Unsaturated fatty acid Isomerization

Positionalisomeri tion occurs most often duting partial hydrogenation of unsaturated fatty acids it also occurs ia strongly basic or acidic solution and by catalysis with metal hydrides or organometaUic carbonyl complexes. Concentrated sulfuric or 70% perchloric acid treatment of oleic acid at 85°C produces y-stearolactone from a series of double-bond isomerizations, hydration, and dehydration steps (57). 

Clay-catalyzed dimerization of unsaturated fatty acids appears to be a carbonium ion reaction, based on the observed double bond isomerization, acid catalysis, chain branching, and hydrogen transfer (8,9,11). 

The carbon chains of samrated fatty acids form a zigzag pattern when extended, as at low temperamres. At higher temperatures, some bonds rotate, causing chain shortening, which explains why biomembranes become thinner with increases in temperamre. A type of geometric isomerism occurs in unsaturated fatty acids, depending on the orientation of atoms or groups around the axes of double bonds, which do not allow rotation. If the acyl chains are on the same side of the bond, it is cis-, as in oleic acid if on opposite sides, it is tram-, as in elaidic acid, the tram isomer of oleic acid (Fig-... 

Lipids from marine products have been studied less frequently. The detection of co-(o-alkylphenyl)alkanoic acids with 16,18 and 20 carbon atoms together with isoprenoid fatty acids (4,8,12-trimethyltetradecanoic acid and phytanic acid) and substantial quantities of bones from fish and molluscs has provided evidence for the processing of marine animal products in vessels [58 60]. C16, C18, and C20 co-(o-alkylphenyl)alkanoic acids are presumed to be formed during the heating of tri-unsaturated fatty acids (C16 3, C18 3 and C20 3), fatty acyl components of marine lipids, involving alkali isomerization, pericyclic (intermolecular Diels-Alder reaction) and aromatization reactions. 

It should be noted that Reaction (4) is not a one-stage process.) Both free radical N02 and highly reactive peroxynitrite are the initiators of lipid peroxidation although the elementary stages of initiation by these compounds are not fully understood. (Crow et al. [45] suggested that trans-ONOO is protonated into trans peroxynitrous acid, which is isomerized into the unstable cis form. The latter is easily decomposed to form hydroxyl radical.) Another possible mechanism of prooxidant activity of nitric oxide is the modification of unsaturated fatty acids and lipids through the formation of active nitrated lipid derivatives. 

First, most naturally occurring unsaturated fatty acids have double bonds in the cis isomeric configuration. .. 

Trans-unsaturated fatty acid from its CIS- and trans-isomeric mixture Na-ZSM-5 [182]... 

The unsaturated fatty acids exhibit geometric isomerism, i.e. there are cis and trans forms (Figure 11.10). The chemical basis for isomerism is discussed in Chapter 3. (Appendix 3.1). 

In lipid metabolism, ds-trans isomerism is particularly important. For example, double bonds in natural fatty acids (see p.48) usually have a as configuration. By contrast, unsaturated intermediates of p oxidation have a trans configuration. This makes the breakdown of unsaturated fatty acids more complicated (see p. 166). Light-induced cis-trans isomerization of retinal is of central importance in the visual cycle (see p.358). 

Unsaturated fatty acids usually contain a cis double bond at position 9 or 12—e.g., linoleic acid (18 2 9,12). As with saturated fatty acids, degradation in this case occurs via p-oxida-tion until the C-9-ds double bond is reached. Since enoyl-CoA hydratase only accepts substrates with trans double bonds, the corresponding enoyl-CoA is converted by an iso-merase from the ds-A, cis- A isomer into the trans-A, cis-A isomer [1]. Degradation by p-oxidation can now continue until a shortened trans-A, ds-A derivative occurs in the next cycle. This cannot be isomerized in the same way as before, and instead is reduced in an NADPH-dependent way to the trans-A compound [2]. After rearrangement by enoyl-CoA isomerase [1 ], degradation can finally be completed via normal p-oxidation. 

In a similar manner, many additions of heteroatom radicals to unsaturated positions have been studied. In many cases, addition reactions of heteroatom radicals to alkenes are reversible and thermodynamically disfavored, but their occurrence is apparent. For example, the rapid addition and elimination of thiyl radicals to unsaturated fatty acid methyl esters results in isomerization reactions from which kinetic parameters can be obtained. Additions of group 14 (IV A) metal-centered... 

