Lipids PDF

Lipids 19 CHAPTER O UT LI NE...

Lipids 19 CHAPTER O UT LI NE 19.1 Structure and Classification of Lipids 654 19.2 Types of Fatty Acids 656 19.3 Physical Properties of Fatty Acids 659 19.4 Energy-Storage Lipids: Triacylglycerols 661 19.5 Dietary Considerations and Triacylglycerols 664 19.6 Chemical Reactions of Triacylglycerols 669 19.7 Membrane Lipids: Dan Guravich/Photo Researchers Phospholipids 674 Chemistry at a Glance Classification Schemes for Fatty Acid Residues Present in Triacylglycerols 676 19.8 Membrane Lipids: Sphingoglycolipids 681 Chemistry at a Glance Terminology for and Structural Fats and oils are the most widely occurring types of lipids. Thick layers of fat help Relationships Among Various Types of insulate polar bears against the effects of low temperatures. Fatty-Acid-Containing Lipids 682 19.9 Membrane Lipids: Cholesterol 682 19.10 Cell Membranes 684 19.11 Emulsification Lipids: T here are four major classes of bioorganic substances: carbohydrates, lipids, Bile Acids 687 proteins, and nucleic acids (Section 18.1). In the previous chapter the first 19.12 Messenger Lipids: Steroid of these classes, carbohydrates, was considered. Attention now turns to the Hormones 689 second of the bioorganic classes, the compounds called lipids. 19.13 Messenger Lipids: Lipids known as fats provide a major way of storing chemical energy and Eicosanoids 692 carbon atoms in the body. Fats also surround and insulate vital body organs, 19.14 Protective-Coating Lipids: Biological Waxes 694 providing protection from mechanical shock and preventing excessive loss of Chemistry at a Glance heat energy. Phospholipids, glycolipids, and cholesterol (a lipid) are the basic Types of Lipids in Terms of How components of cell membranes. Several cholesterol derivatives function as They Function 696 chemical messengers (hormones) within the body. 19.15 Saponifiable and Nonsaponifiable Lipids 697 Chemical Connections 19.1 Structure and Classification of Lipids 19-A The Fat Content of Tree Nuts and Peanuts 666 Unlike carbohydrates and most other classes of compounds, lipids do not have a 19-B Fat Substitutes 668 common structural feature that serves as the basis for defining such compounds. 19-C The Cleansing Action of Soap and Instead, their characterization is based on solubility characteristics. A lipid is Detergents 672 an organic compound found in living organisms that is insoluble (or only sparingly 19-D Trans Fatty Acid Content of Foods 675 19-E Anabolic Steroid use in Competitive Sports 691 Sign in to OWL at www.cengage.com/owl to view tutorials and simulations, develop problem-solving skills, and complete online homework assigned by your professor. 19-F The Mode of Action for Anti-Inflammatory Drugs 694 654  19.1 Structure and Classification of Lipids 655 CH3 Figure 19.1 The structural CH CH3 formulas of these types of lipids CH3 illustrate the great structural CH2 CH CH2 (CH2)29 (CH2)3 diversity among lipids. The CH CH3 defining parameter for lipids is O O O O H3C solubility rather than structure. C O C O C O C O H3 C (CH2)16 (CH2)14 (CH2)12 (CH2)14 CH3 CH3 CH3 CH3 HO A fat A biological wax A steroid + + N(CH3)3 N(CH3)3 CH2 CH2 CH2 CH2 CH2OH O O HO O – – OH O O P O O P O O O OH CH2 CH CH2 CH CH CH CH2 CH CH CH CH2 O O CH OH NH CH OH NH C O C O (CH2)12 C O (CH2)12 C O (CH2)14 (CH2)16 CH3 (CH2)12 CH3 (CH2)16 CH3 CH3 CH3 CH3 A glycerophospholipid A sphingophospholipid A sphingoglycolipid soluble) in water but soluble in nonpolar organic solvents. When a biochemical mate- rial (human, animal, or plant tissue) is homogenized in a blender and mixed with a nonpolar organic solvent, the substances that dissolve in the solvent are the lipids. Figure 19.1 shows the structural diversity that is associated with lipid mole- cules. Some are esters, some are amides, and some are alcohols; some are acyclic, some are cyclic, and some are polycyclic. The common thread that ties all of the compounds of Figure 19.1 together is solubility rather than structure. All are insoluble in water. Two common methods exist for subclassifying lipids into families for the pur- pose of study. One method uses the biochemical function of a lipid as the basis for