Double bonds in fatty acids

As a rule, the anabolic pathway by which a substance is made is not the reverse of the catabolic pathway by which the same substance is degraded. The two paths must differ in some respects for both to be energetically favorable. Thus, the y3-oxidation pathway for converting fatty acids into acetyl CoA and the biosynthesis of fatty acids from acetyl CoA are related but are not exact opposites. Differences include the identity of the acvl-group carrier, the stereochemistry of the / -hydroxyacyl reaction intermediate, and the identity of the redox coenzyme. FAD is used to introduce a double bond in jS-oxidalion, while NADPH is used to reduce the double bond in fatty-acid biosynthesis. 

Figure D2.1.2 Different types of conjugated and nonconjugated double bonds in fatty acids. (A) A polyunsaturated fatty acid (PUFA). (B) A PUFA nonhydroperoxide with a conjugated diene (CD). (C) A PUFA hydroperoxide with a CD. and R2 indicate the remaining alkyl portions of the PUFA. Reprinted from Corongiu and Banni (1994) with permission from Academic Press.

The information includes location of double bonds In fatty acids, indentlficatlon of components in complex lipids, determination of compositions of anionic and cationic surfactants, and identification of long-chained alkyl substituents on phosphonium and ammonium ions. 

Additional analytical methods are appropriate when a more complete characterization of the CLA isomers in biological samples is required. Most often, a combination of GC and silver ion-HPLC is used and permits excellent separation and identification of positional and geometrical isomers of CLA (see Adlof, 2003, and Kramer et al., 2004, for detailed reviews of this approach). In addition, the use of gas chromatography-mass spectrometry (GC-MS) has become increasingly popular and represents a very powerful technique for identification of the position of double bonds in fatty acids (see Dobson, 2003), and the orientation of those bonds in CLA isomers (Michaud et al., 2003). 

In nature almost all double bonds in fatty acids are ci s-bonds. During hydrogenation another kind of double bond is formed, the trans-bond. From a medical point of view the tram-fatty acids are seriously questioned [4],... 

Ozone adds directly to double bonds in fatty acids to form ozonides (183-185). These decompose to lipid alkoxyl and peroxyl radicals that abstract hydrogens to initiate radical chains (186). In the process, internal rearrangements within the original lipid molecule(s) yield hydroxy epoxides and hydroxy epidioxides with 1,3- and 1,4-cyclic hydroperoxides ... 

As the number of carbon-carbon double bonds in fatty acids increases, what is the effect on the melting points ... 

Linoleic acid is an unsaturated fatty acid that is an essential fatty acid in mammals because they cannot synthesize double bonds in fatty acids beyond position 9. Linoleic acid and linolenic acid are thus essential in mammalian diets, since they have double bonds beyond position 9 (at positions 9,12 and at positions 9,12, and 15 for linoleic and linolenic acid, respectively). 

Mammals cannot synthesize double bonds in fatty acids beyond the ninth carbon, so linoleic acid (double bonds at carbons 9 and 12) and linolenic Acid (double bonds at carbons 9,12, and 15) must be provided in mammalian diets. 

Fig. 5.19. Cis and trans double bonds in fatty acid side chains. Note that the cis double bond causes the chain to bend.

Gas chromatography linked to FTIR spectroscopy is an inunensely powerful technique for the determining the cis/trans configuration of double bonds in fatty acids [reviewed elsewhere (22,23)]. In the context of CLA, it enables the identification of cis,trans-, cis,cis- and trans,trans-isomers. Although it does not distinguish between cis,trans- and trans,cis-isomers, assignments can be made with reasonable certainty when GC retention data are taken into account. Unfortunately, few analysts have access to such a facility. 

Yu, Q.T., Liu, B.N., Zhang, J.Y., and Huang, Z.H. (1989) Location of Double Bonds in Fatty Acids of Fish Oil and Rat Testis Lipids. Gas Chromatography-Mass Spectrometry of the Oxazoline Derivatives, Lipids 24, 7983. 

C, fatty acid chain length db, number of double bonds in fatty acid chain. 

It should be recognized that different cell types possess different types of lipids (classes, subclasses, and individual molecular species). For example, plants lack plas-malogen species cholesterol is also unique to mammalian mammalian does not possess the machinery to synthesize essential fatty acids and the numbers and locations of double bonds in fatty acid species in eukaryotes are not randomly present. This nature of lipids in biological samples can be used as a criterion for data quality control. Violation of such a rule indicates something wrong in MS identification of lipid species. 

Mercury adducts can also be reacted with sodium borohydride and converted to methoxy compounds (oxymercuration-demercuration) [321,613], or reacted with halogens in methanol to form methoxyhalogen compounds [611], Such derivatives have been utilised for locating double bonds in fatty acids by mass spectrometry. 

Mass spectrometry of epoxy derivatives has been used to locate the position of double bonds in fatty acids, and as cis- and trans-isomers can be separated by GC, as a means of estimating fatty acids with frans-double bonds (see Chapter 5) [243,244],... 

Aldehydes are more easily identified than are the parent compounds, since a wide range of standards is available from commercial sources or can be prepared synthetically from other lipids. As an example of the full application of this methodology, more than 30 different bases were detected in the sphingolipids of bovine kidney [469]. Mass spectrometry can be utilised as an aid to identification of aldehydes (see also Section B above), although some workers have preferred to reduce them to fatty alcohols and then to prepare acetate or TMS ether derivatives for this purpose [624]. In addition, all the methods for the location of double bonds in fatty acids, such as ozonolysis or hydroxylation with osmium tetroxide and preparation of TMS ethers for MS, have been utilised with aldehydes prepared from sphingoid bases [464,465]. 

Fatty acids that are required by the body but cannot be made in sufficient quantity from other substrates, therefore must be obtained from food and are called essential fatty acids. Essential fatty acids are polyunsaturated fatty acids and are the parent compounds of the omega-6 and omega-3 fatty acid series, respectively. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, Two fatty acids are essential in humans, linoleic acid (LA) and alpha-linolenic acid (ALA). They are widely distributed in plant oils. In addition, fish, flax, and hemp oils contain the longer-chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). 

Modeling of alkyd resin production is a rather formidable task because of the high number of distinguishable chemical groups, the branched structure of the polymer, a nonnegligible amount of intramolecular reaction, and side reactions of the double bonds in fatty acids. The usual problems found in previously discussed polyesterifications, namely lack of data for liquid-vapor and Uquid-sohd equilibria and associated mass transfers, are also present. 

N.M.R. spectroscopy Widely used for lipid structure determination particularly identification and location of double bonds in fatty acids, functional groups (e.g. hydroxyl) on fatty acids and preliminary identification of glycerides, glycolipids and phospholipids... 

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