Linoleic acid, structure

Linoleic acid, structure of, 1062 Linolenic acid, molecular model of. 1063... 

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

Antioxidant capacities of common individual curcuminoids were determined in vitro by phosphomolybdenum and linoleic acid peroxidation methods. Antioxidant capacities expressed as ascorbic acid equivalents (pmol/g) were 3099 for curcumin, 2833 for demethoxycurcumin, and 2677 for bisdemethoxycurcumin at concentrations of 50 ppm. The same order of antioxidant activity (curcumin > demethoxycurcumin > bisdemethoxycurcumin) was observed when compared with BHT (buty-lated hydroxyl toluene) in linoleic peroxidation tests. The antioxidant activity of curcumin in the presence of ethyl linoleate was demonstrated and six reaction products were identified and structurally characterized. The mechanism proposed for this activity consisted of an oxidative coupling reaction at the 3 position of the curcumin with the lipid and a subsequent intramolecular Diels-Alder reaction. ... 

Garssen, G.J., Vliegenthart, J.F.G. and Boldingh, J. (1972). The origin and structures of dimeric fetty acids from the anaerobic reaction between soya-bean lipoxygenase, linoleic acid and its hydroperoxide. Biochem. J. 130, 435-442. 

Soaps are composed of sodium salts of various fatty acids. These acids include those with the general structure CH3-(CH2) -COOH where n = 6 (caprylic acid), 8 (capric acid), 10 (lauric acid), 12 (myristic acid), 14 (palmitic acid), and 16 (stearic acid). Oleic acid (CH3-(CH2)7-CH=CH-(CH2)7-COOH) and linoleic acid (CH3-(CH2)4-CH=CH- H2-CH=CH-(CH2)7-COOH) are also common soap ingredients. These sodium salts readily dissolve in water, but other metal ions such as Ca2+ and Mg2+ form precipitates with the fatty acid anions. For example, the dissolution of the sodium salt of lauric acid and the subsequent formation of a precipitate of the lauric acid anion with calcium ion is given by... 

While little biosynthetic information is available, it has been suggested [38] that 25 and 26 may be formed from AA (24) and EPA (14) via a cyclization mechanism (Scheme 3) similar to that which forms trans-cyclopropyl-containing diol 28 upon treatment of linoleic acid with performic acid [40]. An alternative biogenetic mechanism (Scheme 4), based upon that proposed for the structurally related red algal metabolites constanolactone A and B [41], would involve the formation and opening of an allylic epoxide intermediate created as a result of a 15-/ -LPO acting on either AA or EPA. Related compounds have been isolated from the coral Plexaura homomalla and the mollusc Aplysia kurodai (see below). 

FIGURE B-1 Structures of some fatty acids of neurochemical interest (see also Fig. 3-7 and text). The n minus nomenclature for the position of the double bond(s) is given here. Note that the position of the double bond from the carboxyl end can be indicated by the symbol A, so that linoleic acid may be also be designated as 18 2A9,12. The linolenic acid shown is the a isomer. 

Dietary polyunsaturated fatty acids (PUFAs), especially the n-3 series that are found in marine fish oils, modulate a variety of normal and disease processes, and consequently affect human health. PUFAs are classified based on the position of double bonds in their lipid structure and include the n-3 and n-6 series. Dietary n-3 PUFAs include a-linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) whereas the most common n-6 PUFAs are linoleic acid, y-linolenic acid, and arachidonic acid (AA). AA is the primary precursor of eicosanoids, which includes the prostaglandins, leukotrienes, and thromboxanes. Collectively, these AA-derived mediators can exert profound effects on immune and inflammatory processes. Mammals can neither synthesize n-3 and n-6 PUFAs nor convert one variety to the other as they do not possess the appropriate enzymes. PUFAs are required for membrane formation and function... 

Figure 11.2 Structures of commonly occurring unsaturated fatty acids (i) oleic acid, C18 1 (ii) linoleic acid, C18 2 (iii) a-linolenic acid, C18 3.

The common fatty acids have a linear chain containing an even number of carbon atoms, which reflects that the fatty acid chain is built up two carbon atoms at a time during biosynthesis. The structures and common names for several common fatty acids are provided in table 18.1. Fatty acids such as palmitic and stearic acids contain only carbon-carbon single bonds and are termed saturated. Other fatty acids such as oleic acid contain a single carbon-carbon double bond and are termed monounsaturated. Note that the geometry around this bond is cis, not trans. Oleic acid is found in high concentration in olive oil, which is low in saturated fatty acids. In fact, about 83% of all fatty acids in olive oil is oleic acid. Another 7% is linoleic acid. The remainder, only 10%, is saturated fatty acids. Butter, in contrast, contains about 25% oleic acid and more than 35% saturated fatty acids. 

Figure 1. The chemical structures of linoleic acid (cis-9, cis-12-octadecadienoic acid), and the cis-9, trans-11 CLA isomer (cis-9,trans-l 1-octadecadienoic acid). (Reproduced with permission from the Food Research Institute Annual Report 1990. Copyright Food Research Institute 1991.)...

Castor oil [CO Structure (4.3)] is a triglyceride of ricinoleic (12-hydroxyoleic) acid about 90% of the fatty acid portion of the molecule consists of ricinoleic acid and 10% in the form of non-hydroxy acids consisting largely of oleic and linoleic acids. Small amounts of stearic and dihydroxystearic acids are also found in some industrial grades. 

Melting Points of Lipids The melting points of a series of 18-carbon fatty acids are stearic acid, 69.6 °C oleic acid, 13.4 °C linoleic acid, - 5 °C and linolenic acid, - 11 °C. (a) What structural aspect of these 18-carbon fatty acids... 

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... 

Look up the structure of linoleic acid in Table 24.3, and draw all potential products of its reaction with 2 mol of HC1. 

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