Fatty Acids And Glycerides

To describe the composition of fatty acids it is sometimes useful to use a shorthand designation. In this convention the composition of a fatty acid can be described by two numbers separated by a colon. The first number indicates the number of carbon atoms in the fatty acid chain, the second number indicates the number of double bonds. Thus, 4:0 is short for butyric acid, 16:0 for palmitic acid, 18:1 for oleic acid, etc. The two numbers provide a complete description of a saturated fatty acid. For unsaturated fatty acids, information about the location of double bonds and their stereo isomers can be given as follows: oleic acid (the cis isomer) is 18:lc9; elaidic acid (the trans isomer) is 18: li9. The numbering of carbon atoms in fatty acids starts normally with the carboxyl carbon as number one. In some cases polyunsaturated fatty acids are numbered starting at the methyl end; for instance, linoleic acid is represented as 18:2n-6 and linolenic acid 18:3n-3. These symbols indicate straight-chain, 18-carbon fatty acids with two and three methylene interrupted cis double bonds that start at the sixth and third carbon from the methyl end, respectively. These have also been described as co6 and to3. The reason for this type of description is that the members of each group n-6 or n-3 are related biosynthetically through processes involving desaturation, chain elongation, and chain shortening (Gunstone 1986) (Figure 2-2).

Triglycerides can be abbreviated by using the first letters of the common names of the component fatty acids. SSS indicates tri-stearin, PPP tripalmitin, and SOS a triglyceride with two palmitic acid residues in the 1 and 3 positions and oleic acid in the 2 position. In some cases, glyceride compositions are discussed in terms of saturated and unsaturated component fatty acids. In this case, S and U are used and glycerides would be indicated as SSS for trisaturated glyceride and SUS for a glyceride with an unsaturated fatty acid in the 2 position. In other cases, the total number of carbon atoms in a glyceride is important, and this can be shortened to glycerides with carbon numbers 54, 52, and so on. A glyceride with carbon number 54 could be made up of three fatty acids with 18 carbons, most likely to happen if the glyceride originated from one of the seed oils. A glyceride with carbon number 52 could have two component fatty acids with 18 carbons and one with 16 carbons. The carbon number does not give any information about saturation and unsaturation.

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