Seminar 6 Theoretical part Lipids are a heterogeneous group of naturally occurring organic compounds, classified together on the basis of their common solubility properties. Lipids are insoluble in water, but soluble in nonpolar aprotic organic solvents, including ethers, chloroform and acetone. The classification of lipids is following: Figure 1. Classification of lipids. 1. Simple lipids esters of fatty acids and various alcohols. triglycerides (fats and oils): esters of fatty acids and the trihydroxyl alcohol glycerol waxes: esters of fatty acids and aliphatic alcohols containing long hydrocarbon chains, other than glycerol (e.g. beeswax, esters of vitamins A and D). 2. Compound lipids esters of fatty acids that contain other functional groups in addition to alcohol and the fatty acid. phospholipids: esters of fatty acids containing a phosphate group and a nitrogen base. glycolipids: esters of fatty acids containing a carbohydrate group and nitrogen. 3. Derived lipids derivatives obtained by hydrolysis of the substances mentioned in groups I and II. steroids carotenoids, cholesterol, steroid hormones prostaglandins fat soluble vitamins A, D, E and K precursors of lipids: fatty acids, glycerol and alcohols other than glycerol, monoglycerides, diglycerides, organic bases choline, ethanolamine and sphingosine. Triglycerides are esters of glycerol and fatty acids. Glycerol, also called glycerin, is an alcohol containing three -H groups. The fatty acids are organic acids containing one -CH group. A molecule of triglyceride is formed during esterification reaction as follows:
H H 2 C HC H H + C R 1 H C R 2 H 2 C HC C C R 1 R 2 + 3H2 H 2 C H H H 2 C C R 3 C R 3 glycerol fatty acids a triglyceride R1, R2, R3 are carbon chains of fatty acids. In natural triglycerides R 1 R 2 R 3, in synthetic lipids R 1 = R 2 = R 3. There are two types of fatty acids, saturated and unsaturated. Saturated acids have no double bonds in their carbon chains and unsaturated fatty acids contain one or more than one double bond in their carbon chains (cis isomer predominates, the trans isomer is rare). Those listed in Table 1. are the most important. Table 1. The most important fatty acids that compose animal s fats, vegetable oils and biological membranes. Common name Formula Carbon atoms : double bonds Fat derivative Butyric C 3H 7CH 4 : 0 Tributyrin Caproic C 5H 11CH 6 : 0 Tricaproin Caprylic C 7H 15CH 8 : 0 Tricaprylin Lauric C 11H 23CH 12 : 0 Trilaurin Palmitic C 15H 31CH 16 : 0 Tripalmitin Stearic C 17H 35CH 18 : 0 Tristearin leic C 17H 33CH 18 : 1 Triolein Linoleic C 17H 31CH 18 : 2 Trilinolein Linolenic C 17H 29CH 18 : 3 Trilinolenin Arachidonic C 19H 31CH 20 : 4 Triarachidonin leic acid contains one double bond between 9 and 10 carbon atoms, numbered from the carboxyl end: H 3 C (CH 2 ) 7 CH CH (CH 2 ) 7 CH 18 10 9 1
Linoleic acid contains two double bonds, one between 9 and 10 carbon atoms and one between atoms 12 and 13. H 318 C (CH 2 ) 4 CH 13 CH CH 2 CH CH (CH 2 ) 7 CH 12 10 9 1 Linolenic acid contains three double bonds, one between 9 and 10, 12 and 13, and 15 and 16 carbon atoms, respectively. H 318 C CH 2 CH 16 CH 15 CH 2 CH 13 CH CH 2 CH CH (CH 2 ) 7 CH 12 10 9 1 Arachidonic acid contains four double bonds, one between 5 and 6, 8 and 9, 11 and 12, and 14 and 15 carbon atoms, respectively. Physical properties. When pure, fats are colourless, odourless and tasteless. The yellow colour of many naturally occurring fats is due to the presence of pigments such as carotene or xanthophyll (unsaturated hydrocarbons). Fats are insoluble in water, but soluble in ether, chloroform, benzene and hot alcohol. Because of the flexions present in carbon chains of unsaturated fats, they cannot be compressed very tight. That is why these acids are liquids at room temperature. Unsaturated fatty acids have lower melting points than their saturated counterparts. Melting points are directly proportional to their degree of saturation. Moreover, in unsaturated fatty acids molecules, there are two possibilities for hydrocarbon chain to be arranged around C=C double bond. In cis bonds, the two fragments of carbon chain can be situated both up or both down, in relation to double bond. In trans bonds, the two fragments of molecule are on opposite sides of the double bond, that is, one up and one down across from each other (Fig. 2.). Figure 2. Fatty acid saturation. Chemical properties.
