Continuous bilayers

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

Other electrokinetic chromatography techniques using liposomes (LEKC) and vesicles (VEKC) have also been applied for logP d estimation. Liposomes and vesicles are organized structures containing continuous bilayers of monomers enclosing an aqueous core region. 

All biological membranes are constructed according to a standard pattern. They consist of a continuous bilayer of amphipathic lipids approximately 5 nm thick, into which proteins are embedded, in addition, some membranes also carry carbohydrates (mono- and oligosaccharides) on their exterior, which are bound to lipids and proteins. The proportions of lipids, proteins, and carbohydrates differ markedly depending on the type of cell and membrane (see p. 216). 

Figure 18-5 Schematic representation of (a) a membrane lipid, (b) a bilayer structure formed by lipid molecules in polar media the interior of the bilayer is nonpolar, and (c) a continuous bilayer structure (liposome) with polar interior and exterior...

There is a close resemblance between fatty-acid salts and phospholipids (p. 790) in that both possess long hydrocarbon tails and a polar head. Phospholipids also aggregate in a polar medium to form micelles and continuous bilayer structures such as shown in Figure 18-5. The bilayer lipid structure is very important to the self-sealing function of membranes and their impermeability to very polar molecules. 

There have been reports that the gastric surface is coated with a phospholipid, which Is In principle similar to the surfactant found in the lung. If present as a continuum-and this Is difficult to denrxjnstrate-such a phospholipid could prevent access of protons to the gastric surface. In the fundus, under add secretion, it is unlikely that such a continuous bilayer would persist, but It may do so In the antrum. 

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