classification, and the other method is based on whether or not a lipid can be bro- ken down into smaller units through basic hydrolysis, that is, reaction with water under basic conditions. A hydrolysis reaction that occurs in basic solution is called a saponification reaction (Section 16.16). Based on biochemical function, lipids are divided into five categories: 1. Energy-storage lipids (triacylglycerols) 2. Membrane lipids (phospholipids, sphingoglycolipids, and cholesterol) 3. Emulsification lipids (bile acids) 4. Messenger lipids (steroid hormones and eicosanoids) 5. Protective-coating lipids (biological waxes) Based upon whether or not saponification occurs when a lipid is placed in basic aqueous solution, lipids are divided into two categories: 1. Saponifiable lipids (triacylglycerols, phospholipids, sphingoglycolipids, and biological waxes) 2. Nonsaponifiable lipids (cholesterol, steroid hormones, bile acids, and eicosanoids) Saponifiable lipids are converted into smaller molecules when hydrolysis occurs. Nonsaponifiable lipids cannot be broken up into smaller units since they do not react with water. 656 Chapter 19 Lipids Some textbooks, including this one, use the first of these two classification sys- tems (biochemical function) as the basis for lipid classification, whereas others use the second system (hydrolysis). The decision of which system to use is arbitrary; both systems have their merits. Because the saponification classification system is also widely used, the last section in this chapter reformats important chapter con- siderations in terms of the saponification classification system. This reformatting will serve as a useful review of the chapter’s lipid considerations. A parallel exists between carbohydrate chemistry of the last chapter and lipid chemistry. A fundamental premise of carbohydrate chemistry is the con- cept that monosaccharides are the basic structural unit, or building block, from which carbohydrate molecules are made. In a like manner, basic building blocks for lipid molecules exist. Because of the structural diversity found in lipid mol- ecules, several different building blocks are needed. The most frequently encoun- tered lipid building block is the structural unit called a fatty acid. Consideration of the structural characteristics and physical properties of fatty acids is the start- ing point for development of the subject of lipid chemistry. All energy-storage lipids, the most abundant type of lipid, contain fatty acid building blocks. Most membrane lipids, the second most abundant type of lipid, also contain this building block. 19.2 Types of Fatty Acids Fatty acids were first isolated from A fatty acid is a naturally occurring monocarboxylic acid. Because of the pathway naturally occurring fats; hence the by which they are biosynthesized (Section 25.7), fatty acids nearly always contain designation fatty acids. an even number of carbon atoms and have a carbon chain that is unbranched. In terms of carbon chain length, fatty acids are characterized as long-chain fatty acids (C12 to C26), medium-chain fatty acids (C8 and C10), or short-chain fatty acids (C4 and C6). Fatty acids are rarely found free in nature but rather occur as part of the structure of more complex lipid molecules. Saturated and Unsaturated Fatty Acids The carbon chain of a fatty acid may or may not contain carbon–carbon double bonds. On the basis of this consideration, fatty acids are classified as saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), or polyunsaturated fatty acids (PUFAs). A saturated fatty acid is a fatty acid with a carbon chain in which all carbon– carbon bonds are single bonds. The structural formula for the 16-carbon SFA is H H H H H H H H H H H H H H H O A A A A A A A A A A A A A A A B HO COCOCOCOCOCOCO COCOCOCOCOCOCO CO COOH A A A A A A A A A A A A A A A H H H H H H H H H H H H H H H IUPAC name: hexadecanoic acid Common name: palmitic acid The structural formula for a fatty acid is usually written in a more condensed form than the preceding structural formula. Two alternative structural notations for palmitic acid are O B CH3 O (CH2 )14 OCOOH and COOH (Line-angle structural formulas were first encountered