Lipids

Classification By structure: Simple: fats, oils, waxes, steroids. Complex: phospholipids, spingolipids, glygolipids. They derivatives: hormones, fat-solubility vitamins On the basis of whether they undergo hydrolysis reactions in alkaline solution: Saponifiable lipids can be hydrolyzed under alkaline conditions to yield salts of fatty acids. Nonsaponifiable lipids do not undergo hydrolysis reactions in alkaline solution.

LIPIDS Lipids are a class of biological molecules defined by low solubility in water and high solubility in nonpolar solvents. As molecules that are largely hydrocarbon in nature, lipids represent highly reduced forms of carbon and, upon oxidation in metabolism, yield large amounts of energy. Lipids are thus the molecules of choice for metabolic energy storage.

Lipids Lipids (fixed oils, fats, and waxes) are esters of long-chain fatty acids and alcohols, or of closely related derivatives. The chief difference between these substances is the type of alcohol; in fixed oils and fats, glycerol combines with the fatty acids; in waxes, the alcohol has a higher molecular weight, e.g., cetyl alcohol[CH3(CH2)15OH].

Fats and oils are made from two kinds of molecules: glycerol (a type of alcohol with a hydroxyl group on each of its three carbons) and three fatty acids joined by dehydration synthesis. Since there are three fatty acids attached, these are known as triglycerides.

triglyceride where R, R', and R" are long alkyl chains; the three fatty acids RCOOH, R'COOH and R"COOH can be all different, all the same, or only two the same.

Structure of Fatty Acids The “tail” of a fatty acid is a long hydrocarbon chain, making it hydrophobic. The “head” of the molecule is a carboxyl group which is hydrophilic. Fatty acids are the main component of soap, where their tails are soluble in oily dirt and their heads are soluble in water to emulsify and wash away the oily dirt. However, when the head end is attached to glycerol to form a fat, that whole molecule is hydrophobic.

The terms saturated, mono-unsaturated, and poly-unsaturated refer to the number of hydrogens attached to the hydrocarbon tails of the fatty acids as compared to the number of double bonds between carbon atoms in the tail. Fats, which are mostly from animal sources, have all single bonds between the carbons in their fatty acid tails, thus all the carbons are also bonded to the maximum number of hydrogens possible.

Since the fatty acids in these triglycerides contain the maximum possible amount of hydrogens, these would be called saturated fats. The hydrocarbon chains in these fatty acids are, thus, fairly straight and can pack closely together, making these fats solid at room temperature.

Oils, mostly from plant sources, have some double bonds between some of the carbons in the hydrocarbon tail, causing bends or “kinks” in the shape of the molecules. Because some of the carbons share double bonds, they’re not bonded to as many hydrogens as they could if they weren’t double bonded to each other. Therefore these oils are called unsaturated fats.

Because of the kinks in the hydrocarbon tails, unsaturated fats (or oils) can’t pack as closely together, making them liquid at room temperature.

In unsaturated fatty acids, there are two ways the pieces of the hydrocarbon tail can be arranged around a C=C double bond (cis and trans). In cis bonds, the two pieces of the carbon chain on either side of the double bond are either both “up” or both “down,” such that both are on the same side of the molecule. In trans bonds, the two pieces of the molecule are on opposite sides of the double bond, that is, one “up” and one “down” across from each other.

Naturally-occurring unsaturated vegetable oils have almost all cis bonds, but using oil for frying causes some of the cis bonds to convert to trans bonds.

If oil is used only once like when you fry an egg, only a few of the bonds do this so it’s not too bad. However, if oil is constantly reused, like in fast food French fry machines, more and more of the cis bonds are changed to trans until significant numbers of fatty acids with trans bonds build up. The reason for this concern, is that fatty acids with trans bonds are carcinogenic, or cancer-causing.

Although most vegetable oils are liquid at ordinary temperatures and most animal fats are solid, there are notable exceptions, such as cocoa butter, which is a solid vegetable oil, and cod liver oil, which is a liquid animal fat.

