Cubic phase detergent

Lipid cubic (51) and sponge (52) phases, as well as bicelles (53), are alternatives to detergents that have been applied successfully to membrane protein crystallization. In these instances, the protein is embedded in a lipid bilayer environment, which is considered more natural compared with the detergents that form micellar phases. In the recent high-resolution crystal structure of the human 32 adrenergic G-protein-coupled receptor, lipid cubic phase was used with necessary cholesterol and 1,4-butandiol additives (54). The cholesterol and lipid molecules were important in facilitating protein-protein contacts in the crystal. 

The cubic phases can also be dispersed by amphiphilic proteins. Caseins, for example, which also are very effective as emulsifiers, can disperse the cubic phase just as do simple surfactants, such as bile salts. The mechanism is exactly the same as the solubilisation of bilayers by detergents at the cmc - the mixed system has a different surfactant parameter, i.e. local curvature, and the system takes on a different structure. 

Use of liquid crystalline phases Surfactants produce liquid crystalline phases at high concentrations. Three main types of Hquid crystals can be identified hexagonal phase (sometimes referred to as middle phase) cubic phase and lamellar (neat phase). All of these structures are highly viscous and also show elastic responses. If produced in the continuous phase of suspensions, they can eliminate sedimentation of the particles. These Hquid crystalline phase are particularly useful for application in liquid detergents which contain high surfactant concentrations. Their presence reduces sedimentation of the coarse builder particles (phosphates and silicates). 

Figure 15 Oscillatory behaviour of the cubic phase of a branched nonionie detergent.

Note that this phase sequence, can be universally observed in the thermotropic phase sequence of block copolymers and low-molecular-mass amphiphiles as well as in lyotropic detergent solvent systems. In addition to the phase sequence described above, several additional intermediate phases can occur at the transitions between different mesophase morphologies. The bicontinuous cubic phases Cub are often also included in these intermediate phases. 

The lamellar lyotropic liquid crystal phase is often formed in detergent solutions. When subjected to shear lamellae can, under certain conditions, curve into closed shell structures called vesicles (Section 4.11.4). These are used in pharmaceutical and cosmetic products to deliver molecules packed into the core. Selective solubilization in micelles finds similar applications, although micelles tend to break down more rapidly than vesicles when diluted. Applications for hexagonal and cubic structures may stem from the recent discovery that they can act as templates for inorganic materials such as silica, which can be patterned into an ordered structure with a regular... 

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