Acylglycerol

Acylglycerols are the fatty acid esters of glycerol. Because every alcohol function of glycerol can be esterified, the molecules can contain one, two, or three fatty acids, termed mono-, di-or triacylglycerols, respectively. Characterization of an acylglycerol molecule requires not only the identification of its component fatty acids but also their positions in the glycerol molecule (positional isomer). 

The general procedure allowing the characterization of an acylglycerol consists of hydrolyzing the acylglycerol that has been purified previously by TLC or HPLC in order 

Electron ionization (25 eV) mass spectra of picolinic esters from Go-octadenoic acid and ante-isononadecanoic acid. The fragment ion at mfz 151 corresponds to a McLafferty rearrangement product. Reproduced (modified) from Harvey D.J., Biomed. Mass Spectrom., 9,33,1982, with permission. 

At present, mass spectrometry allows the analysis of an acylglycerol by identifying its various component fatty acids and their positions in the glycerol without necessitating prior chromatographic separation. The quantity of sample that is used during a mass spectrometric analysis is about 1 picomole. [288] 

Various ionization techniques, such as El, [289] Cl, [290] DCI, [291] PD, [292] FAB, [293] APCI [294] and ESI, [295] have been used successfully. Overall, the acylglycerol 

Triacylglycerols in cocoa butters have been separated by high performance liquid chromatography (HPLC) or gas chromatography (GC). These methods 

mass spectrometry offers an attractive alternative as a detector to HPLC. Newer techniques for linking HPLC systems with mass spectrometers directly via atmospheric pressure chemical ionization (APCI) and electrospray interfaces should see an expansion of this analytical tool in the analysis of confectionery fats, a field in which it has not yet been applied. Triacylglycerols 

With low polarity columns, separation is based on the number of carbon atoms, with unsaturation of the acyl substituent not detected. The degree of separation can be improved by the use of medium-polarity columns when the number of double bonds in the acyl molecule does have an effect on the separation. More recently there has been a move towards the use of longer capillary columns but even this does not give a very substantial improvement in separation of triacylglycerols than can be achieved by HPFC. 

The Padley and Timms (1980) method has been the subject of a collaborative trial which showed the method to be suitable for use by competent laboratories for the analysis of fats that did not contain added milk fat, hazelnut oil or the CBE Calvettta , which has a rather different triacylglycerol composition to other CBEs (FSA, 2001). 

The precision of the Padley and Timms (1980) and Young s (1984) procedures has been improved by the incorporation of data derived from sterol degradation product analysis which helps to identify the type of fat present and therefore narrows the range along the C54-C50 band in which the fat falls (Macarthur el al., 2000). These improvements to the method have been proposed for adoption into the Codex standard (Codex Alimentarius Commission, 2001). 

Included in this class of compounds are the mono-, di- and tri-acylglycerols in which the hydroxyl groups of the glycerol molecule 

Argentation HPLC using silver nitrate-loaded silica stationary phases has been used for the analysis of the positional isomers of triglycerides including 2-unsaturated-l,3-disaturated (SUS), 1-un-saturated-2,3-disaturated (USS), fully saturated (SSS) and fully unsaturated (UUU) isomers (Smith et al., 1980 Hammond, 1981). In these separations the silica was loaded with 10% silver nitrate and the mobile phases used were either benzene (Smith et al., 1980) or toluene (Hammond, 1981). The resolution of the lipid species was found to 

Some of the earUest bonded phase chromatographic separations of triacylglycerols used a stationary phase of hydroxyalkoxypropyl Sep-hadex under low pressure conditions in combination with mobile phases of either acetone-water-heptane (87 13 10) plus 1% pyridine to reduce acid hydrolysis (Curstedt and Sjovall, 1974) or a gradient system of 2-propanol-chloroform-hexane-water (115 2 15 35) and heptane-acetone-rwater (5 15 1) (Lindqvist et al., 1974). In these separations good resolution of Cg to C54 triglycerides was reported 

A series of alternative reversed phase systems have been used for the separation of individual triglyceride species on the basis of their chain length and degree of unsaturation. For example, the first conventional reversed phase HPLC separation utiUsed a mobile phase of 60% aqueous methanol (Pei et al., 1975). It should be noted that the substitution of a methanol-acetone mobile phase for an acetonitrile-acetone phase causes small changes in retention time, with the introduction of one double bond in a fatty acid being 

The term carbohydrate includes a wide range of molecules which in many cases are quite complex structures. In chemical terms a carbohydrate is either a polyhydroxy aldehyde, a polyhydroxy ketone or, alternatively, it is a compound that can be hydrolysed to such a structure. The smallest unit that cannot be hydrolysed any further is called a monosaccharide. Glucose (Fig. 11.4.1) is the most abundant monosaccharide known and is by far the most important. Other examples include fructose and mannose. A carbohydrate which has been hydrolysed to two monosaccharide units is cabled a disaccharide and the best known example is sucrose, which is composed of a unit of glucose covalently linked to a unit of fructose. The nomenclature system continues in a logical fashion until one can refer to an ohgosaccharide as being a chain composed of several monosaccharide units. Depending upon whether a monosaccharide is an aldehyde or a 

In the presence of a suitable catalyst, e. g. Ni, hydrogen can be added to the double bond of an acyl lipid. This heterogeneous catalytic hydrogenation occurs stereo selectively as a cis-addition. Catalyst-induced isomerization from an isolene-type fatty acid to a conjugated fatty acid occurs with fatty acids with several double bonds  

Since diene fatty acids form a more stable complex with a catalyst than do monoene fatty acids, the former are preferentially hydrogenated. Since nature is not an abundant source of the solid fats which are required in food processing, the partial and selective hydrogenation, just referred to, plays an important role in the industrial processing of fats and oils (cf. 14.4.2). 

