Cholesterol benzoate

Figure D1.3.1 A typical HPLC chromatogram of cholesterol benzoate derived from dried egg yolk.

Methanol used for the HPLC measurement should be of HPLC grade with low absorbance at UV wavelengths below 230 nm. Dried-proc-essed foods usually include a relatively large amountof oxidized cholesterol analogs (Smith, 1996). The benzoate derivatives of these oxides do not separate from cholesterol benzoate by HPLC. Therefore, the GC measurement is more suitable for the determination of cholesterol in dried products with a high level of the oxidized cholesterol. 

For GLC and HPLC analysis, the extraction of total lipids from food requires overnight separation of the organic layer. If the sample volume is small, however, centrifugation of the extract may shorten the sample preparation time. Cholesterol derivatization requires <2 hr depending on the number of samples. The peak of TMS ether and cholesterol benzoate derivatives will be eluted within 10 min after the injection of the sample. 

Chemists who synthesized cholesterol benzoate about 100 years ago, a routine synthesis of a derivative of a known compound, had no way of knowing that they had opened a route to the creation of innumerable and various devices in which liquid crystals are used. This new state of a material was unexpectedly discovered in the course of studies which were narrowly focused at the preparation of various derivatives from the readily available natural compound cholesterol. Similarly, the epoch of modern chemotherapy originated with the discovery of sulfa drugs , which happened as an absolutely unexpected consequence in a broad investigation aimed at the synthesis of hundreds of most diversified derivatives of aromatic compounds, potentially useful as components of azo dyes. 

Many strategies have been followed to control levels of serum cholesterol. A compound that incorporates the carbon skeleton of cholesterol itself, colestolone, is thought to inhibit cholesterol synthesis in a late step in its biosynthesis by acting as a product feedback inhibitor. The synthesis starts with bromination of the allylic C position in cholesterol benzoate 22-1 by means of A-bromosuccinimide (22-2) (Scheme 8.22). Dehydrobromination, for example with collidine, leads to the endocyclic 5,7-diene 22-3. In the presence of strong acid, the bonds migrate to form the transoid... 

In 1888, Austrian botanist Friedrich Reinitzer (1857-1927) was synthesizing esters of cholesterol when he discovered a very curious phenomenon. Investigating the melting point of cholesterol benzoate, he discovered that at 293°F (145°C) it did not really melt but turned milky ... 

Many works deal with the variation of the helicoidal pitch in cholesteric phases as a function of temperature and composition. The best way to elucidate the origin of the twist seems to be to compare the pitch variations in lyotropic and in thermotropic systems. The first accurate work in this field is due to Robinson (1958-66) who studied PBLG (polybenzyl-L-glutamate) a synthetic polypeptide in organic solvents as dioxane, ethylic alcohol, chloroform etc. and Cano (1967) who made measurements of the pitches of nematic paraazoxyphenetol with different amounts of cholesterol benzoate. 

In 1888, the botanist Friedrich Reinitzer [9.5] was studying cholesterol in plants. He had the idea of observing a cholesterol benzoate crystal through the microscope as it underwent melting. At 145.5°C, the crystal became cloudy but fluid. It then transformed into a transparent liquid, very like water, at 178.5°C. The natural inclination of any physicist would have been to doubt the purity of the sample. Indeed, this had been the conclusion of those scientists who had previously made the same observation. Reinitzer, however, trusted in the quality of his compound. He therefore introduced the idea that melting could take place in two stages, and in this way opened up a new area of research the study of liquid crystals, intermediate states between liquids and solids. 

Details first observation by LeClerc Reinitzer described properties based on observations of cholesterol benzoate ... 

Even the first thermotropic liquid crystals (cholesterol benzoate and cholesterol acetate) were chiral molecules, and their chiral mesophases were observed [2]. Here, the chirality is taken from the chiral pool. Because liquid crystals are needed in gram scales for physical research and technical applications, they should be prepared by short synthetic pathways using easily available starting materials. Thus, synthetic strategies based on the chiral pool are more often used than asymmetric synthesis or chiral separation techniques. 

Cholesterol benzoate ( CB , Figure 4.1) [5] was the first chiral liquid crystal. Cholesterol chloride ( CC ), cholesterol nonanoate ( CN ), and cho-lesteryl oleyl carbonate ( COC ) have been reported in hundreds of papers. 

Figure 2. Spherical cholesteric germs of MBBA, plus a small amount of cholesterol benzoate. There is a disclina-tion similar to the T pattern shown in Fig. 25 and a spiral decoration in another spherulite. One polarizer only. Scale bar 20 pm.

Figure 34. Cholesteric textures (a-c) MBBA plus Canada balsam (d) MBBA plus cholesterol benzoate, (a, b) Two views at the coverslip and sUde levels of a fan texture, with lozenges associating two +w and two -1C disclina-tions, linked by pairs of focal lines, (c) Planar domains separated by walls of nearly vertical layers, (d) One of the walls is associated with horizontal and vertical focal lines, appearing as intercalated black spots, (a, b) Crossed polarizers (c, d) natural light. Scale bars (a, b) 20 pm ...

Figure 36. A cholesteric texture simulating the presence of focal parabolae, (a, b) Optical sections at the coverslip and slide levels. MBBA plus cholesterol benzoate, crossed polarizers. Scale bar 20 pm.

Spherulites showing concentric layers present a disclination radius or diameter, but this structure is due to a topological constraint and does not seem to be linked to liquid crystal growth. Very rapid growth of cholesteric phases often generates screw dislocations of the two types shown in Fig. 24i and j, and this has been filmed by Rault in p-azoxyanisol added to cholesterol benzoate [98, 99]. Slow growth does not result in the production of these defects. 

Traditionally, the discovery of liquid crystals has been attributed to Friedrich Reinitzer in 1888. Reinitzer was a scientist at the German University of Prague in the Institute of Plant Physiology while studying the properties of cholesterol compounds, he found that cholesterol benzoate appeared to have two different melting points. Another scientist of the time, Otto Lehman, a crystallographer at the University of Aachen, first proposed the liquid crystal state to be a distinct new state of matter. ... 

FIGURE 2.9 Cholesterol derivatives were the first liquid crystals investigated. Cholesterol benzoate shown here is a chiral molecule and forms a cholesteric phase. 

The best known of the chiral liquid crystal phases is the cholesteric phase or chiral nematic (N ). Here an asterisk is used to indicate a chiral phase. The cholesteric phase (Figure 2.10) was the first liquid crystal to be discovered by Reinitzer in 1888. Reinitzer observed pure cholesterol benzoate under the microscope and noticed two apparent melting points solid crystal to the phase that is now known as cholesteric and then a second melting point to the isotropic liquid phase. Cholesterol is a chiral molecule. 

The first observation of a liquid crystalline phenomenon was made in 1888 by the Austrian botanist Reinitzer, in Graz. He noticed an opaque appearance in Uquid cholesterol benzoate upon melting which turned into the common transparent Uquid state when further increasing temperature. Only decades later it was realized that liquid crystals are not suspended solid-state microcrystals but represent a novel class of materials exhibiting long-range molecular order within their Uquid... 

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