Fatty acids geometric isomers

In a partially hydrogenated fat, the number of fatty acid isomers is dramatically increased by the appearance of geometrical isomers (i.e., cis and trans configurations) and various positional isomers (i.e., different positions of double bonds within the fatty acid chain). Thus the in-... 

Dutton, H. and Emken, E.A. (1979) Geometrical and Positional Fatty Acid Isomers, American Oil Chemists Society, Champaign. 

There are a myriad of possible isomers, including positional and geometrical isomers of the double bond(s) as well as stmctural isomers resulting from head-to-head or head-to-tad alignment of the reacting fatty acids. 

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-... 

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. 

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. 

A number of mammalian and fungal enzyme systems, whole cells and entire animals have been shown to respond differently to fatty acids which vary in number and position of unsaturation, geometrical isomers, or cyclopropyl ring positions (110,116). 

The usual diet of ruminants consists of fresh and preserved herbage and cereals. As a result of microbial activity in the rumen, esterified dietary fatty acids are hydrolyzed, short chain fatty acids are produced by fermentation of cellulose and other polysaccharides, unsaturated fatty acids are hydrogenated and/or converted to geometric (trans) and positional isomers, and microbial lipids are synthesized. These activities account in part for the enormous diversity of fatty acids in milk and the unique features short-chain and a high proportion of long chain saturated fatty acids. (Patton and Jensen, 1976 Christie, 1979B). 

Hay and Morrison (1970) identified the monoenoic positional and geometric isomers in milk fat and determined the amounts of each total acid class and percentage of trans isomers. The geometric and positional isomers of the monoenes are primarily the result of biohydrogenation of polyunsaturated fatty acids in the rumen. Stearate is also produced, and cis-9-18 l accounts for most of the monoenes. The several positional isomers in trans 16 1 and 18 1 are due to the positional isomerization of double bonds which accompanies elaidinization. 

Silver ion chromatography is a useful technique for separating geometrical isomers of fatty acids (as the methyl ester derivatives) for subsequent analysis by GC. On the other hand, a stable ion-exchange column loaded with silver ions has been developed for HPLC that has proved of value in the simplification of complex mixtures of fatty acids (FAs) of natural origin for subsequent identification by GC-MS and for separating molecular species of triacylglycerols (41). 

Silver ion chromatography has, for several decades, been utilized for the separation of geometrical isomers of lipids, such as fatty acids and TGs (89-95). This type of separation is possible due to the formation of ir-complexes between double bonds and silver ions. 

Christie (96) has demonstrated such a separation of some geometrical isomers of TGs into the groups SSE1, SSM, SEIM, and SMM (S = saturated fatty acid, M = monounsaturated fatty acid) using an ion-exchange column impregnated with silver ions. 

Table 4.6 Formation of geometrical isomers of y-linolenic acid in borage oil deodorized at different temperatures as determined by gas liquid chromatography of fatty acid isopropyl esters on a 30 m long DB-wax capillary column...

The term conjugated linoleic add (CLA) refers to a mixture of positional and geometric isomers of linoleic add with a conjugated double bond system milk fat can contain over 20 different isomers of CLA. CLA isomers are produced as transient intermediates in the rumen biohydrogenation of unsaturated fatty acids consumed in the diet. However, cis-9, trans-11 CLA, known as rumenic acid (RA), is the predominant isomer (up to 90% of total) because it is produced mainly by endogenous synthesis from vaccenic acid (VA). VA is typically the major biohydrogenation intermediate produced in the rumen and it is converted to RA by A9-desaturase in the mammary gland and other tissues. 

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