in Section 12.9.) A monounsaturated fatty acid is a fatty acid with a carbon chain in which one carbon–carbon double bond is present. In biochemically important MUFAs, the  19.2 Types of Fatty Acids 657 configuration about the double bond is nearly always cis (Section 13.5). Different ways of depicting the structure of a MUFA follow. H H H A A H H A OCP COC A AH H H A C A OCO A H H A OCO A H A CO A H H A C A H H A CO A H H A OCO A H H A CO A H More than 500 different fatty acids H A OC A H H A C A have been isolated from the lipids A C A H H A CO A H H A OCO of microorganisms, plants, animals, CO A H H A C KO and humans. These fatty acids dif- HO A H H H fer from one another in the length H OH of their carbon chains, their degree of unsaturation (number of double O bonds), and the positions of the B CH3 O (CH2 )7 O CHPCHO (CH2 )7 OCOOH double bonds in the chains. COOH IUPAC name: cis-9-octadecenoic acid Common name: oleic acid The first of these structures correctly emphasizes that the presence of a cis double The fatty acids present in naturally bond in the carbon chain puts a rigid 308 bend in the chain. Such a bend affects the occurring lipids almost always have physical properties of a fatty acid, as discussed in Section 19.3. the following three characteristics: A polyunsaturated fatty acid is a fatty acid with a carbon chain in which two or 1. An unbranched carbon chain more carbon–carbon double bonds are present. Up to six double bonds are found in 2. An even number of carbon atoms in the carbon chain biochemically important PUFAs. 3. Double bonds, when present Fatty acids are nearly always referred to using their common names. IUPAC in the carbon chain, in a cis names for fatty acids, although easily constructed, are usually quite long. These configuration two types of names for an 18-carbon PUFA containing cis double bonds in the 9 and 12 positions are as follows: IUPAC name: cis,cis-9,12-octadecadienoic acid Common name: linoleic acid Unsaturated Fatty Acids and Double-Bond Position A numerically based shorthand system exists for specifying key structural param- eters for fatty acids. In this system, two numbers separated by a colon are used to specify the number of carbon atoms and the number of carbon–carbon double bonds present. The notation 18:0 denotes a C18 fatty acid with no double bonds, whereas the notation 18:2 signifies a C18 fatty acid in which two double bonds are present. To specify double-bond positioning within the carbon chain of an unsaturated fatty acid, the preceding notation is expanded by adding the Greek capital letter delta (D) followed by one or more superscript numbers. The notation 18:3(D9,12,15) denotes a C18 PUFA with three double bonds at locations between carbons 9 and 10, 12 and 13, and 15 and 16. 18 16 15 13 12 10 9 7 5 3 COOH 17 14 11 8 6 4 2 9 MUFAs are usually D acids, and the first two additional double bonds in PUFAs are generally at the D12 and D15 locations. [A notable exception to this generalization is the biochemically important arachidonic acid, a PUFA with the structural parameters 20:4(D5,8,11,14).] Denoting double-bond locations using this “delta notation” always assumes a numbering system in which the carboxyl carbon atom is C-1. Several different “families” of unsaturated fatty acids exist. These family rela- tionships become apparent when double-bond position is specified relative to the methyl (noncarboxyl) end of the fatty acid carbon chain. Double-bond positioning 658 Chapter 19 Lipids determined in this manner is denoted by using the Greek lowercase letter omega (v). An omega-3 fatty acid is an unsaturated fatty acid with its endmost double bond three carbon atoms away from its methyl end. An example of an omega-3 fatty acid is 1 3 COOH -3 (20:5) 2 An omega-6 fatty acid is an unsaturated fatty acid with its endmost double bond six carbon atoms away from its methyl end. The following three acids all belong to the omega-6 fatty acid family. 