Cholesterol mesomorphous phases

Ekwall and Baltcheffsky [265] have discussed the formation of cholesterol mesomorphous phases in the presence of protein-surfactant complexes. In some cases when cholesterol is added to these solutions a mesomorphous phase forms, e.g. in serum albumin-sodium dodecyl sulphate systems, but this does not occur in serum albumin-sodium taurocholate solutions [266]. Cholesterol solubility in bile salt solutions is increased by the addition of lecithin [236]. The bile salt micelle is said to be swollen by the lecithin until the micellar structure breaks down and lamellar aggregates form in solution the solution is anisotropic. Bile salt-cholesterol-lecithin systems have been studied in detail by Small and coworkers [267-269]. The system sodium cholate-lecithin-water studied by these workers gives three paracrystalline phases I, II, and III shown in Fig. 4.37. Phase I is equivalent to a neat-soap phase, phase II is isotropic and is probably made up of dodecahedrally shaped lecithin micelles and bile salts. Phase III is of middle soap form. The isotropic micellar solution is represented by phase IV. The addition of cholesterol in increasing quantities reduces the extent of the isotropic... 

Lewis RNAH, McElhaney RN. The mesomorphic phase behaviour of lipid bilayers. In The Structure of Biological Membranes. Yeagle PE, ed. 1991. CRC Press, Boca Raton, FL. pp. 73-155. Ipsen JH, Karlstrom G, Mouritsen OG, Wennerstrom H, Zuk-ermann MJ. Phase equilibria in phosphatidylcholine-cholesterol system. Biochim. Biophys. Acta 1987 905 162-172. 

Ordinarily a crystalline solid melts sharply at a single, well-defined temperature to produce a Uquid phase that is amorphous and isotropic. A different behavior is exhibited by a class of organic compounds known as liquid crystals. The oldest examples are cholesterol derivatives, e.g., cholesteryl benzoate. This substance, for instance, does not have a sharp transition to amorphous Uquid at 145.5°C, but changes to a cloudy liquid, which becomes clear and isotropic only at 178.5°C. This cloudy intermediate state that possesses an ordered stmcture with some resemblance to a crystaUine soUd, while still in the liquid state, is called a mesophase or mesomorphic phase from the Greek mesos, meaning in between or intermediate. 

Some binary mixtures of protic molecules have also been reported to form mesophases that are not present in either pure component. For instance, Lawrence observed mesophase formation in 1 2 molar ratios of cholesterol and 1-alkanols containing 12,13,14,16 and 18 carbon atoms [151]. Mixtures of 1-hexadecanol and dodecanoic acid exhibits a lamellar liquid crystalline (smectic) phase [152]. Intermolecular hydrogen bonding must be an important contributing factor to maintain the necessary aggregation for mesomorphism in these cases. For applications of this concept to PLCs, see Chapter 3. 

In the case of Cm, the cormection of straight mesogens, such as cholesterol [18] or ferrocenomesogens [19] indeed produced mesophases, although in most cases the Hquid-crystaUine phases were either transient or monohopic [20]. Conversely, the use of mesomorphic dendritic addends led to liquid-crystalline materials with accessible and stable mesomorphic properties. 

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