Lipid-water phases, cross section

Figure 1. Cross section through a number of lipid-water phases. Frozen (gel) chain phases (b, c), lamellar phases (a, b, c), and hexagonal phases (d, e) are shown.

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... 

Crystals of most polar lipids can swell in the presence of water. The corresponding phases, gel-phases, with lamellarly packed lipid, and water layers, are sometimes thermodynamically stable (Larsson, 1994, p. 41). Also, the hydrocarbon chain packing of gel-phases usually show some axial rotational disorder. The alkyl chain cross-sectional area is close to 20 A2 in a plane perpendicular... 

Figure 5. A cross section through a Hz/ phase that illustrates that the interstitial areas (stipled) break the axial symmetry of the Hu tubes. The water cores (hatched) are in the center. A few lipids are shown in the leftmost cylinder for orientation. Lipid hydrocarbon chains fill the interstitial as well as the annular regions. Deviations from the mean hydrocarbon chain-segment density or in the mean extended length of the chains correspond to an entropically expensive set of chain configurations.

A cubic phase of space group laid (which was the first cubic structure to be solved [161] and is among the most commonly observed [162]). The structure of laid belongs to a body-centered space group of rods (which are essentially a surfactant bilayer with a circular cross section) connected 3 x 3 to generate two interwoven but unconnected 3D networks [162]. A chiral cubic phase of space group PA Il has been observed so far in only one lipid-protein-water system [163]. Its proposed structure is similar to that of laid. It has one water-lipid network interwoven with one network of quasi-spherical inverse micelles that encloses the protein molecules. 

From data available on lipid-water L -phases it is known that the cross-section area per chain is always near 30 A, and when the chain disorder is exceptionally high (for example at high temperatures) the value... 

This situation is shown in Fig. III. 11. The pore has a diameterD smaller than the natural size Rf of the chains. [The precise shape of the cross-section (circular, square, etc.) is not important for our scaling arguments.] Physically, this might be achieved in ternary solutions lipid + water -I- polymer, where the lipid tends to make a hexagonal phase with long, parallel tubes. 

---