Fatty acid double bond, configuration

The specific behaviour of unsaturated fatty acids under oxidation is determined by the position and the number of double bonds in the fatty acid molecule. The stepwise oxidation of an unsaturated acid to the position of a double bond in it proceeds in a manner similar to that of saturated acid oxidation. If the double bond retains the same configuration (trans-configuration) and position (A2,3) as those of the enoyl-CoA, which is produced during the oxidation of saturated fatty acids, the subsequent oxidation proceeds via conventional route. Otherwise, the oxidation reaction proceeds with the involvement of an accessory enzyme, A3,4-CiS-A2,3jrans-enoyl-CoA isomerase this facilitates the translocation of the double bond to an appropriate position and alters the double-bond configuration from cis to trans. 

On the other hand, BF3 (see Basic Protocol 1), as well as other acidic catalysts, will change the double-bond configuration of fatty acids that contain conjugated dienes. As research on conjugated linoleic acid (CLA) and other conjugated fatty acids becomes more popular, it is essential not to provide misinformation about compositional analysis due to improper application of a methylation protocol (Li and Watkins, 1998). The basic catalysts perform better on lipids rich in fatty acids with unique conjugated diene structures. Isomerization and artifacts are not produced when sodium methoxide or TMG are used as transesterification agents... 

Standard Solutions Run through the chromatograph a commercially available standard containing a mixture of fatty-acid methyl esters. Fatty acids and methyl esters with a wide range of carbon numbers and double-bond configurations can be purchased. The calculated concentration should compare to that claimed within 2(7, where cr is the standard deviation calculated from at least 10 replicate determinations, preferably made over a period of several days. 

Fig. (2). double bond configuration in fatty acid chains, (a) stearic acid (Cis-o) saturated (b) oleic acid (C jjin-9) monounsaturated containing one cisi-double bond, (c) elaidic acid (Cis i) monounsaturated containing one trans-doubie bond... 

Preparative TLC is still very much in use as a preliminary separation technique when there is interference of peaks in mixtures to be analysed by GC or HPLC. An example of this is the characterisation of the double-bond configurations of fatty acid methyl esters derived from hydrogenated soybean oil and margarine which required preparative TLC using silver nitrate/silica gel coated plates to separate the mixture prior to GC analysis [10],... 

Mossoba, M.M., M.P.Yuracwecz, J.A.G. Roach, H.S. Lin, R.E. McDonald, B.D. Flickinger, and E.G. Perkins, Rapid Determination of Double Bond Configuration and Position along the Hydrocarbon Chain in Cyclic Fatty Acid Monomers, Lipids 29 893-896(1994). 

Mossoba, M. M., Yuracwecz, M. P., Roach, J. A. G. et al. (1994) Rapid determination of double bond configuration and position along the hydrocarbon chain in cyclic fatty acid monomers. Lipids, 29 (12), 893-6. 

Hydroxy fatty acids may be intermediates of F. a.b which have been diverted (3-hydroxy fatty acids with D-configuration). They may also arise by oxygenation catalysed by cytochrome P-450 systems (especially (fl-oxidation in eukaryotes see Fatty acid degradation). Hydroxy fatty acids can also be biosynthesized by hydration of double bonds in unsaturated fatty acids specific fatty acid hydratase systems catalysing this reaction have been characterized in bacter-... 

The double bonds in almost all naturally occurring fatty acids have cis configurations. The triglycerides of animal fats are richer in saturated fatty acids, whereas the triglycerides of plant oils (e.g., corn, soybean, canola, olive, and palm oils) are richer in unsaturated fatty acids. 

The structural requirements in the fatty acid moiety of the native elicitor necessary for elicitation have been investigated.Saturated and unsaturated 16- and 18-carbon fatty acids were inactive. A 20-carbon chain and free carboxyl group were necessary for activity. Of the five 20-carbon acids tested, those with a minimum double bond configuration of A 5,8,11 were by far the most active (Table 1). 

Measurement of Unsaturation. The presence of double bonds in a fatty acid side chain can be detected chemically or through use of instmmentation. Iodine value (IV) (74) is a measure of extent of the reaction of iodine with double bonds the higher the IV, the more unsaturated the oil. IV may also be calculated from fatty acid composition. The cis—trans configuration of double bonds may be deterrnined by infrared (59) or nmr spectroscopy. Naturally occurring oils have methylene-intermpted double bonds that do not absorb in the uv however, conjugated dienes maybe deterrnined in an appropriate solvent at 233 nm. 

The double bonds found in fatty acids are nearly always in the cis configuration. As shown in Figure 8.1, this causes a bend or kink in the fatty acid chain. This bend has very important consequences for the structure of biological membranes. Saturated fatty acid chains can pack closely together to form ordered, rigid arrays under certain conditions, but unsaturated fatty acids prevent such close packing and produce flexible, fluid aggregates. 

Some of the fatty acids found in the diets of developed nations (often 1 to 10 g of daily fatty acid intake) are trans fatty acids— fatty acids with one or more double bonds in the trans configuration. Some of these derive from dairy fat and ruminant meats, but the bulk are provided by partially hydrogenated vegetable or fish... 

If the double bond is on an odd carbon, (3 oxidation removes 2-car-bon fragments until it gets to the structure with a 3-cis double bond [R-CH=CH-CH2-C(=0)-SCoA]. A new double bond can t be placed between C-2 and C-3 because there s already a double bond at C-3. In this situation, the activity of an isomerase simply moves the double bond from C-3 to C-2 and at the same time makes sure that the configuration is trans. From this point on, the metabolism is just like normal (3 oxidation (hydration, oxidation, cleavage). If you re counting ATPs, these unsaturated fatty acids produce 2 fewer ATPs for each double bond since there is no FADH2 produced by putting in the double bond (see Fig. 13-6). 

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. 

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

Studies of the reaction of ozone with simplified lipid systems have shown that malonaldehyde can be produced by direct ozonolysis. The use of malonaldehyde assay as an index of lipid peroxidation is therefore invalid in ozone studies. Liposomes formed from egg lecithin and prepared in aqueous media were quite resistant to ozone, but the contribution of polyconcentric spheres to this resistance has not been fully assessed. However, the bilayer configuration, with the susceptible unsaturated fatty acids shielded from ozone by the hydrophilic areas of the molecule, may be resistant. In hexane, where the fatty acid moieties are exposed, ozone reacts stoichiometrically with the double bonds. The experiments with aqueous suspensions of phosphatidylcholine gave no evidence of the formation of lipid peroxides,nor did experiments with films of fatty acids exposed to ozone. ... 

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). 

Figure 4-2. Structures of naturally occurring fatty acids. All the double bonds i these structures are of the cis configuration.

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