Lipid-containing solvent

The lipid is dissolved in a solvent which evaporates easily and is not miscible with water (usually chloroform, CHCI3). After the hydrophilic solid substrate has been moved into the pure water subphase, drops of the lipid-containing solvent are set carefully onto the water surface between the movable barriers by a syringe ( spreading ). After solvent evaporation the monolayer is compressed to the desired pressure (usually some 20-40 mN/m, in the LC phase). 

Hydrophobic substances are soluble in nonpolar solvents, whereas their solubiUty in water is very limited. Many of these substances are also soluble in fats and Hpids and are also called hpophile compounds. Such substances have a tendency to avoid contact with water and to associate with a nonpolar, nonaqueous environment, such as a surface, eg, an organic particle, a particle containing organic material, or the lipid-containing biomass of an organism. 

From your previous studies of solubility, you might hypothesize that lipid molecules must have large hydrocarbon parts, since they are soluble in hydrocarbon solvents. Your hypothesis would be correct. Lipids contain large hydrocarbon chains, or other large hydrocarbon structures. 

Lipids are a class of biomolecules defined by the fact that they are insoluble in water and similar solvents. Lipids include the fats in foods and the fats stored in our bodies, waxes, and steroids. Importantly for the body, the membranes that surround all cells are made of lipids. Like the carbohydrates starch and glycogen, lipids also serve as energy storage compounds, but per gram, lipids contain more than twice as much energy as carbohydrates and proteins. Excess nutrients not needed for energy are stored as body fat. 

Most of the one-pot syntheses of organically functionalized mesoporous silicates have been done under basic conditions. Only hexagonal phases (2d, p6m) were reported so far, except one very recent example of a phenyl-functionalized cubic phase [17], analogous to the bicontinuous MCM-48 phase (la3d) [8]. The cubic phase prepared under acidic conditions from PTES and TEOS is indeed related to a different type of cubic mesophases, micellar mesophases, reported in the literature for various surfactant/solvent systems [23] as well as for lipid-containing systems [24]. 

Saponification. Before solvent is added for extraction, saponification (alkaline hydrolysis) is a step used in most extractions of tocopherols and tocotrienols. It should be noted that acetate forms of tocopherols or tocotrienols in a sample are changed to free tocopherols and tocotrienols after saponification. This process breaks down the ester bonds of lipids and sample matrices as well. In most extraction procedures, a 60% to 80% (w/v) aqueous solution of KOH is used to perform the saponification. The volume of KOH required varies according to the amount of lipid contained in the sample. Also, ethanol is needed to stabilize the saponified solution and prevent the precipitation of soap material. Usually, the ratio of KOH, ethanol, and fat (in sample) during saponification is 3 (g) 15 (ml) 1 (g), respectively (Ball, 1988). The ratio may need to be adjusted based on the types of fats in the sample. Although ethanol concentration has no effect on the extraction of a-tocopherol by hexane, a concentration above 30% may cause lower recoveries of other tocopherols (Ueda and Igarashi, 1990). For most food samples, saponification for 30 min at70°C is sufficient. 

We conclude the discussion of the organic molecules in biological systems by turning our attention to lipids, biomolecules that are soluble in organic solvents. Unlike the carbohydrates in Chapter 27 and the amino acids and proteins in Chapter 28, lipids contain many carbon-carbon and carbon-hydrogen bonds and few functional groups. 

The term lipid or lipoid is used to describe a wide variety of natural origin chemical substances. All lipids contain mostly non-polar groups. This feature causes the most obvious common property of these molecules, which is the similar behavior to some organic solvents (i.e. chloroform, diethylether, hexane etc)... 

