A,/3-Diglycerides

Both the a,/3-diglyceride and the phosphatidic acid may be used for the synthesis of phospholipids. The choline and ethanolamine required respectively for the synthesis of lecithins and cephalins must be available in an active form as their cytidine diphosphate derivatives. 

These finding brought triglyceride synthesis in line with the synthesis of phospholipids, which was shown by Smith et al. (1957) to proceed by removal of the phosphate from phosphatidic acid, with the formation of an a )3-diglyceride. Diglyceride can now be converted into phospholipid, by cytidine-diphosphate-choline or ethanolamine (Kennedy and Weiss 1956) or into neutral triglycerides by the addition of one more fatty acid, derived from fatty acyl-coenzyme A. (Weiss and Kennedy 1956, 1960). 

Figure D1.6.3 TLC-FID separation of a range of standard lipids. The mobile phase was 91 6 3 1 (v/v/v/v) hexane/ethyl acetate/diethyl ether/formic acid. Time refers to scanning time of the Chromarod. Abbreviations CE, cholesterol ester CHO, cholesterol DG, 1,2-diglyceride 1,3-DG, 1,3-diglyceride FFAU, highly unsaturated free fatty acid FFAS, less unsaturated free fatty acid MG, 1-monoglyceride PL, phosphatidylcholine TGU, highly unsaturated triglyceride TGS, saturated triglyceride. Tripalmitin and palmitic acid were used to complement trilinolenin and linolenic acid. Reproduced from Ackman and Heras (1997) with permission from AOCS Press.

Draw the general structural formula of each of the following classes of compound (a) a (3-amino acid, (b) a nucleotide, (c) a plasmalogen, (d) an a-amino acid, (e) a tripeptide and (f) a diacylglycerol (diglyceride). 

Diacetyl Tartaric Acid Esters of Mono- and Diglycerides occur over a range in appearance from sticky, viscous liquids through a fatlike consistency to a waxy solid, depending on the iodine value of the oils or fats used in their manufacture. They are the reaction product of partial glycerides of edible oils, fats, or fat-forming fatty acids with diacetyl tartaric anhydride. The diacetyl tartaroyl esters are miscible in all proportions with oils and fats. They are soluble in most common fat solvents, in methanol, in acetone, and in ethyl acetate, but are insoluble in other alcohols, in acetic acid, and in water. They are dispersible in water and resistant to hydrolysis for moderate periods of time. The pH of a 3% dispersion in water is between 2 and 3. 

Monoglycerides can be made by treatment of the vinyl ester of a fatty acid with glycerol (without solvent) in 100% conversion.208 A lipase from PeniciUium roquefor-tii has been used to react unsaturated fatty acid vinyl esters with glycerol to give more than 95% selectivity to 1,3-diglycerides in 85% yield.209 Diols can be converted selectively to monoacetates.210 An example is given in reaction 9.21. 

The first diglyceride crystal structure to be determined contained heavy-atom-substituted fatty acids, the symmetric diglyceride of 3-thiododecanoic acid (Larsson, 1963). The structure is shown in Fig. 8.48. The most important feature is that the molecule is V-shaped so that the polar groups are arranged in a sheet between the hydrocarbon chain layers formed by the two acyl groups. The hydrogen-bond system links the molecules into infinite chains in one direction which are parallel with the needle direction of the crystals. The same type of crystal structure was also found in the 1,3-diglyceride of 11-bromoundecanoic acid (Hyble and Dorset, 1971). 

The determination of thermal data for diglycerides is seriously complicated by the rapid isomerization due to acyl migration. A few hours in the liquid state is enough for a pure diglyceride to form an equilibrium mixture of about equal amounts of the 1,2-and 1,3-isomers. 

The preparation of glycerol-based Ca-synthons and intermediates represents an interesting example where enzymatic transesterification is widely exploited. Alcoholysis of tributyrin with PPL was already shown to produce a chiral diglyceride (Fig. 3). Alcoholysis of trityl-protected dibutyrin with methanol in organic solvents shows some essential features about regio- and enantioselectivity in bifunctional compounds (Fig. 18) (25). Most lipases, such as lipases PS and AK and Novozym 435, first regioselectively produce the primary alcohol from the diester. With lipase AK, the reaction leads to effective kinetic resolution (A), whereas Novozym 435-catalyzed reaction (B) is not enantioselective. However,... 

In the biosynthesis of many of the glycerophosphatides, phosphatidic acid is first dephosphorylated. Smith, Weiss and Kennedy (1957) showed that liver contains an enzyme, phosphatidic acid phosphatase (L-a-phosphatidate phospho-hydrolase EC 3.1.3.4), which removes phosphate from phosphatidic acid with the formation of D-a, j8-diglyceride (II) ... 

Similar enzymes occur in many tissues. D-a,j8-diglyceride so formed may be converted into many of the glycerophosphatides (see below) or, as demonstrated by Weiss, Kennedy and Kiyasu (1960), it may be esterified by a diglyceride acyltransferase (Acyl-CoA 1,2-diglyceride 0-acyltransferase EC 2.3.1.d) to give triglyceride ... 

Thus CDP-choline is the precursor of three phosphatides (Figure 3). It may donate phosphorylcholine to D-a,/5-diglyceride with the formation of lecithin... 

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