Vernolic a-Hydroxy palmitate ra-Hydroxy palmitate
Produced by Lactobacilli,
Found in plants of the Malvales family Used for leprosy treatment
Epoxy derivative of oleate
Found in galactosyl-cerebrosides
Constituent of suberin coverings of plants
Represents > 90% of the total acids of castor bean oil major components of surface waxes, cutin and suberin of plants (Table 2.4).
The major polyunsaturated fatty acids all contain cis-methylene-interrupted sequences and for years it was thought that most conjugated systems were artefacts of isolation. However, many such acids have now been firmly identified and are found in sources as diverse as seed oils, some micro-organisms and some marine lipids (especially sponges).
An example of one such acid would be a-eleostearic acid (Table 2.3).
Many tissues contain appreciable amounts of fatty alcohols or aldehydes whose chain length and double bond patterns reflect those of the fatty acids from which they can be derived. Sometimes the alcohols form esters with fatty acids and these 'wax esters' are important in marine waxes such as sperm whale oil or in its plant equivalent, jojoba wax.
The short-chain fatty acids (i.e. of chain lengths up to 8C) are poorly water-soluble although in solution they are associated and do not exist as single molecules. Indeed, the actual solubility (particularly of longer chain acids) is often very difficult to determine because it is influenced considerably by the pH and also because the tendency for fatty acids to associate leads to monolayer or micelle formation. Micelle formation is characteristic of many lipids. McBain and Salmon many years ago introduced the concept of micelles. The most striking evidence for the formation of micelles in aqueous solutions of lipids lies in extremely rapid changes in physical properties over a limited range of concentration, the point of change being known as the critical micellar concentration or CMC, and this exemplifies the tendency of lipids to self-associate rather than remain as single molecules. The CMC is not a fixed value but a small range of concentration and is markedly affected by the presence of other ions, neutral molecules, etc. The value of the CMC can be conveniently measured by following the absorbance of a lipophilic dye such as rhodamine in the presence of increasing 'concentrations' of the lipid. The tendency of lipids to form micelles or other structures in aqueous solution often means that study of enzyme kinetics is difficult. Thus, for example, enzymes metabolizing lipids often do not display Michaelis-Menten kinetics and phrases such as 'apparent' Km must be used. In some cases the enzymes prefer to work at interfaces rather than with free solutions (Section 7.2.1).
Fatty acids are easily extracted from solution or suspension by lowering the pH to form the uncharged carboxyl group and extracting with a non-polar solvent such as light petroleum. In contrast, raising the pH increases solubility because of the formation of alkali metal salts, which are the familiar soaps. Soaps have important properties as association colloids and are surface-active agents.
The influence of fatty acid structure on its melting point has already been mentioned with branch chains and cis double bonds lowering the melting points of equivalent saturated chains. Interestingly, the melting point of fatty acids depends on whether the chain is even or odd numbered (Table 2.5).
Saturated fatty acids are very stable, but unsa-turated acids are susceptible to oxidation; the more double bonds the greater the susceptibility. Unsa-turated fatty acids, therefore, have to be handled under an atmosphere of inert gas (e.g. nitrogen) and kept away from (photo) oxidants or substances giving rise to free radicals. Anti-oxidant compounds have to be used frequently in the biochemical laboratory just as organisms and cells have to utilize similar compounds to prevent potentially harmful attacks on acyl chains in vivo (Sections 2.3.4 and 220.127.116.11).
Melting point (°C)
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