6 v -6 (14:1) 1 COOH 6 COOH v -6 (18:2) 1 6 v -6 (20:3) 1 COOH The structural feature common to these omega-6 fatty acids is highlighted with color in the preceding structural formulas. All the members of an omega family of fatty acids have structures in which the same “methyl end” is present. Table 19.1 gives the names and structures of the fatty acids most commonly encountered as building blocks in biochemically important lipid structures, as well as the “delta” and “omega” notations for the acids. Table 19.1 Selected Fatty Acids of Biological Importance Structural Notation Common Name Structure Saturated Fatty Acids 12:0 lauric acid COOH 14:0 myristic acid COOH 16:0 palmitic acid COOH 18:0 stearic acid COOH 20:0 arachidic acid COOH Monounsaturated Fatty Acids 16:1 D9 v-7 palmitoleic acid COOH 18:1 D9 v-9 oleic acid COOH Polyunsaturated Fatty Acids COOH 9,12 linoleic acid 18:2 D v-6 COOH 9,12,15 linolenic acid 18:3 D v-3 COOH 20:4 D5,8,11,14 v-6 arachidonic acid COOH 5,8,11,14,17 EPA (eicosapentaenoic acid) 20:5 D v-3 22:6 D4,7,10,13,16,19 v-3 DHA (docosahexaenoic acid) COOH  19.3 Physical Properties of Fatty Acids 659 EX AM PLE 19. 1 Classifying Fatty Acids on the Basis of Structural Characteristics Classify the fatty acid with the following structural formula in the ways indicated. COOH a. What is the type designation (SFA, MUFA, or PUFA) for this fatty acid? b. On the basis of carbon chain length and degree of unsaturation, what is the numerical shorthand designation for this fatty acid? c. To which “omega” family of fatty acids does this fatty acid belong? d. What is the “delta” designation for the carbon chain double-bond locations for this fatty acid? Solution a. Two carbon–carbon double bonds are present in this molecule, which makes it a polyunsaturated fatty acid (PUFA). b. Eighteen carbon atoms and two carbon–carbon double bonds are present. The shorthand numerical designation for this fatty acid is thus 18:2. c. Counting from the methyl end of the carbon chain, the first double bond encoun- tered involves carbons 6 and 7. This fatty acid belongs to the omega-6 family of fatty acids. d. Counting from the carboxyl end of the carbon chain, with C-1 being the carboxyl group, the double-bond locations are 9 and 12. This is a D9,12 fatty acid. Practice Exercise 19.1 Classify the fatty acid with the following structural formula in the ways indicated. COOH a. What is the type designation (SFA, MUFA, or PUFA) for this fatty acid? b. On the basis of carbon chain length and degree of unsaturation, what is the numerical shorthand designation for this fatty acid? c. To which “omega” family of fatty acids does this fatty acid belong? d. What is the “delta” designation for the carbon chain double-bond location for this fatty acid? Answers: a. MUFA (monounsaturated fatty acid); b. 12:1 fatty acid; c. omega-3 fatty acid (v-3); d. delta-9 fatty acid (D9) 19.3 Physical Properties of Fatty Acids The physical properties of fatty acids, and of lipids that contain them, are largely determined by the length and degree of unsaturation of the fatty acid carbon chain. Water solubility for fatty acids is a direct function of carbon chain length; sol- Fatty acids have low water solubili- ubility decreases as carbon chain length increases. Short-chain fatty acids have a ties, which decrease with increasing carbon chain length; at 308C, lauric slight solubility in water. Long-chain fatty acids are essentially insoluble in water. acid (12:0) has a water solubility of The slight solubility of short-chain fatty acids is related to the polarity of the 0.063 g/L and stearic acid (18:0) a carboxyl group present. In longer-chain fatty acids, the nonpolar nature of the solubility of 0.0034 g/L. Contrast hydrocarbon chain completely dominates solubility considerations. this with glucose’s solubility in water Melting points for fatty acids are strongly influenced by both carbon chain at the same temperature, 1100 g/L. length and degree of unsaturation (number of double bonds present). Figure 19.2 shows melting-point variation as a function of both of these variables. As carbon chain length increases, melting point increases. This trend is related to the greater surface area associated with a longer carbon chain and to the increased opportu- nities that this greater surface area affords for intermolecular attractions between fatty acid molecules. 