Free fatty acids configuration

Total fatty acids were liberated by subjecting Salmonella minnesota Re lipopolysaccharide (or free lipid A) to acidic (4 N HC1, 5 h, 100°C) followed by alkaline (1 N NaOH, 1 h, 100°C) hydrolysis. After extraction (chloroform), the free fatty acids were converted into their methyl esters (diazomethane) and analysed by combined gas-liquid chromatography/mass spectrometry. Alternatively, the fatty acids of lipid A are transesterified by treatment of lipopolysaccharide with methanolic HC1 (2 N HC1 in water-free CHaOH, 18 h, 85°C). By these procedures the following fatty acids were identified (in approximate amounts relative to 2 moles glucosamine) dodecanoic (12 0, 1.1 mole), tetradecanoic (14 0, 0.8 mole), hexadecanoic (16 0, 0.9 mole), 2-hydroxytetradecanoic (2-OH-l4 0, 0.1 mole), and 3-hydroxytetradecanoic acid (3-OH-14 0, 4 moles). In total, therefore, approximately 7 moles of fatty acids are present per mole of lipid A backbone. The stereochemistry of the hydroxylated fatty acids was determined by gas-liquid chromatography of their diastereomeric methoxyacyl-L-phenylethylamide derivatives (24). It was found that 2-hydroxyte-tradecanoic acid possesses the-Ts), and the predominating 3-hydroxytetradecanoic acid the (R) configuration. 

The simplest and most efficient technique for obtaining 0)3 PUFA concentrates in the form of free fatty acids is urea complexation. This technique is well established for elimination of saturated and monounsaturated fatty acids (70). In this method, the saturated and monounsaturated fatty acids can easily complex with urea after hydrolysis of TAG with aUcaline, and crystallize out on cooling and may subsequently be removed by filtration (70). This method is favored by many researchers because complexation depends on the configuration of the fatty acid moieties because of the presence of multiple double bonds, rather than of pure physical properties such as melting point or solubility (10). 

The pathway for formation of DG from TG in the fat body is unknown. The lipolytic activity in fat body homogenates from L. migraloria (Tietz and Weintraub, 1978) and M. sexta (Arrese and Wells, 1992) converts TG primarily to free fatty acids (FFAs), whereas in the desert locust Schisto-cerca gregaria (Spencer and Candy, 1976) and the cockroach P. americana (Hoffman and Downer, 1979b) the end products were DG and FFA. Micro-somes from the L. migratoria fat body can acylate 2-MG to produce DG (Tietz et al., 1975). It has been shown that the DG released from the fat body has the 5n-l,2 configuration (Lok and Van der Horst, 1980 Tietz and Weintraub, 1980), therefore either a pathway involving de novo synthesis of DG via phosphatidic acid or the stereospecific hydrolysis of TG could be involved. 

Unlike synthetic elastomers such as polybutadiene with a high level of cis configuration or polychloroprene, natural rubber crystallizes very slightly at cold temperature without strain. Kawahara et a/. suggested that the presence of free fatty acids in natural rubber might be conducive to cold crystallization. 

Silver nitrate may be incorporated in the adsorbent slurry (25 g l-1) giving a final concentration of about 5% in the dry plate. The silver ions bind reversibly with the double bonds in the unsaturated compounds, resulting in selective retardation, and the lipids are separated according to the number and configuration (cis or trans) of their double bonds. This technique is extremely useful in fatty acids, mono-, di- and particularly triacylglycerol analyses when even positional isomers may be resolved. Borate ions may also be incorporated in the silica gel and these plates are used to separate compounds with adjacent free hydroxyl groups. 

The fatty acid free derivative, II, is usually optically active and strongly suggests that the two glycerophosphate residues have the same stereochemical configuration. Otherwise this derivative would have had a meso configuration (with no optical activity). The peroxidation of II will yield 2 mol of formaldehyde per mole of phosphorus together with a dialdehyde, III, which on reaction with dimethylhydrazine will yield glycerol-1,3-diphosphoric acid, IV. 

Silver-ion TLC is the modification with the most important impact on the development of lipid chemistry and has been of immense importance for the understanding of lipid structure. It is used to resolve the molecular species of a single lipid class. The separation is based on the ability of unsaturated fatty acid moieties in lipid molecules to form weak reversible charge-transfer complexes with silver ions. The complexation includes the formation of a a-type bond between the occupied 2p orbitals of the oleflnic double bond in the fatty acid (FA) moiety and the free 5s and 5p orbitals of the silver ion, and a (probably weaker) rr-acceptor backbond between the occupied 4d orbitals of the silver ion and the free antibonding Ipv orbitals of the olefinic bond. Thus, Ag TLC separates lipid classes into molecular types depending on the number, configuration, and, occasionally, the position of the double bond in the fatty acid moieties. 

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