Thermotropic cholesterol

Thermotropic Cholesterol-Containing Liquid Crystalline Polymers... 

T. E. (1978). Studies on anomalous thermotropic behavior of aqueous dispersions of dipalmitoyl phosphatidylcholine-cholesterol mixtures. Biochemistry. H, 1984-1989,... 

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

Thermotropic cholesterics were officially discovered2 in 1888 by the Austrian botanist Friederich Reinitzer, while studying the melting of cholesterol esters cholesteryl benzoate first melted to give a cloudy liquid that, at higher temperature, turned into an ordinary clear liquid. The cloudy liquid (the mesophase) was a thermotropic cholesteric liquid crystal. These phases... 

The so ealled a phase of the fatty alcohols is a thermotropic type smectic B liquid crystal with a hexagonal arrangement of the moleeules within the double layers. It is initially formed from the melt during the manufaeturing proeess and normally transformed into a erystalline modifleation on eooling. However, the erystallization of the gel matrix ean be avoided if the a phase ean be kept stable as it eools to room temperature. This ean be achieved by eombining appropriate surfaetants, such as myristyl or lauryl alcohol and cholesterol, a mixture of whieh forms a lamellar liquid... 

Thermotropic liquid crystals come in two types calamitic and discotic. Calamitic phases (from the Greek for tube ) are all those that are caused by rod-like mesogens. The more recently characterised discotic phases are caused by disc-like species. Calamitic phases may be either nematic (from the Greek for thread ), smectic (from the Greek for soap ) or cholesteric (named after the cholesterol derivatives such as 13.4, which exhibit this behaviour). 

The influence of cholesterol concentration at various temperatures on the thermotropic phase behavior and organization of saturated PE bilayers has been studied by combining DSC, FT-IR, and 31P-NMR. It was found that incorporation of low levels of cholesterol into the bilayer caused a progressive reduction in the temperature, enthalpy, and overall cooperativity of the lipid hydrocarbon chain melting transition... 

Figure 1.53. A Cgo-cholesterol conjugate, representing the first fullerene-containing thermotropic liquid crystal.

The occurrence of cholesterol and related sterols in the membranes of eukaryotic cells has prompted many investigations of the effect of cholesterol on the thermotropic phase behavior of phospholipids (see References 23-25). Studies using calorimetric and other physical techniques have established that cholesterol can have profound effects on the physical properties of phospholipid bilayers and plays an important role in controlling the fluidity of biological membranes. Cholesterol induces an intermediate state in phospholipid molecules with which it interacts and, thus, increases the fluidity of the hydrocarbon chains below and decreases the fluidity above the gel-to-liquid-crystalline phase transition temperature. The reader should consult some recent reviews for a more detailed treatment of cholesterol incorporation on the structure and organization of lipid bilayers (23-25). 

The effect of cholesterol on the thermotropic phase behavior of PC bilayer also varies significantly with the structure, particularly the degree of unsaturation, of the hydrocarbon chains, with more highly unsaturated PCs exhibiting a reduced miscibility with cholesterol and other sterols. Moreover, the structure of the lipid polar headgroup is also important in determining the effect of cholesterol on the host lipid, as is the structure of the sterol molecule itself. For more information on the application of DSC to the biologically important area of lipid-sterol interactions, the reader is referred to recent reviews (23-25). 

McMullen TPW, Lewis RNAH, McElhaney RN. Differential scan- 45. ning calorimetric study of the effect of cholesterol on the thermotropic phase behavior of a homologous series of linear saturated phosphatidylcholines. Biochemistry 1993 32 516-522. 

A remarkable property of lipid bilayers is their structural phase transitions (thermotropic polymorphism). For example, fully hydrated pure diacyl-phosphatidyl cholines exibit one fluid phase. La and three crystalline phases Pp/, Lp/. and Lc (12). Because of the high degree of disorder caused by defects, the Pp/ and Lp/ phases usually are called gel phases. The Pp/ phase is sometimes called a ripple phase, because the surface of the bilayer is rippled (13) and presents a wave-like appearance in electron micrographs (Fig. 2). Depending on the nature of the lipid and the presence of additional components (cholesterol etc.), the Pp/ phase may be present or absent in the phase diagram, and a tilted gel Lp/ could be replaced by the Lp phase, which has similar physical properties but no tilt of the hydrocarbon chains. 

Whether quasicrystalline structures are limited to alloys remains an open question. It is possible that their occurrence is much more widespread than had been previously thought. Indeed there is evidence for quasicrystallinity in both thermotropic and lyotropic liquid crystals. Diffraction patterns of decagonal symmetry have been recorded in lyotropic liquid crystals [K. Fontell, private communication], (Fig. 2.19), and there is theoretical evidence for the existence of a quasicrystalline structure within the blue phase of cholesterol (Chapters 4, 5). (The decagonal structure has quasisymmetry perpendicular to the tenfold axes, and translation symmetry along them.) Viruses crystallise in icosahedral clusters and the list continues to grow. In addition to five-fold symmetry, it has been shown that eight and ten- fold quasisymmetry is possible. ... 

The arrangement of chiral molecules in thermotropic liquid crystals is more complex, since entire volumes of space - rather than the bounded twisted ribbons discussed above - must be ed subject the constraint of a preferred twist between neighbouring molecules. The simplest examples of such mesophases are the cholesteric liquid crystals, discovered last century, (c/. section 5.1.8). This class of thermotropic liquid crystals derives its generic name from chiral cholesterol derivatives (shown below), which were found a century ago to exhibit peculiar optical changes as they were heated. 

A second category of liquid crystals is the type produced when certain substances, notably the esters of cholesterol, are heated. These systems are referred to as thermotropic liquid crystals and, although not formed by surfactants, their properties will be described here for purposes of comparison. The formation of a cloudy liquid when cholesteryl benzoate is heated to temperatures between 145 and 179°C was first noted in 1888 by the Austrian botanist Reinitzer. The name liquid crystal was applied to this cloudy intermediate phase because of the presence of areas with crystal-like molecular stmcture within this solution. 

Differential Scanning Calorimetiv has been used for studying the interaction of labdanes 1 (labd-13(E)-ene-8a,15-diol), 2 (labd-13(E)-cne-8a-oI-15-yl-acetate) and 3 (Iabd-14-ene-8,13-diol (Sclareol)) Fig. (12) [76-77] with DPPC model lipid bilayers. The thermotropic changes caused by the labdanes in DPPC bilayers could be used for evaluating the incoiporation efficiency of labdanes in DPPC liposomes. Furthermore in these studies [29, 76], a comparison with the structurally similar molecule to cholesterol was carried out based on their thermotropic behavior. The development of liposomal formulations incorporating 1 and 2 has been achieved, wliile the stability studies of the liposomal formulations based on ,-potential and on liposomal size have been evaluated. 

Polyisocyanides with cholesterol-containing pendant groups have been reported to display thermotropic liquid crystalline properties.223... 

The influence of the amount of cholesterol at various temperatures on the thermotropic phase behavior and organization of saturated phosphatidylethanolamine bilayers has been... 

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