660 Chapter 19 Lipids Figure 19.2 The melting point 90 of a fatty acid depends on the length of the carbon chain and 80 on the number of double bonds Stearic acid present in the carbon chain. 70 60 Temperature (°C) Saturated fatty acids 50 40 30 Room temperature 20 Oleic acid (1 double bond) 10 0 Linoleic acid (2 double bonds) –10 Linolenic acid (3 double bonds) –20 4 6 8 10 12 14 16 18 20 22 24 Number of carbon atoms A trend of particular significance is that saturated fatty acids have higher melting points than unsaturated fatty acids with the same number of carbon atoms. The greater the degree of unsaturation, the greater the reduction in melting points. Figure 19.2 shows this effect for the 18-carbon acids with zero, one, two, and three double bonds. Long-chain saturated fatty acids tend to be solids at room temperature, whereas long-chain unsaturated fatty acids tend to be liquids at room temperature. The decreasing melting point associated with increasing degree of unsaturation in fatty acids is explained by decreased molecular attractions between carbon chains. The double bonds in unsaturated fatty acids, which generally have the cis config- uration, produce “bends” in the carbon chains of these molecules (Figure 19.3). These “bends” prevent unsaturated fatty acids from packing together as tightly as saturated fatty acids. The greater the number of double bonds, the less efficient the packing. As a result, unsaturated fatty acids always have fewer intermolecular attractions, and therefore lower melting points, than their saturated counterparts. Figure 19.3 Space-filling models of four 18-carbon fatty acids, which differ in the number of double bonds present. Note how the presence of double bonds changes the shape of the molecule. Stearic acid (18:0) Oleic acid (18:1) Linoleic acid (18:2) Linolenic acid (18:3)  19.4 Energy-Storage Lipids: Triacylglycerols 661 19.4 Energy-Storage Lipids: Triacylglycerols With the notable exception of nerve cells, human cells store small amounts of en- ergy-providing materials for use when energy demand is high. The most widespread energy-storage material within cells is the carbohydrate glycogen (Section 18.15); it is present in small amounts in most cells. Lipids known as triacylglycerols also function within the body as energy-storage materials. Rather than being widespread, triacylglycerols are concentrated primarily in special cells (adipocytes) that are nearly filled with the material. Adipose tissue containing these cells is found in various parts of the body: under the skin, in the abdominal cavity, in the mammary glands, and around various organs (Figure 19.4). Manfred Kage/Peter Arnold, Inc. Triacylglycerols are much more efficient at storing energy than is glycogen because large quantities of them can be packed into a very small volume. These energy- storage lipids are the most abundant type of lipid present in the human body. In terms of functional groups present, triacylglycerols are triesters; three ester functional groups are present. Recall from Section 16.10 that an ester is a compound produced from the reaction of an alcohol with a carboxylic acid. The alcohol involved in triacylglycerol formation is always glycerol, a three-carbon alcohol with three hydroxyl groups. Figure 19.4 An electron micro- CH2 OOH A graph of adipocytes, the body’s CHOOH triacylglycerol-storing cells. Note A the bulging spherical shape. CH2 OOH Glycerol Fatty acids are the carboxylic acids involved in triacylglycerol formation. In the esterification reaction producing a triacylglycerol, a single molecule of glycerol re- acts with three fatty acid molecules; each of the three hydroxyl groups present is esterified. Figure 19.5 shows the triple esterification reaction that occurs between glycerol and three molecules of stearic acid (18:0); note the production of three molecules of water as a by-product of the reaction. Two general ways to represent the structure of a triacylglycerol are O Triacylglycerols do not actually con- G Fatty acid tain glycerol and three fatty acids, as CH2 O C R Ester the block diagram for a triacylglycerol l linkage y O implies. They actually contain a c glycerol residue and three fatty acid Fatty acid CH O C R9 residues. In the formation of the e