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Lipids

 Lipids form a broad category of biomolecules which constitute an essential part of living organisms in addition to carbohydrates and proteins. In this section, we will introduce lipid related substances such as fatty acids and their derivatives. The biosynthesis of fatty acids involves the condensation of malonyl-CoA (or methylmalonyl CoA) with acyl CoA as a primer.1) Carboxylic acids with chains of 4 or more carbons are referred to as fatty acids while those with 10 or more carbons are called higher fatty acids.2) Lipids and related substances can be classified as illustrated in this section.3)
 Steroids or terpenes which are occasionally considered as lipid related substances will be explained in other sections.

■Functional Roles
Fatty acids exist in living organisms mainly as esters of glycerol and triacylglycerols which occur as a major form of energy storage in adipose tissue. Triacylglycerols are found in living organisms as mixtures of acyl groups with different number of carbons and are difficult in most cases to isolate as a single substance. Moreover, fatty acids also exist in living organisms in the form of cholesteryl esters which constitute an essential component of cell membranes where it is required to establish proper membrane permeability and fluidity.
Free fatty acids are known to suppress cell-growth at an order of 0.1mM and above, therefore considerable attention should be paid during their administration to cells.4)

■Derivatives
Fatty acid methyl esters are the most widely used fatty acid derivatives in analytical chemistry due to their ease of handling in organic solvents as compared to the highly polar free fatty acids. Moreover their analysis by TLC, gas chromatography (GC) and liquid chromatography (LC) can be improvised by suppressing tailing.  
Although fatty acid ethyl esters are rarely used for analysis as compared to the methyl esters, they offer an advantage since their method of preparation from fatty acids involves the use of the less toxic ethanol instead of methanol. For instance, eicosapentaenoic acid which is used as a hyperlipemia medicine is being supplied as ethyl ester.
Ethyl esters of lower to middle-chain fatty acids can be also employed in the fragrance industry.  Sodium salts of fatty acids can be obtained as saponification products of lipids and are widely used in daily life as an ingredient of soap owing to their amphiphilicity and surfactant properties.

■Analysis
GC is the most frequently used technique for the analysis of fatty acids. However, their direct analysis appears to be difficult owing to their low volatility in electron impact (EI) ionization mass spectrometry under normal condition. They can however be easily detected by GC-MS method upon derivatization to their methyl esters.
The several methylation methods include the treatment of the free fatty acids with boron trifluoride-ether complex in methanol or with trimethylsilyldiazomethane, etc. 
The GC on-column method is a simple method that uses a methanolic solution of trimethylsulfonium hydroxide or 3-(trifluoromethyl)phenyltrimethylsilylammonium hydroxide to analyze the fatty acid component of lipids such as glycerolipids. Please refer to "GC Derivatizing Reagents" for protocols.

■Solubility
In general, the solubility of fatty acids in water decreases as the carbon number increases.
Fatty acid esters and glycerolipids are insoluble in water but soluble in ethanol, chloroform and diethyl ether. They can be added to the buffer solution as a dimethyl sulfoxide solution to examine their activity in living organisms. Please take caution that the solution becomes suspended as the concentration level of the dissolved substance increases. It is recommended to define the optimal concentration level and volume of addition in advance. Sodium salts of fatty acid are more water-soluble than the free fatty acids. Long-chain fatty acid salts tend to form micelles.

■Storage Precautions
Unsaturated fatty acids like oleic acid are known to undergo aerial oxidation to produce peroxides. Opened bottles of unsaturated fatty acids and their derivatives should be stored in the refrigerator or frozen with inert gas such as nitrogen or argon. Moreover the tendency to oxidation increases as the degree of unsaturation in the fatty acids increases.

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