Hexagonal II phase

The adaptations that thwart inappropriate phase changes have been presented as theories of homeophasic adaptation (McElhaney, 1984) and dynamic phase behavior (Hazel, 1995). In essence, both theories stress the same primary points the lipid composition of the bilayer must be modified in the face of temperature change to conserve the appropriate phase structure. The liquid-crystalline phase must be conserved at low temperatures. At higher temperatures the propensity to form the hexagonal II phase must not become too great. Thus, Tm and Th must be adjusted during adaptation. 

Li L. Zheng LX. Yang FY. Effect of propensity of hexagonal II phase formation on the activity of mitochondrial ubiquinol-cytochrome c reductase and H(-i-)-ATPase. Chem Phys Lipids 1995 76 135-144. Litman B J, Lewis EN, Levin IW. Packing characteristics of highly unsatrrrated bUayer lipids Raman spec-... 

Fig. 10. Molecular arrangements of phospholipids in the bilayer and hexagonal II phases with their characteristic P-NMR spectra and freeze-fracture morphology. From [97].

The other mesophases which may be formed are the viscous isotropic, cubic phase and the hexagonal II phase described in Chapter 8. 

The cubic phase is formed instantly when highly unsaturated monoglycerides from soybean oil or sunflower oil, etc. are mixed with water at room temperature (see Fig. 5.9(b)). The hexagonal II phase is formed by heating a cubic phase, or by aqueous blends of mono-, di- and tri-glycerides, like most of the commercial saturated 35-55% monoglycerides, or other complex mixtures of polar and non-polar glycerides. 

Neither the cubic nor the hexagonal II phase has found a direct application, but they may be part of interfacial structures formed during processing of food emulsions. 

The mono-olein-water phase diagram (Figure 15c) shows the formation of lamellar liquid crystalline structure at room temperature (20 °C) at water content between 2 and 20%. At higher water concentrations, a cubic phase is formed, which above 40% water exists in equilibrium with water. If the temperature of the cubic phase is increased above 90 °C, a hexagonal II phase is produced. 

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