r O triacylglycerol, three molecules of o water have been removed from the l Fatty acid CH2 O C R0 structural components of the tri- acylglycerol, leaving residues of the The first representation, a block diagram, shows the four subunits (building blocks) reacting molecules. present in the structure: glycerol and three fatty acids. The second representation, a general structural formula, shows the three ester linkages present in a triacylglyc- erol. Each of the fatty acids is attached to glycerol through an ester linkage. H O H O Figure 19.5 Structure of the + H2O simple triacylglycerol produced H C O H H O C H C O C from the triple esterification reaction between glycerol and three molecules of stearic acid O O (18:0 acid). Three molecules of H C O H H O C H C O C + H2O water are a by-product of this reaction. O O H C O H H O C H C O C + H2O H H Glycerol Three fatty acids Triester of glycerol Three water molecules 662 Chapter 19 Lipids Figure 19.6 Structure of a mixed H O triacylglycerol in which three (18:0 fatty acid) different fatty acid residues are H C O C present. O (18:1 fatty acid) H C O C O (18:2 fatty acid) H C O C H Formally defined, a triacylglycerol is a lipid formed by esterification of three fatty acids to a glycerol molecule. Within the name triacylglycerol is the term acyl. An acyl group, previously defined and considered in Section 16.1, is the portion of a carboxylic acid that remains after the !OH group is removed from the carboxyl carbon atom. The structural representation for an acyl group is O B RO C O An acyl group Thus, as the name implies, triacylglycerol molecules contain three fatty acid resi- dues (three acyl groups) attached to a glycerol residue. An older name that is still frequently used for a triacylglycerol is triglyceride. The triacylglycerol produced from glycerol and three molecules of stearic acid (shown in Figure 19.5) is an example of a simple triacylglycerol. A simple triacyl- glycerol is a triester formed from the esterification of glycerol with three identical fatty acid molecules. If the reacting fatty acid molecules are not all identical, then the result is a mixed triacylglycerol. A mixed triacylglycerol is a triester formed from the esterification of glycerol with more than one kind of fatty acid molecule. Figure 19.6 shows the structure of a mixed triacylglycerol in which one fatty acid is saturated, another monounsaturated, and the third polyunsaturated. Naturally occurring simple triacylglycerols are rare. Most biochemically important triacylglycerols are mixed triacylglycerols. E XAMP L E 19. 2 Drawing the Structural Formula of a Triacylglycerol Draw the structural formula of the triacylglycerol produced from the reaction between glycerol and three molecules of myristic acid. Solution Table 19.1 shows that myristic acid is the 14:0 fatty acid. Draw the structure of glyc- erol and then place three molecules of myristic acid alongside the glycerol. The fatty acid placements should be such that their carboxyl groups are lined up alongside the hydroxyl groups of glycerol. Form an ester linkage between each carboxyl group and a glycerol hydroxyl group with the accompanying production of a water molecule. O O CH2 OH HO C CH2 O C O O CH OH + HO C CH O C + 3H2O O O CH2 OH HO C CH2 O C Glycerol Fatty acids (14:0) Triacylglycerol  19.4 Energy-Storage Lipids: Triacylglycerols 663 Practice Exercise 19.2 Draw the structural formula of the triacylglycerol produced from the reaction between glycerol and three molecules of lauric acid. Answer: O O CH2 OH HO C CH2 O C O O CH OH + HO C CH O C + 3H2O O O CH2 OH HO C CH2 O C Glycerol Fatty acids (12:0) Triacylglycerol Fats and Oils Fats are naturally occurring mixtures of triacylglycerol molecules in which many different kinds of triacylglycerol molecules are present. Oils are also naturally occurring mixtures of triacylglycerol molecules in which there are many differ- ent kinds of triacylglycerol molecules present. Given that both are triacylglycerol mixtures, what distinguishes a fat from an oil? The answer is physical state at room temperature. A fat is a triacylglycerol mixture that is a solid or a semi-solid at room temperature (258C). Generally, fats