Structures

lhc characteristic feature of fatty acids is thecarboxv function attached to an aliphatic body. Fatty acids in plants and animals most commonly have an even number of carbons, but a small percentage of odd-numbered fatty acids is also present in many species. Short-chain fatty acids contain 2. 3 or 4 carbons, medium-chain fatty acids between 6 and 12 carbons. Long-chain fatty acids are those with 14 IS carbons, but a larger number is possible (very long-chain fatty acids). While most fatty acids have straight chains, branching does occur. Some fatty acids have double bonds linking carbons. Their number, type and position influence physical and metabolic properties. It is characteristic of els double bonds that the hydrogens at the two involved carbons are on the same side (that is what cis means in Latin) and that the acyl chain is bent at a stark angle with some freedom of movement. In trans fatty acids the hydrogens are at different side of the chain and the acyl chain is more or less straight and rigid. Fatty acids without douhlc bonds are called saturated. Monounsaturated fatty acids have one double bond polyunsaturated fatty acids have several.

All fatty acids have systematic names that consist of the (¡reek number term for the chain length followed by the ending -ic and acid or just the ending -ate. The saturated

Table 6.1 S4tura[#d fatty aods

Trivial name

Short-chain fatty adds

Medium chain fatty audi Caproitr acid Caprylic acid Capric acid Laurie acid

Long-chain fatty audi Myristic acid

Palmitic acid Mjrgannn acid Stearic acid Arachidic acid Behenic acid Lignoceric acid Cerotenic add Branched-chain fatty ¡lads

Phytanic acid Prístante acid

Systematic name

Acetic acid Propionic acid Butyric acid

He*anoic acid Oct a no ic acid Decanoic acid Dodrcanoic acid

Teiradecanoic acid Pentadfcanoic acid Mtrxadecanoic acid Heptadecanoic acid Octadecanoic artd Eicosanoic acid Docosanoic acid Tetracosanoic acid Hexacosanoic acid

Iso-tetradecanoic acid Iso-pentadecanoic acid Ameiso-pcniadecanoic acid tso-heiadecanoic acid lso- he pi ad «a note ac id Anteno-htptadecanoic acid 1 ft^methyle icosa mc acid 3,7,J1,1 s-tetramethythexadicanoic acid 2.6,tQ.14-tetramet:hylpi:ntadccanoic acid

Carbons: double bonds

C140 C15:0 C16:0 C17:0 CI 8:0 C20:0 C22:0 C24:IJ C26:0

Iso-CI 4:0

Iso-ClSrO

Iso-Cl 6:0

lso*C17:0

Anteisa-C17:0

Anteiso-C21:0

fetty acid Willi 18 carbons is called octadecartoic acid or octadecanoate. therefore. If the chain contains a double bond, this is indicated by adding to the Greek number term to the number of the first double-bonded carbon (counting from the carboxyl end I and the endings -enic or -enate. Thus, the 18-carbon fatty acid with a double bond between carbons 4 and ID has the systematic name of octadec-9-enoic acid or octa-9-enoate. If there is more than one double bond all carbon positions arc indicated and followed by the Greek number term <di-. tri-. tetra-. penta-. hex a-, etc.) referring to the number of double bonds followed by the endings -enic or -enate. This would make the 18-carbon fatty acid with double bonds at carbons L) and 12 an octa-9.l2-dtenoic acid or octa-9,12-dienoate.

The usual assumption is that these are cis double bonds unless indicated otherwise. If there are trans double bonds or the presence of cis double bonds is to be emphasized the terms cis or trans are inserted before the position number. To abbreviate this lengthy notation, the letters c or t are often used, instead. The common fatty acids have trivial names that avoid the burdensome use of the systematic names. It is certainly

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