Many of the compounds derived from enzyme-catalysed oxidative breakdown of unsaturated fatty acids may also be produced by autoxidation [23]. While the enzymatically produced hydroperoxides in most cases yield one hydroperoxide as the dominant product, non-enzymatic oxidation of unsaturated fatty acids yields a mixture of hydroperoxides which differ in the position of the peroxide group and in the geometrical isomerism of the double bonds [24]. As the number of double bonds increases, the number of oxidation and oxygen-addition sites increases proportionally and thus the number of possible volatile... 

Oxidation of unsaturated fatty acids requires two additional enzymes enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. Odd-number fatty acids are oxidized by the /3-oxidation pathway to yield acetyl-CoA and a molecule of propionyl-CoA This is carboxylated to methylmalonyl-CoA, which is isomerized to succinyl-CoA in a reaction catalyzed by methylmalonyl-CoA mutase, an enzyme requiring coenzyme B12. 

Smith, L. M., Dunkley, W. L., Franke, A. and Dairiki, T. 1978. Measurement of trans and other isomeric unsaturated fatty acids in butter and margarine. J. Am. Oil Chem. Soc. 55, 257-261. 

The hydroperoxides formed in the autoxidation of unsaturated fatty acids are unstable and readily decompose. The main products of hydroperoxide decomposition are saturated and unsaturated aldehydes. The mechanism suggested for the formation of aldehydes involves cleavage of the isomeric hydroperoxide (I) to the alkoxyl radical (II), which undergoes carbon-to-carbon fission to form the aldehyde (III) (Frankel et al. 1961). 

A closely related E. coli protein is a 79-kDa multifunctional enzyme that catalyzes four different reactions of fatty acid oxidation (Chapter 17). The amino-terminal region contains the enoyl hydratase activity.32 A quite different enzyme catalyzes dehydration of thioesters of (3-hydroxyacids such as 3-hydroxydecanoyl-acyl carrier protein (see Eq. 21-2) to both form and isomerize enoyl-ACP derivatives during synthesis of unsaturated fatty acids by E. coli. Again, a glutamate side chain is the catalytic base but an imidazole group of histidine has also been implicated.33 This enzyme is inhibited irreversibly by the N-acetylcysteamine thioester of 3-decynoic acids (Eq. 13-8). This was one of the first enzyme-activated inhibitors to be studied.34... 

Radical X , which initiates the reaction, is regenerated in a chain propagation sequence that, at the same time, produces an organic peroxide. The latter can be cleaved to form two additional radicals, which can also react with the unsaturated fatty acids to set up the autocatalytic process. Isomerization, chain cleavages, and radical coupling reactions also occur, especially with polyunsaturated fatty acids. For example, reactive unsaturated aldehydes can be formed (Eq. 21-14). 

Unsaturated fatty acids such as oleoyl-CoA can also be degraded to acetyl-CoA in mitochondria. Three cycles of /3 oxidation result in an acyl-CoA (A3-cis-dodecenoyl-CoA) that is not a substrate for acyl-CoA dehydrogenase. This problem is circumvented by isomerization of the double bond to the t -tmns position by enoyl-CoA isomerase. Complete oxidation of the remainder of the molecule by the enzymes of f3 oxidation is now possible. 

As the name anaerobic implies, the double bond of the fatty acid is inserted in the absence of oxygen. Biosynthesis of monounsaturated fatty acids follows the pathway described previously for saturated fatty acids until the intermediate /3-hydroxydecanoyl-ACP is reached (fig. 18.15). At this point, a new enzyme, /3-hydroxydecanoyl-ACP dehydrase, becomes involved. This dehydrase can form the a-j8 trans double bond, and saturated fatty acid synthesis can occur as previously discussed. In addition, this dehydrase is capable of isomerization of the double bond to a cis /3-y double bond as shown in figure 18.15. The /3-y unsaturated fatty acyl-ACP is subsequently elongated by the normal enzymes of fatty acid synthesis to yield pal-mitoleoyl-ACP (16 1A9). The conversion of this compound to the major unsaturated fatty acid of E. coli, cA-vacccnic acid (18 1A11), requires a condensing enzyme that we have not previously discussed, /3-ketoacyl-ACP synthase II, which shows a preference for palmitoleoyl-ACP as a substrate. The subsequent conversion to vaccenyl-ACP is cata-... 