are obtained from animal sources. An oil is a triacylglycerol mixture that is a liquid at room temperature (258C). Generally, oils are obtained from plant sources. Because they are mixtures, no fat or oil can be rep- resented by a single specific chemical formula. Many different fatty acids are pres- ent in the triacyl-glycerol molecules found in the mixture. The actual composition of a fat or oil varies even for the species from which it is obtained. Composition de- pends on both dietary and climatic factors. For example, fat obtained from corn-fed hogs has a different overall composition than fat obtained from peanut-fed hogs. Flaxseed grown in warm climates gives oil with a different composition from that obtained from flaxseed grown in colder climates. Additional generalizations and comparisons between fats and oils follow. 1. Fats are composed largely of triacylglycerols in which saturated fatty acids Petroleum oils (Section 12.15) are predominate, although some unsaturated fatty acids are present. Such triac- structurally different from lipid oils. The former are mixtures of alkanes ylglycerols can pack closely together because of the “linearity” of their fatty and cycloalkanes. The latter are mix- acid chains (Figure 19.7a), thus causing the higher melting points associated tures of triesters of glycerol. with fats. Oils contain triacylglycerols with larger amounts of mono- and polyunsaturated fatty acids than those in fats. Such triacylglycerols cannot pack as tightly together because of “bends” in their fatty acid chains (Figure 19.7b). The result is lower melting points. Figure 19.7 Representative triacylglycerols from (a) a fat and (b) an oil. (a) (b) 664 Chapter 19 Lipids Figure 19.8 Percentages of Canola oil 6% 58% 36% saturated, monounsaturated, Safflower oil 9% 13% 78% and polyunsaturated fatty acids in the triacylglycerols of various Sunflower oil 11% 20% 69% dietary fats and oils. Avocado oil 12% 74% 14% Corn oil 13% 25% 62% Dietary Oil or Fat Olive oil 14% 77% 9% Soybean oil 15% 24% 61% Peanut oil 18% 48% 34% Cottonseed oil 27% 19% 54% Pork fat 41% 47% 12% Palm oil 51% 39% 10% Beef fat 52% 44% 4% Butterfat 66% 30% 4% Coconut oil 92% 6% Saturated Monounsaturated Polyunsaturated 2% 2. Fats are generally obtained from animals; hence the term animal fat. Although fats are solids at room temperature, the warmer body temperature of the living animal keeps the fat somewhat liquid (semi-solid) and thus allows for movement. Oils typically come from plants, although there are also fish oils. A fish would have some serious problems if its triacylglycerols “solidified” when it encountered cold water. 3. Pure fats and pure oils are colorless, odorless, and tasteless. The tastes, odors, and colors associated with dietary plant oils are caused by small amounts of other naturally occurring substances present in the plant that have been car- ried along during processing. The presence of these “other” compounds is Fats contain both saturated and usually considered desirable. unsaturated fatty acids. Oils also contain both saturated and unsatu- Figure 19.8 gives the percentages of saturated, monounsaturated, and poly- rated fatty acids. The difference unsaturated fatty acids found in common dietary oils and fats. In general, a higher between a fat and an oil lies in degree of fatty acid unsaturation is associated with oils than with fats. A notable which type of fatty acid is more exception to this generalization is coconut oil, which is highly saturated. This oil prevalent. In fats, saturated fatty acids are more prevalent; in oils, is a liquid not because it contains many double bonds within the fatty acids but unsaturated fatty acids are more because it is rich in shorter-chain fatty acids, particularly lauric acid (12:0). prevalent. 