Figure 3 Structural polymorphism of lipid assemblies, (a) Lipid head group and alkyl tails have similar cross-sectional areas (cylinder), (b) Cross-sectional area of the alkyl tail region is less than that of the head group (cone), (c) Cross-sectional area of the head group is less than that of the tail, (d) Cross section of a planar bilayer in the L

The spectra upon membrane binding still contained residual sharp spectral components allocated to unbound ASYN. The spectra can be corrected for this component by subtraction of the spectrum of the free label. Analyzing the resulting spectra originating exclusively from membrane bound ASYN labeled within the repeat region exhibited line shapes indicating lipid- or solvent-exposed helix surface sites. 

To make these membranes, a suitable phospholipid, lipid or mixture of lipids is dissolved in an organic solvent (say n-decane) the mixture is gently brushed across a circular orifice in a piece of machined teflon, which itself forms a partition between the two sides of a chamber filled with suitable electrolyte (say 0.1 M NaCl). The diameter of the orifice is usually 1 or 2 mm, and over a few seconds the lipid-containing solution thins down, the excess lipid remaining as a torus around the hole, until a black lipid bilayer remains covering the orifice and separating two salt solutions. 

According to Oomah et al. (1996), extraction of residual oil from commercially available flaxseed meal with hexane alone resulted in a higher oil recovery than methanol/hexane or methanol-ammonia-water/hexane. However, it should be noted that their starting material was meal and not seeds of flax. Analysis of the extracted lipids from the meal showed (Table 3) that hexane preferentially extracted the neutral lipids primarily composed of triacylglycerols (TAG). Neutral lipid fraction of oil extracted with methanol/hexane solvent system contained more monoacylglycerols (MAG), diacylglycerols (DAG) and free fatty acids (FFA) than the lipids extracted by other solvent systems (Table 3). Methanol/hexane solvent system extracted more polar lipids than methanol-water-am-monia/hexane system and this lipid fraction was composed mainly of phosphotidylcholine (PC). Polar lipids of methanol-ammonia-water/hexane extracted lipids contained also phosphotidylethanolamine (PE), lysophosphatidylcholine (LPC) and some unidentified compounds. 

Technically important Fatty Acid Derivatives. Nitrogen-containing lipids such as primary, secondary and tertiary amines, quaternary ammonium bases, amides and nitriles, all of which find many varied applications in the plastic and textile industries, can be separated into classes on silica gel G by using ammonia-containing solvents (see Table 68) [19, 121, 162]. The first four solvents in Table 68 have been used in succession in the stepwise development technique (see p. 87) for analysing complex mixtures [121]. 

Solvents. Table 71 contains solvent mixtures suitable for fractionating the most important lipid classes. Besides these solvents, the following have also been often used the methyl esters of unsaturated, positionally isomeric fatty acids have been separated from one another by triple development at —15° C using toluene [142] triglycerides have been fractionated with chloroform-carbon tetrachloride-methanol-acetic acid (50 -I- 50 + 1.5 -1- 0.5) [6, 7]. The separation of lecithins on silica gel H, impregnated with silver nitrate [5], is very satisfactory using chloroform-methanol-water (65 -1- 25 + 4) [5] (see also [57, 88]). 

Mercuric acetate adducts of lipids containing more than three double bonds are fractionated on silica gel G-kieselguhr G (30 + 70), using isobutanol-formic acid-water (100 + 0.5+ 15.7) [211, 212]. The solvent should be placed in the chamber 5 hours before chromatography in order to saturate the atmosphere it migrates about 16—17 cm in ca. 5 h. 

Lipids are a varied group of organic compounds that share one property they are not very soluble in water. Lipids contain a high proportion of C—H bonds, and they dissolve in nonpolar organic solvents, such as ether, chloroform, and benzene. 

One of the simplest and most efficient approaches for aroma isolation is direct solvent extraction. The major limitation of this method is that it is most useful on foods that do not contain any lipids. If the food contains lipids, the lipids will also be extracted along with the aroma constituents, and they must be separated from each other prior to further analysis. Aroma constituents can be separated from fat-containing solvent extracts via techniques such as molecular distillation, steam distillation, and dynamic headspace. 

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