Hydrolysis. When triglycerides are heated with acids (acid hydrolysis) or alkalis (base hydrolysis), they split into fatty acids and glycerol. Triglycerides may also be hydrolyzed in the presence of digestive enzymes (enzymatic hydrolysis); this is the most common enzymatic process of triglycerides digestion in the alimentary tract of animals. Enzymatic or acidic hydrolysis results as follows we obtain glycerol and fatty acids: H 2 C HC H 2 C C C C R 1 R 2 R 3 + 3H 2 H + or enzymes H H H C R 1 C R 2 C R 3 + H 2 C HC H H H 2 C H a triglyceride fatty acids glycerol If an alkali is used, it reacts with the triglycerides to form alkaline salts of the fatty acids, called soaps. This reaction is called saponification: Reduction of fatty acid chains. Triglycerides rich in oleic, linoleic and other unsaturated fatty acids are generally liquid at room temperature and are called oils, for example olive oil. Triglycerides rich in palmitic, stearic and other saturated fatty acid are generally semisolids or solids at room temperature and are called fats, as for example butter fat. The process of conversion oils to fats is called hardening of oils and involves catalytic reduction: (liquid, unsaturated) (solid, saturated)
Soaps and detergents Soaps are sodium or potassium salts of fatty acids. Soaps owes its remarkable cleansing properties to its ability to act as an emulsifying agent. ne end of a soap molecule, the hydrophilic carboxylate group (e.g. -C -, which is bonded to Na + or K + ), is soluble in water and the other end, the hydrophobic (lipophylic) hydrocarbon chain (e.g. -C 17H 35 ), is soluble in oil (dirt). Such molecules are termed amphiphilic (gk. amphi = both) or amphipathic (Fig. 1.). Figure 1. Structure of soap molecule. Fatty acids made up of ten or more carbon atoms are nearly insoluble in water, and because of their lower density, float on the surface when mixed with water. These fatty acids spread evenly over an extended water surface, eventually forming a monomolecular layer in which the polar carboxyl groups are hydrogen bonded at the water interface, and the hydrocarbon chains are aligned together away from the water. This behavior is illustrated in Figure 2. micelle. Figure 2. Soap molecules in water. Figure 3. Soap molecules as a Substances that accumulate at water surfaces and change the surface properties are called surfactants. Most dirt particles adhere to the skin or to fabrics by being dispersed in oils, which are insoluble in water. Soaps remove the particles of fat and dirt by emulsifying them (with its hydrocarbon chains). After their formation the emulsified particles containing dirt are readily removed by water. The surfactant molecules reversibly assemble into polymolecular aggregates called micelles, (Fig. 3.) with nonpolar carbon chains and dirt molecules in their centers and -C - groups outside. To summarize, the presence of a soap or a detergent in water facilitates the wetting of all parts of the object to be cleaned, and removes water-insoluble dirt by incorporation in micelles. Soaps also have their disadvantages. Foremost among them, they form insoluble salts when used in water containing Ca (II), Mg (II) or Fe (III) ions (hard water), for example (C 17H 35C) 2Ca.
The action of detergents in removing dirt procedure is similar to that described for soaps. A molecule of detergent contains the hydrophilic and the hydrophobic groups as well, and removes dirt from surfaces by forming an oil-water emulsion, and all the dirt is dissolved into the oil phase. Detergents differ from soaps in one aspect: the water-soluble end of the molecule is a sulphate group instead of a carboxylate group present in soaps. The general formula for detergents and its comparison with a soap molecule is present below: Nonpolar hydrocarbon tail polar head group Figure 4. A soap and a detergent molecule. Since detergents do not form insoluble salts with calcium or magnesium ions (as soaps do), they can be used effectively in hard water. All natural fats contain the glyceryl group. They differ from each other in chain length and the degree of saturation of the fatty acid they contain. Two chemical tests used to inform about the chemical nature of fats, are the saponification number and the iodine number. Saponification number of a fat is the number of milligrams of KH required to saponify 1 g of fat. The longer chain of fatty acid, the smaller amount of KH needed for saponification. Iodine number is the number of grams of iodine absorbed by 100 g of fat. The iodine combines with fatty acid at its double bonds, thus the amount of iodine taken up is a measure of the degree of unsaturation of the fatty acids.
