Lipid-water interaction and liquid-crystalline phases

For more than two thousand years liquid-crystalline phases of soaps have been used for cleaning. The physical structure of these phases was revealed only recently. This was mainly due to the opinion that soap molecules should be regarded as rigid rods. In an infrared absorption spectroscopy study of anhydrous soaps, Chapman (1958) concluded that a high-temperature phase transition was caused by melting of the chains. Later Luzzati and co-workers (1960) conclusively demonstrated the liquid nature of the hydrocarbon chains as a fundamental feature in the structure of liquid-crystalline phases. They were also able to determine the structures of the most common liquid-crystalline phases. 

More commonly in lipids, however, a liquid-crystalline phase is formed in the presence of water, as shown in Fig. 8.9. Above a critical hydrocarbon chain melting temperature water penetrates the polar region and a lamellar lipid-water structure is formed with water layers alternating with lipid bilayers. 

Two other structures, which together with the lamellar phase are the most important liquid-crystalline phases in lipid-water systems, are shown in Fig. 8.9. It should be pointed out that the classification of lipids into polar and non-polar is best defined from their interaction with water. Lipids which do not give lipid-water phases are thus non-polar whereas those forming aqueous phases are classified as polar lipids. 

The two hexagonal structures shown in Fig. 8.9 are temed Hi and Hu. The /fj-phase consists of infinite cylinders of lipid molecules with their polar groups exposed on the surface and with a core of hydrocarbon chains in a disordered liquid-like conformation. These cylinders are hexagonally arranged in a water medium. The //n-phase is the inverse of Hi, i.e. water forms cylinders and hydrocarbon chains form the continuous medium with the polar groups at the water/hydrocarbon chain interface. 

The lamellar and hexagonal phases can be identified unambiguously from their X-ray diffraction patterns. The lamellar phase shows a series of reflections corresponding to one-dimensional periodicity. From the X-ray data it is possible to determine the thickness of the lipid bilayer, the cross-section per polar head-group and the water layer thickness. In the same way the diameter of the 

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Lipid-water interaction and liquid-crystalline phases... 

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