Lipids cholesteric

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

The Helfrich-Prost model was extended in a pair of papers by Ou-Yang and Liu.181182 These authors draw an explicit analogy between tilted chiral lipid bilayers and cholesteric liquid crystals. The main significance of this analogy is that the two-dimensional membrane elastic constants of Eq. (5) can be interpreted in terms of the three-dimensional Frank constants of a liquid crystal. In particular, the kHp term that favors membrane twist in Eq. (5) corresponds to the term in the Frank free energy that favors a helical pitch in a cholesteric liquid crystal. Consistent with this analogy, the authors point out that the typical radius of lipid tubules and helical ribbons is similar to the typical pitch of cholesteric liquid crystals. In addition, they use the three-dimensional liquid crystal approach to derive the structure of helical ribbons in mathematical detail. Their results are consistent with the three conclusions from the Helfrich-Prost model outlined above. 

However, there is a structure consistent with both the required space group and the optical properties. The gyroid surface, which occurs frequently in lipid-water systems, provides such a possibility. If we assume that cholesterol skeletons form rod-like infinite helices, this structure represents an effective three-dimensional packing of such helices. Thus, the rods form a body-centered arrangement as shown in Fig. 5.5. In this structure, there is a helical twist between the rods, in addition to the cholesteric twist within each rod. The h)rperbolic structure is a consequence of the chirality of the esters, which induces torsion into the packing arrangement. A racemic mixture does not exhibit this phase natural cholesteric esters contain a single enantiomer only. 

The structure of the blue phase is of some importance. Among the lipoproteins carrying lipids in the blood, low-density lipoproteins (LDL) have attracted much attention. They are the factors mainly responsible for plaque formation, which ultimately leads to atheriosclerotic changes and heart disease. The major components of the LDL-particles are cholesterol fatty acid esters. A remarlmble property is the constant size of LDL particles [28], which indicates that the interior must possess some degree of order. It seems probable that the structure proposed above for cholesterol esters in the cholesteric liquid-crystalline structure should occur also in the LDL-particle. In that case the LDL particle can be viewed as a dispersed blue phase, whose size is related to the periodicity of the liquid-crystalline phase, and the protein coat at the surface is oriented parallel to adjacent specific crystallographic planes of the blue phase. These amphiphilic proteins will expose lipophilic segments inwards emd expose hydrophilic groups towards tiie enviroiunent. 

Quaternary structure is akin to the mesostructure of lipid or surfactant self-assemblies, such as the aggregates characteristic of mesh-structures in bacterial protein coats (described in Chapter 4), or the cholesteric liquid crystals found in... 

Cholesteryl esters (cholesterol esters). As a monofunctional secondary alcohol cholesterol is able to form esters. As lipids these esters are very important for the construction of cell membranes (see cholesterol). Esters of C. with aliphatic or aromatic carboxylic acids are also important as liquid crystals because of their special properties cholesteric phases). 

Helpful tools for this structurization of liquid crystal research were temperature dependent X-ray investigations [36] of natural and synthetic lipids, and the discovery that mesophases may be identified by their different textures appearing in the microscope using crossed polarizers [37]. In the decade starting in about 1957 systematic screening of the concentration and temperature dependency of the major lyotropic mesophases was done and models of the molecular arrangement in the different phases were developed [38-45] (e.g., the so-called middle or neat phases [38], the cholesteric phase of polypeptides and nucleopep-tides [44]). 

The distribution of defects in mesophases is often regular, owing to their fluidity, and this introduces pattern repeats. For instance, square polygonal fields are frequent in smectics and cholesteric liquids. Such repeats occur on different scales - at the level of structural units or even at the molecular level. Several types of amphiphilic mesophase can be considered as made of defects . In many examples the defect enters the architecture of a unit cell in a three-dimensional array and the mesophase forms a crystal of defects [119]. Such a situation is found in certain cubic phases in water-lipid systems [120] and in blue phases [121] (see Chap. XII of Vol. 2 of this Handbook). Several blue phases have been modeled as being cubic centred lattices of disclinations in a cholesteric matrix . Mobius disclinations are assumed to join in groups of 4x4 or 8x8, but in nematics or in large-pitch cholesterics such junctions between thin threads are unstable and correspond to brief steps in recombinations. An isotropic droplet or a Ginsburg decrease to zero of the order parameter probably stabilizes these junctions in blue phases. 

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