Properties of Triglycerides Hydrogenation Unsaturated compounds react with H2 Ni or Pt catalyst C=C bonds C–C bonds Hydrolysis Split by water and acid or enzyme catalyst Produce glycerol and 3 fatty acids

Hydrogenation

Product of Hydrogenation Hydrogenation converts double bonds in oils to single bonds. The solid products are used to make margarine and other hydrogenated items.

Hydrolysis Triglycerides split into glycerol and three fatty acids (H+ or enzyme catalyst)

Saponification and Soap Hydrolysis with a strong base Triglycerides split into glycerol and the salts of fatty acids The salts of fatty acids are “soaps” KOH gives softer soaps

Saponification

Production of fixed oils and fats Fixed oils and fats of vegetable origin are obtained by: Extraction by expression Fixed oils are obtained by expression in hydraulic presses. If the expression is carried out in the cold, the oil is known as a "virgin oil" or a "cold-pressed oil." In contrast, if the expression is carried out in heat, the oil is known as a "hot-pressed oil." Extraction by solvents Sometimes organic solvents are used for the extraction of oils.

Animal fats are separated from other tissues by rendering with steam, with or without pressure. The heat melts the fat, which rises to the top and may be separated by decantation.

Biosynthesis of lipids The biosynthesis of saturated and unsaturated fatty acids is from combinations of acetate units (acetate pathway).

Applications of fixed oils and fats Soap manufacture Suppositories, tablet coating Dietary supplements Emulsifying agents Manufacture of paints, varnishes and lubricants Therapeutic uses (castor oil).

Examples Castor oil Olive oil Peanut oil Soybean oil Sesame oil Almond oil Cottonseed oil Corn oil Safflower oil Cocoa butter

Waxes wax is а monoester formed from the reaction of а long-chain monohydroxy alcohol with а fatty acid molecule. Example Biological role: They serve as protective coatings on leaves, stems, and fruit of plants and the skin and fur of animals.

Waxes Like fats, waxes are esters of fatty acids. The alcohol, however, is not glycerol but usually a long-chain, high-molecular weight alcohol. In plants, waxes are generally found covering the external parts, like the epidermis of leaves and fruits, where their main function is to prevent the loss of water.

Wax is also produced by insects, e.g. the honeycombs of bees and wasps. USES OF WAX Wax is used in pharmacy to make soft ointments harder and to prepare lip salves. The technical uses of waxes are substantial, e.g. in shoe polishes and car waxes.

Waxes  fixed oils and fats Wax has a melting point above approximately 45 °C (113 °F) (which differentiates waxes from fats and oils). Fats and oils my be saponified by means of either aqueous or alcoholic alkali but waxes are only saponified by alcoholic alkali. (this fact is used for the detection of fats when added as adulterants to waxes).

Examples Jojoba wax (Simmondsia chinensis) Carnauba wax (Copernicia cerifera) Beeswax (Apis mellifera)

Characterization of fats. Acid number. It is the number of milligrams of potassium hydroxide required to neutralise the free fatty acids in 1 g of the oil or fat. Saponification number. It is number of milligrams of potassium hydroxide required tо completely saponify l00 g of the oil or fat. Iodine number. It is the number of grams of iodine that combine with 100 g of oil or fat. It is а measure of the degree of unsaturation of а fat or oil; а high iodine number indicates а high degree of unsaturation of the fatty acids of the fat. Reichert -Meissl number. (R. M. number). It is the number of millilitres of Cn=10 potassium hydroxide required to neutralise the distillate (obtained by saponification, acidification and steam distillation of the fat) оf 5 g of the fat. Unsapopnificable matter: It is substances such as sterols, stigmasterol which remain after saponification of oil Peroxide value: amount of peroxide formed in lipid oxidation. Value indicates rancidity of oil.

THANK YOU -PHARMA STREET