Phospholipids (glycerophospholipids or phosphoglycerides), are fatty acids esters that contain a phosphate group. The basic structure of phospholipids is very similar to that of the triacylglycerides except that carbon C-3 of the glycerol backbone is esterified to phosphoric acid. The building block of the phospholipids is phosphatidic acid which results when the X substitution in the basic structure (shown in the Figure 5 below) is a hydrogen atom. Substitutions include ethanolamine (phosphatidylethanolamine, also called cephalin), choline (phosphatidylcholine, also called lecithin), serine (phosphatidylserine) or glycerol (phosphatidyl-glycerol). Phospholipids are the main constituents of cell membranes. Figure 5. Basic composition of a phospholipid. X can be a number of different substituents. Lecithins when hydrolysed with diluted acid or alkali by boiling, they yield glycerol, fatty acid, phosphoric acid and choline. The formula for this lipid is as follows: Figure 6. Lecithin molecule. The two fatty acids tend to be non-identical. They may differ in length and/or the presence or absence of double bonds. Choline, present in lecithin, is a nitrogenous base: Figure 7. Choline molecule. The structural formula for cephalin is as follows:
Figure 8. Cephalin molecule. Glycolipids are lipids that contain sugar instead of the phosphorus present in phospholipids. An example of sugar commonly found in combination with lipid is galactose. Figure 9. Glycolipid molecule. Steroids are a group of plant and animal lipids that have the tetracyclic ring system of steran shown below: Figure 10. Steran. The major classes of steroids are: 1. Cholesterol is white, water-insoluble, waxy solid present in blood plasma and all animal tissues. This substance act as integral part of a human metabolism in two ways: a) it is essential component of biological membranes and b) it is the compound from which sex hormones, adrenocorticoid hormones, bile acids, and vitamin D are synthesized (thus, cholesterol is, in a sense, the parent steroid). The structural formula for cholesterol is as follows: Figure 11. Glycolipid molecule. Cholesterol is insoluble in blood plasma, but can be transported as a plasma soluble complex formed by cholesterol with proteins called lipoproteins. Low-density lipoproteins (LDL) transport cholesterol from the place of its synthesis in the liver to the various tissues and cells of the body where it is used. It is primarily
cholesterol attached to LDLs that builds up in atherosclerotic deposits in blood vessels. Highdensity lipoproteins (HDL) transport excess and not used cholesterol from cells back to the liver for its degradation to bile acids and eventual excretion in the faeces. It is thought that HDLs retard or reduce atherosclerotic deposits. 2. Steroid hormones are: Androgens (male sex hormones), for example testosterone and androsterone, are synthesized in the testes; responsible for development of male secondary sex characteristics. Estrogens (female sex hormones), for example progesterone or estrone, synthesized in the ovaries; responsible for development of female secondary sex characteristics and control of the menstrual cycle. Glucocorticoid hormones (e.g. cortisone, cortisol) are synthesized in the adrenal cortex; they regulate metabolism of carbohydrates, decrease inflammation and are involved in the reaction to stress. Mineralocorticoid hormones (e.g. aldosterone) are synthesized in the adrenal cortex; they regulate blood pressure and volume by stimulating the kidneys to absorb Na +, Cl - and HC 3-. Prostaglandins are a family of compounds all containing the 20-carbon skeleton of prostanoic acid: Figure 12. Prostanoic acid. Prostaglandins are not stored as such in target tissues. They are synthesized in response to specific physiological triggers. Starting materials for the biosynthesis of prostaglandins are polyunsaturated fatty acids of 20 carbon atoms (for example arachidonic acid), stored until needed as membrane phospholipid esters. In response to a physiological trigger, the ester is hydrolysed, the fatty acid is released and the synthesis of prostaglandins is initiated. The diversity of receptors means that prostaglandins act on an array of cells and have a wide variety of effects such as: cause constriction or dilation in vascular smooth muscle cells, cause aggregation or disaggregation of platelets, sensitize spinal neurons to pain, regulate inflammation, regulate calcium movement, regulate hormones, control cell growth, acts on thermoregulatory centre of hypothalamus to produce fever, increase glomerular filtration rate and other.
Seminar 6 problems. 1. Define the term lipid and describe the two major classes of lipids. 2. Arrange the following fatty acids in order of increasing melting point: stearic acid linolenic acid 18:3(Ʌ 9,12,15 ) linoleic acid oleic acid 3. In the formation of a triglyceride, what functional groups chemically combine and what functional group is formed? 4. Draw the triacylglycerol composed of glycerol and lauric acid. Lauric Acid = CH3(CH2)10C2H Glycerol = CH2HCHHCH2H 5. Unsaturated fats are preferred over saturated fatty acids for nutritional reasons. Would you expect a deep sea fish (living in cold water) or a shallow water fish (living in warmer water) to have a higher percentage of unsaturated fatty acids in its tissues? 6. The following triacylglycerol is treated with excess H2 and Pd. Write the structure of product. 7. Draw the structure of the product which results when the following compound is completely hydrogenated using excess H2: 1-palmitoyl, 2-linoleoyl, 3-oleoyl glicerol 8. What do we obtain in reaction of hydrolysis of tripalmitin? 9. Provide the structures of the products which result when the following molecule is heated in aqueous solution of sodium hydroxide: 1-stearoyl, 2-linoleoyl, 3-oleoyl glycerol 10. Define the term hydrophobic. Identify the hydrophobic and hydrophilic regions of a sodium soap. 11. Draw the structural formula of a lecithin containing molecules of palmitic acid. 12. Draw structures for each of the following: a) palmitate ion b) sodium oleate c) arachadonic acid - 20:4(Ʌ 5,8,11,14 ) d) phosphatidate (2-linoleoyl, 3-stearoyl) e) phosphatidyl choline (2-lauroyl, 3-myristoyl) f) cholesterol 13. Which of the following terms best describes the compound below? 1
a) a phosholipid b) a triglyceride c) a steroid d) a prostaglandin e) a sphingolipid 14. Check any statements in the following list that are TRUE. All comments concern the fluid mosaic membrane model Molecules in a lipid bilayer are stationary unless they need to perform a function Molecules in a lipid bilayer are in constant motion Molecules move faster at higher temperatures Cholesterol lowers the melting point of a membrane Cholesterol raises the melting point of a membrane Cholesterol has little effect on the melting point of a membrane 15. What is a iodine number for the fat? 2