Fatty acids trans isomers

Gas chromatography is the most accurate means of quantifying the trans isomers present in fatty acid mixtures. In the GC trace the trans peaks appear broader because they contain isomers whose complete resolution is difficult. Virgin olive oils may show, in the sum of the trans-oleic acid-isomers, a maximum value of 0.05% and in the sum of /ran.v-linoleic and linolenic acid the same maximum value of 0.05%. Lower grade olive oils and refined olive oils are allowed wider limits for fatty acid trans isomers (see Table 2.2). 

Fatty acid triesters of glycerol containing the unnatural trans isomers of fatty acids. Trans fats are often formed as by-products in the partial hydrogenation of vegetable oils to produce margarine and vegetable shortening, (p. 1205)... 

During the course of transformation of the unsaturated fatty acids to saturated fatty acids, positional isomers of unsaturated fatty acids are produced. These unsaturated fatty acid isomers are known as trans-fatty acids (62), as shown in Figure 6. 

Mossoba, M.M., R.E. McDonald, J.A.G. Roach, D.D. Fingerhut, M.E Yurawecz, and N. Sehat. Spectral Confirmation of Trans Monounsaturated Fatty Acid Positional Isomers.74-. 125-130 (1997). 

Cis and trans isomers of dietary fats and oils are suspected to play a role in human health. All unprocessed fats and oils contain only cis isomers. During processing, some of the cis isomers are converted to trans isomers, called trans fatty acids. Trans fatty acids are suspected of raising the amount of serum cholesterol in the blood stream, which can cause health problems. 

In addition to geometrical isomerism (cis and trans), unsaturated fatty acids also exhibit positional isomerism, where the double bonds can occur in different positions along the chain in fatty acids which have identical chemical formulae. As with cis fatty acids, trans fatty acids also occur as mixtures of positional isomers. 

Valenzuela A, Morgado N Trans fatty acid isomers in human health and the food industry. Biol Res 1999 32 273. 

On the other hand, isomerization of sil-trans P-carotene was found to be comparatively faster in a model containing methyl fatty acid and chlorophyll heated at 60°C (Table 4.2.6), resulting in 13-cw-P-carotene as the predominant isomer. The first-order degradation rate of P-carotene significantly decreased with the increased number of double bonds in the methyl fatty acid, probably due to competition for molecular oxygen between P-carotene and the fatty acid. Since the systems were maintained in the dark, although in the presence of air, the addition of chlorophyll should not catalyze the isomerization reaction. 

Tran. -isomers are much rarer than cis-isomers. Many different positional isomers of monoenoic acids may be present in a single, natural lipid and this is not a comprehensive list. Palmitoleic and oleic acids are quantitatively the commonest unsaturated fatty acids in most organisms. Odd-chain monoenoic acids are minor components of animal lipids but are more significant in some fish and bacterial lipids. 

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. 

Argentation thin-layer chromatography is an extemely useful procedure for the separation of methyl esters of fatty acids. Saturated fatty acids have the highest Rf values, which decrease with the increasing degree of unsaturation, and for a particular acid, the trans isomer usually travels ahead of its corresponding cis isomer. The solvents most commonly used contain hexane and diethyl ether (9 1) although a mixture of 4 6 is used to separate compounds with more than two double bonds. In order to separate positional isomers of the same acid, conditions must be carefully controlled and multiple development in toluene at low temperatures is often necessary. 

It is usual to convert fatty acids to their methyl ester derivatives before separation by GLC, although it may be possible to analyse those with short chain lengths (two to eight carbon atoms) as the free fatty acids. Polar or non-polar stationary phases can be used and capillary (open-tubular) or SCOT columns will separate positional and geometric isomers. The cis isomers have shorter retention times than the corresponding trans isomers on a non-polar phase and visa versa on a polar phase. 

PERCENTAGE CHANGES OF ALL-TRANS CAROTENE AND ITS CIS ISOMERS IN THE PRESENCE OF CHL A AND FATTY ACID ESTERS, AND THE PERCENTAGE CHANGES OF CHL A AND ITS ISOMERS IN THE PRESENCE OF /1-CAROTENE AND FATTY ACID ESTERS DURING ILLUMINATION FOR VARIED LENGTHS OF TIMEa... 

Some of the unsaturated fats ingested by ruminants are partially hydrogenated by bacteria in the rumen. In consequence, milk fat, dairy products, as well as beef and mutton fat, also contain small amounts of trans isomers, about 2 to 9%. However, in fat from ruminants the main trans fatty acid is vaccenic (18 1 t 11), while in hydrogenated fats it is elaidic (18 1 t 9) (Figure 13.14). 

Figure 13.13 Fatty acid isomers in partially hydrogenated rapeseed oil (a) all fatty acids, (b) C18 l cis fatty acids, (c) C18 l trans fatty acids, (d) structure of C18 l A 5 trans fatty acid, and (e) structure of C18 l A15 trans fatty acid. (Courtesy of A. Stolyhwo.)...

Figure 13.14 Trans isomers of C18 l fatty acids of (a) partially hydrogenated rapeseed oil and (b) milk fat. (Courtesy of A. Stolyhwo.)...

Note that the term trans fats refers to fats containing trans fatty acids. At a couple of points earlier, 1 emphasized that aU the common unsaturated fatty acids possess cis carbon-carbon double bonds. It is true that some common foods—beef and dairy products—contain very small amounts of trans fatty acids but it is also true that the vast preponderance of dietary trans fats come from processed foods. Here is one example of a normal cis fatty acid—oleic acid—and its trans isomer ... 

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. 

Upcn hydrogenation, fish oils, especially the longer chain fatty acids (C22 s), form many new positional and geometric isomers. In some cases, partially hydrogenated fish oils may contain as many as 50% of the docosenoic acids in the trans form as opposed to the naturally occurring cis form. It is these trans isomers which may be responsible for the lipidosis of heart and skeletal muscle found in monkeys. Epidemiological studies of populations with high intakes of docosenoic acids. 

The presence of carbon-carbon double bonds leads to the possibility of isomers. Double bonds are rather restrictive and limit molecular movement. Groups on the same side of the double bond tend to remain in that position (cis), while groups on opposite sides tend to remain across the bond from each other (trans). You can see an example of each in Figure 1-1. However, if the two groups attached to either of the carbon atoms of the double bond are the same, cis-trans isomers are not possible. Cis isomers are the normal form of fatty acids, but processing tends to convert some of the cis isomers to the trans isomers. 

The first step in the biohydrogenation of linoleic acid to stearic acid by the mmen microorganism, Butyrivibrio fibrisolvens, is the formation of the cis-9, trans-11 CLA isomer (Figure 1). This reaction is catalyzed by a membrane-bound enzyme, linoleate isomerase, which acts only on fatty acids possessing cis double bonds in positions and and a free carboxyl group (72). 

Unsaturated fatty acids may occur as cis or trans isomers trans isomers, which have higher melting points than the corresponding cis isomers, are considered to be nutritionally undesirable. Bovine milk fat contains a low level (5%) of trans fatty acids in comparison with chemically hydrogenated (hardened) vegetable oils, in which the value may be 50% due to non-stereospecific hydrogenation. 

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