However, branch points can also be found in other positions. The presence of the side-chain has a similar effect on fluidity as the presence of a cis double bond (i.e. lowers the melting temperature). Branched-chain acids occur widely, but mainly at low concentrations in animal fats and some marine oils. They are rarely found in plant lipids. Butter fats, bacterial and skin lipids contain significant amounts. In the latter class, the uropygial (preen) gland of birds is a major source. Branched-chain fatty acids are major components of the lipids of Gram-positive bacteria and more complex structures with several branches may be found in the waxy outer coats of mycobacteria.
2.1.3 Unsaturated fatty acids
188.8.131.52 Monoenoic (monounsaturated) fatty acids
Over one hundred naturally occurring monoenoic acids have been identified but most of these are extremely rare. In general, the more common compounds have an even number of carbon atoms, a chain length of 16-22C and a double bond with the cis configuration. Often the cis bond begins at the A9 position. Trans isomers are rare but do exist, one of the most interesting being trans-3-hexadecenoic acid, a major fatty acid esterified to phosphatidylglycerol in the photosynthetic membranes of higher plants and algae.
The presence of a double bond causes a restriction in the motion of the acyl chain at that point. Furthermore, the cis configuration introduces a kink into the average molecular shape [Fig. 2.1(b)] while the trans double bond ensures that the fatty acid has an extended conformation and properties nearer to that of an equivalent chain length saturated acid (Tables 2.1 and 2.2). Because the cis forms are less stable thermodynamically than the trans forms, they have lower melting points than the latter or their saturated counterparts.
In addition to the normal ethylenic double bonds, some fatty acids possess acetylenic bonds. These occur in a number of rare seed oils and a few mosses.
Common name Melting point Occurrence
Table 2.2 Some naturally occurring monoenoic fatty acids
No. of carbon Systematic name atoms l6 frans-3-hexadecenoic l6 cis-5-hexadecenoic l6 cis-7-hexadecenoic l6 cis-9-hexadecenoic Palmitoleic lS cis-6-octadecenoic Petroselenic lS cis-9-octadecenoic Oleic lS frans-9-octadecenoic Elaidic lS frans-ll-octadecenoic frans-Vaccenic lS cis-ll-octadecenoic Vaccenic
20 cis-ll-eicosenoic Gondoic
22 cis-l3-docosenoic Erucic
184.108.40.206 Polyenoic (polyunsaturated) fatty acids
All dienoic acids are derived from monoenoic acids, the position of the second double bond being a function of the biochemical system. Thus, mammals have desaturases that are capable of removing hydrogens only from carbon atoms between an existing double bond and the carboxyl group. Because of this, further desaturations may need to be preceded by chain elongation. Higher plants on the other hand carry out desaturation mainly between the existing double bond and the terminal methyl group. In either case, the double bonds are almost invariably separated from each other by a methylene grouping (Table 2.3).
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