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Othersa,b

Othersa

Othersc

a Pigments. b Waxes, cutin. c Lipopolysaccharide.

a Pigments. b Waxes, cutin. c Lipopolysaccharide.

Sphingolipids and sterols are present in leaves but are minor components. Sterols in animals are predominantly cholesterol.

Total fatty acid analysis is usually conducted by forming volatile derivatives (such as methyl esters) for gas chromatography. Special techniques have to be used occasionally to avoid destroying unusual functional groups such as cyclopropene rings. Moreover, the danger of auto-oxidation means that analysis of samples containing polyunsaturated fatty acids has to be especially careful. For complete identification of individual acyl groups, degradation or derivatization usually has to be employed -for example, when double bond positions have to be assigned.

In order to determine the positional distribution of acyl groups on, say, the glycerol backbone, enzymic cleavage is usually utilized. For example, phospholipases are available which have a specificity for either the sn-1 or the sn-2 position. Use of such enzymes will release fatty acids from one position and these can be separated from the partly deacylated product. Analysis of fatty acids and deacylated lipid will then reveal which fatty acids are present at each glycerol carbon.

Molecular species of lipids can be separated on the basis of size and/or unsaturation. In the past it has often been necessary to derivatize or remove the polar part of the lipid making analyses time-consuming. For example, because the charge on the head-group of phosphatidylcho-line is large in relation to differences in acyl unsaturation, it was usually necessary to degrade such phosphoglycerides to diacylglycerols before analysis by chromatography. Modern methods of High Performance Liquid Chromatography (HPLC) have, however, rendered such methods unnecessary provided that adequate methods of detection are available.

Single lipid classes can be separated from each other by methods which make use of differences in their size and charge. They can often be provisionally identified by co-chromatography with authentic standards in various systems. Important constituent groups will be revealed by spectro-scopic techniques or with specific colour reagents. However, unambiguous identification may require that the various products of hydrolysis be isolated, characterized and quantified. When enzymes are used to cause hydrolysis, their action (or otherwise)

may also provide information about the stereochemistry of particular linkages.

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