The hardening effect of fatty oil isomerization processes is limited by the fact that cis-trans conversion of the double bond of mono-unsaturated fatty acids is an equilibrium reaction in the case of oleic and elaidic acid esters the equilibrium mixture consists of 67% elaidic acid and 33% oleic acid this equilibrium ratio is practically independent of the isomerization temperature77. 

Typical examples are unsaturated fatty acids (Ferreri et al. 1999 Sprinz et al. 2000, 2001 Adhikari et al. 2001). The equilibrium constants for the oleic and linoleic systems are in the order of 10 dm3 mol1 and the reverse reaction in the order of 106 s"1 (Sprinz et al. 2000 and Sprinz, pers. comm.). In polyunsaturated fatty acids, such isomerizations could, in principle, also occur by an H-abstrac-tion/H-donation mechanism as discussed above. However, the rate of H-dona-tion of RSH to the pentadienylic radicals must be verylow (see above), and isomerization has been considered to occur only by the addition/elimination pathway (Sprinz et al. 2000). With the nucleobases, any thiyl addition can only be detected when the short-lived adduct is trapped by a fast reaction (Chap. 10.10). 

Adhikari S, Sprinz H, Brede 0 (2001) Thiyl radical induced isomerization of unsaturated fatty acids determination of equilibrium constants. Res Chem Intermed 27 549-559 Adhikary A, Bothe E, Jain V, von Sonntag C (2000) Pulse radiolysis of the DNA-binding bisbenzimid-azole derivatives Hoechst 33258 and 33342 in aqueous solution. Int J Radiat Biol 76 1157-1166 Akhlaq MS, von Sonntag C (1986) Free-radical-induced elimination of H2S from dithiothreitol. A chain reaction. J Am Chem Soc 108 3542-3544... 

Sprinz H, Schwinn J, Naumov S, Brede O (2000) Mechanism of thiyl radical-catalyzed isomerization of unsaturated fatty acids in homogeneous solution and in liposomes. Biochim Biophys Acta 1483 91-100... 

Although E double bonds are uncommon in unsaturated fatty acids of animals, many have been found among the unsaturated components of lepidopteran pheromones. Early investigations of pheromone biosynthetic pathways showed that E isomers are not produced by isomerization of the Z double bonds, but rather are produced directly by the catalytic activity of desaturases possessing... 

The production of a,m-diesters from fatty esters can be realized via their SM as already explained, but it can also be performed by CM with methyl acrylate. The bulk CM of several unsaturated fatty acid methyl esters containing double bonds in different positions with methyl acrylate was studied by Rybak and Meier (Scheme 6) [43], C4 and C5 displayed very good activities with high conversions and CM selectivities. Among them, C5 showed the best performance for both methyl oleate (97% conversion, 92% selectivity, with 0.2 mol%) and methyl 10-undecenoate (99% conversion, 99% selectivity, with 0.1 mol%). The same conditions were successfully applied to methyl erucate and methyl petroselinate. The reaction conditions were further optimized, also considering the effect of 1,4-benzoquinone as additive for the reduction of double-bond isomerization [39], The CM of methyl 10-undecenoate and methyl acrylate worked with full conversions and high selectivity if five- to tenfold excess of methyl acrylate is used. Furthermore, using a 1 1 ratio between both reactants led, after optimization of the reaction... 

The systematic names for these acids are given in parentheses the systematic terms are preferable to the trivial names when it is necessary to describe the geometric isomerism of an unsaturated fatty acid or the exact location of a substituent group. 

Inclusion protects unsaturated fatty acids from aging however, this protection is not perfect, as amylose itself also complexes oxygen753 756 (see Fig. 48). Unsaturated acids have a specific pattern of the carbon chain because at least two sp2-hybridized carbon atoms are present, causing geometrical isomerism. The frans-isomer does not fit perfectly into the cavity of amylose. Another report indicates that starch complexation with tannic acid has no effect on its activity against experimental ulcers in laboratory rats.757... 

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