To obtain polyunsaturated fatty acids, the double bonds are introduced by a stepwise process. A fundamental difference exists between mammals and plants. In the former, oleic acid synthesis is possible, and, also, additional double bonds can be inserted towards the carboxyl end of the fatty acid molecule. For example, y-hnolenic acid can be formed from the essential fatty acid linoleic acid and, also, arachidonic acid (Fig. 3.5) can be formed by chain elongation of y-linolenic acid. In a diet deficient in linoleic acid, oleic acid is dehydrogenated to isolinoleic acid and its derivatives (Fig. 3.5), but none of these acquire the physiological function of an essential acid such as linoleic acid. 

Plants can introduce double bonds into fatty acids in both directions towards the terminal CH3-group or towards the carboxyl end. Oleic acid (oleoyl-CoA ester or P-oleoyl-phosphatidylcholine) is thus dehydrogenated to linoleic and then to linolenic acid. In addition synthesis of the latter can be achieved by another pathway involving stepwise dehydrogenation of lauric acid with chain elongation reactions involving C2 units (Fig. 3.5). 

The biosynthetic precursors of unsaturated fatty acids are saturated fatty acids in an activated form (cf. a biochemistry textbook). These are aer-obiocally and stereospecifically dehydrogenated by dehydrogenase action in plant as well as animal tissues. A flavoprotein and ferredoxin are in- 

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Acylglycerols can be hydrolyzed by heating with acid or base or by treatment with lipases. Hydrolysis with alkali is called saponification and yields salts of free fatty acids and glycerol. This is how soap (a metal salt of an acid derived from fat) was made by our ancestors. One method used potassium hydroxide potash) leached from wood ashes to hydrolyze animal fat (mostly triacylglycerols). (The tendency of such soaps to be precipitated by Mg and Ca ions in hard water makes them less useful than modern detergents.) When the fatty acids esterified at the first and third carbons of glycerol are different, the sec-... 

Olid carbon is asymmetric. The various acylglycerols are normally soluble in benzene, chloroform, ether, and hot ethanol. Although triacylglycerols are insoluble in water, mono- and diacylglycerols readily form organized structures in water (discussed later), owing to the polarity of their free hydroxyl groups. 

M. Leclmer, C. Bauer-Plank and E. Lorbeer, Determination of acylglycerols in vegetable oil methyl esters by on-line noimal phase LC-GC , J. High Resolut. Chromatogr. 20 581-585 (1997). 

In some polysaccharides, the reducing terminal is linked, through a phosphoric diester linkage, to O-1 of a 2,3-di-6 -acylglycerol. This structural feature has been demonstrated for some capsular polysaccharides from E. coli and Neisseria species, - but is probably more common than that. Non-covalent linkage between the lipid part and the cell membrane may explain why extracellular polysaccharides often occur as capsules, and the high (apparent) molecular weight observed for these polysaccharides may be due to micelle formation in aqueous solution. 

Because they are uncharged, acylglycerols (glycerides), cholesterol, and cholesteryl esters are termed neutral lipids. 

The triacylglycerols (Figure 14—6) are esters of the tri-hydric alcohol glycerol and fatty acids. Mono- and di-acylglycerols wherein one or two fatty acids are esteri-fied with glycerol are also found in the tissues. These are of particular significance in the synthesis and hydrolysis of triacylglycerols. 

These compounds constimte as much as 10% of the phospholipids of brain and muscle. StmcmraUy, the plasmalogens resemble phosphatidylethanolamine but possess an ether link on the sn- carbon instead of the ester link found in acylglycerols. Typically, the alkyl radical is an unsamrated alcohol (Figure 14-10). In some instances, choline, serine, or inositol may be sub-stimted for ethanolamine. 

The membranes of the endoplasmic reticulum contain the enzyme system for acylglycerol synthesis, and the ribosomes are responsible for protein synthesis. 

Figure 24-1. Overview of acylglycerol biosynthesis. (PAF, platelet-activating factor.)...

The free fatty acids (FFA, nonesterified fatty acids, im-esterified fatty acids) arise in the plasma from hpolysis of triacylglycerol in adipose tissue or as a result of the action of hpoprotein hpase during uptake of plasma tri-acylglycerols into tissues. They are found in combination with albumin, a very effective solubilizer, in concentrations varying between 0.1 and 2.0 ieq/mL of plasma. Levels are low in the ftiUy fed condition and rise to 0.7-0.8 leq/mL in the starved state. In uncontrolled diabetes mellitus, the level may rise to as much as 2 Ieq/mL. 

Hypolipoproteinemias Abetaiipoproteinemia No chylomicrons, VLDL, or LDL are formed because of defect in the loading of apo B with lipid. Rare blood acylglycerols low intestine and liver accumulate acylglycerols. Intestinal malabsorption. Early death avoidable by administration of large doses of fat-soluble vitamins, particularly vitamin E. 

Besides water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins), fiber (for roughage), minerals (elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in small amounts for essential metabolic and physiologic functions). The polysaccharides, tri-acylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before they can be absorbed and utifized. 

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