Fatty acid acetylenic groups

There is a seemingly endless variety of fatty acids, but only a few of them predominate in any single organism. Most fatty acid chains contain an even number of carbon atoms. In higher plants the C16 palmitic acid and the C18 unsaturated oleic and linoleic acids predominate. The C18 saturated stearic acid is almost absent from plants and C20 to C24 acids are rarely present except in the outer cuticle of leaves. Certain plants contain unusual fatty acids which may be characteristic of a taxonomic group. For example, the Compositae (daisy family) contain acetylenic fatty acids and the castor bean contains the hydroxy fatty acid ricinoleic acid. 

Another alteration of unsaturated fatty acids is the formation of acetylenic groups (-OC-). This apparently occurs by dehydrogenation of -CH=CH- Examples of naturally occurring acetylenes are crepenynic acid (p. 381), alloxanthin (p. 1240), and the following remarkable hydrocarbon from the common cornflower Centaurea cyanusm ... 

The reader is referred to the section on fatty acids (3.8), which includes brominated fatty acids containing multiple acetylene groups. 

Many unsaturated compounds found in nature contain one or more acetylenic bonds, and these are predominantly produced by further desaturation of olefinic systems in fatty acid-derived molecules. They are surprisingly widespread in nature, and are found in many organisms, but are especially common in plants of the Compositae/Asteraceae, the Umbelliferae/Apiaceae, and fungi of the group... 

The Effect of Ethylenic and Acetylenic Groups on the Properties of Fatty Acid Monolayers... 

Twenty carbon fatty acids with 1-3 acetylenic groups also proved to be inhibitors of cyclooxygenase. Triple bonds in positions 9 and 12 resulted in more potent inhibitors than at position 5 of the fatty acid. 

Fatty acids containing acetylenic bonds, double bonds close to the carboxyl group, conjugated unsaturation, or other reducible (e.g., keto) groups rarely add the... 

A shorthand nomenclature is in common usage for fatty acids. They are written as two numbers separated by a colon. The number before the colon indicates the carbon chain length, and the figure after corresponds to the number of double bonds. Additional figures in parentheses show the position of double bonds, and the letters c, t, a, and e, show whether the bond is c/5-olefinic, rra 5-olefinic, acetylenic, or ethylenic. The position of the double bonds is numbered fi om the carboxyl group, but where it is more meaningful to number from the methyl end it is prefixed by o>. These abbreviations... 

Some plants contain fatty acids only found in a few species. These are termed unusual fatty acids and are often located in seed oils. However, when found, these bizarre structures frequently represent major components of the individual seed being studied. The unusual fatty acids can be conveniently divided into five groups nonconjugated ethylenic, conjugated ethylenic, acetylenic, substituted, and branched-chain. The structures and distribution of some of these acids are shown in Tables II-IV. For a more complete description the reader is referred to Galliard (1974), Hitchcock (1975), Hitchcock and Nichols (1971), Hopkins (1972), and Smith (1970). 

The following shorthand designation may be used to describe the chemical structure of fatty acids the number of carbon atoms in the chain separated by a colon from the number of unsaturated bonds is followed by a set of parentheses containing the numbers of the double-bond positions and the letters c, t, or a to indicate whether a bond is cis-olefinic, trans-olefinic, or acetylenic. The position of double bonds is numbered from the carboxyl group. 

Acetylenes are formed from fatty acids. In higher plants linoleic acid is the most common precursor. The acetylene groups are formed from double bonds by dehydrogenation (Fig. 66). 

Most acetylenes are formed when the carboxylic group of the precursor fatty acid is removed by decarboxylation. The carbon chain may also be shortened by oc- and /S-oxidations (D 3.2). 

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