd3m (No 227), in two different systems of hydrated binary lipid mixtures. One of these systems consists of mixtures of phosphatidylcholine with diacylglycerol; the other is an acid-soap mixture of an unsaturated fatty acid with its alkali salt. This Fd3m cubic phase in both systems occurs between the inverse hexagonal (HII) phase and the inverse micellar solution (L2), with increasing concentration of the lipid component with the less strongly hydrophilic headgroup. They surmise that the average mean curvature of the polar/nonpolar interface in this Fd3m cubic phase is more negative than that of the neighbouring HII phase; this is quite different from the inverse bicontinuous cubic phases, where it has a value intermediate between those of the lamellar and HII phases. They conclude that the structure of this Fd3m cubic phase most probably consists solely of closed inverse micellar aggregates.