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  Chemistry of lipids, Biology tutorial

Introduction:

Lipids are group of compounds which are insoluble in water but soluble in organic solvents. Lipids are generally oily or greasy organic substances extractable from cells and tissues utilizing solvents such as chloroform or ether. Most abundant kind of lipid is triglyceride (Triacylglycerol).

Chemical composition of lipids:

Most of lipids contain fatty acids as their building blocks. Fatty acids are long chain organic acids (carboxylic acids). They contain carbon atoms from 4 - 36, a single (-COOH) carboxyl group and long non-polar tail that is responsible for water - insolubility and oily or greasy nature of most lipids.

Kinds of fatty acids:

Hydrocarbon tail of fatty acids may be either saturated, have one double bond (monounsaturated) or more (up to six) double bonds (polyunsaturated fatty acids). Double bonds are almost always in cis-configuration in unsaturated fatty acids. In polyunsaturated fatty acids double bonds are never conjugated (-CH - CH - CH = CH -), rather they are divided by methylene group (- CH =CH = CH2- CH = CH -). This arrangement is known as pentadiene structure.

Properties of fatty acids:

Similar to any other organic compound, fatty acids also contain their physical and chemical properties that are dependent on chemical nature of fatty acids.

Physical property of fatty acid:

Physical property of fatty acids is mainly determined by length of fatty acid and degree of unsaturation of hydrocarbon chain. Non polar hydrocarbon chains accounts for poor solubility of fatty acids in water. Longer the hydrocarbon chain and the fewer the double bonds the lower the solubility of fatty acids, e.g. Lauric acid with 12 carbon atoms contains greater solubility than Palmitic acid with 16 carbon atoms.

Melting points are also affected by length and degree of unsaturation of hydrocarbon chain. At room temperature (25oC), saturated fatty acids from 12 carbon to 24 carbon atoms have waxy nature, while unsaturated fatty acids of the chain length are oily liquids. This difference in melting points is because of deference in degree of packing of fatty acid molecules. For the given fatty acid chain, melting point reduces as number of double bond increases.

Chemical properties of fatty acid:

  • Ester bond formation: carboxyl group (-COOH) in fatty acids can react with hydroxyl groups (-OH) of compounds similar to Glycerol to form ester. This kind of reaction leads to formation of triacylglycerides and phospholipids. Glycerol + Fatty Acid = Mono, Di or Triglyceride, depending on number of fatty acids.
  • Methyl group of non-conjugated pentadiene structure in polyunsaturated fatty acids can be directly attacked by O2 to create free radicals.

Nomenclature of fatty acids:

The general rule in nomenclature of fatty acids first considers number of carbon atoms, then number of double bonds if any and lastly position of double bonds counting from -COOH carbon as carbon number 1.

For instance, palmitic acid, CH3(CH2)14COOH, the saturated fatty acid is written as 16:0, that means fatty acid contains 16 carbon atoms and no double bonds .

Oleic acid, CH3(CH2)7CH=CH(CH)7COOH, is monounsaturated fatty acid is written as 18 : 1 (Δ9) that means fatty acid contains 18 carbon atoms and one double bond or carbon atom number 9. Notation (Δ) simple mean carbon atom number. For polyunsaturated fatty acid (PUFA) arachidonic acid, its nomenclature is written as 20:4 (Δ5,8,11,14), meaning that it is a 20 carbon atom fatty acid with double bond at positions/carbon atoms 5,8,11 and 14. It is significant to note that usually the cis configuration of double bonds in fatty acids is supposed when their nomenclatures are written.

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