19.5 Dietary Considerations and Triacylglycerols In recent years, considerable research has been carried out concerning the role of dietary factors as a cause of disease (obesity, diabetes, cancer, hypertension, and A grain- and vegetable-rich diet that atherosclerosis). Numerous studies have shown that, in general, nations whose contains small amounts of extra- citizens have high dietary intakes of triacylglycerols (fats and oils) tend to have virgin olive oil (three to four tea- higher incidences of heart disease and certain types of cancers. This is the reason spoons daily) has been found to help for concern that the typical American diet contains too much fat and the call for people with high blood pressure Americans to reduce their total dietary fat intake. reduce the amount of blood pressure medication they require, on average, Contrary to the general trend, however, there are several areas of the world by 48%. Substitution of sunflower oil where high dietary fat intake does not translate into high risks for cardiovascu- for the olive oil resulted in only a lar disease, obesity, and certain types of cancers. These exceptions, which include 4% reduction in medication dosage. some Mediterranean countries and the Inuit people of Greenland, suggest that The blood-pressure-reduction relationships between dietary triacylglycerol intake and risk factors for disease benefits of olive oil do not relate to the triacylglycerols present but rather involve more than simply the total amount of triacylglycerols consumed. come from other compounds natu- rally present, namely from antioxi- “Good Fats” Versus “Bad Fats” dant polyphenols olive oil contains. These antioxidants help promote the In dietary discussions, the term fat is used as a substitute for the term triacylglyc- relaxation of blood vessels. erol. Thus a dietary fat can be either a “fat” or an “oil.” Ongoing studies indicate  19.5 Dietary Considerations and Triacylglycerols 665 that both the type of dietary fat consumed and the amount of dietary fat consumed are important factors in determining human body responses to dietary fat. Current dietary fat recommendations are that people limit their total fat intake to 30% of total calories—with up to 15% coming from monounsaturated fat, up to 10% from polyunsaturated fat, and less than 10% from saturated fats. These recommendations imply correctly that different types of dietary fat Freshly pressed extra-virgin olive oil have different effects. In simplified terms, research studies indicate that saturated contains a compound that has the same pharmacological activity as fats are “bad fat,” monounsaturated fats are “good fat,” and polyunsaturated fats the over-the-counter pain reliever can be both “good fat” and “bad fat.” In the latter case, fatty acid omega clas- ibuprofen (Section 16.4). This find- sification (Section 19.2) becomes important, a situation addressed later in this ing suggests a possible explanation section. Studies indicate that saturated fat can increase heart disease risk, that for some of the various health ben- monounsaturated fat can decrease both heart disease and breast cancer risk, and efits attributed to a Mediterranean diet that typically is rich in olive that polyunsaturated fat can reduce heart disease risk but promote the risk of oil. It is estimated that this olive oil certain types of cancers. compound, called oleocanthal, is Referring to Figure 19.8, note the wide variance in the three general types of present in a typical Mediterranean fatty acids (SFAs, MUFAs, and PUFAs) present in various kinds of dietary fats. diet in an amount equivalent to Dietary fats high in “good” monounsaturated fatty acids include olive, avocado, about 10% of the ibuprofen dose recommended for headache relief. and canola oils. Monounsaturated fatty acids help reduce the stickiness of blood platelets. This helps prevent the formation of blood clots and may also dissolve clots once they form. O O Many people do not realize that most tree nuts and peanuts are good sources B B G O H of MUFAs. The focus on relevancy feature Chemical Connections 19-A on the HO H B next page looks at recent research on the fat content of nuts. O Oleocanthal Omega-3 and Omega-6 Fatty Acids In the 1980s, researchers found that the Inuit people of Greenland exhibit a low incidence of heart disease despite having a diet very high in fat. This contrasts markedly with studies on the U.S. population, which show a correlation between a high-fat diet and a high incidence of heart disease. What accounts for the dif- ference between the two peoples? The Inuit diet is high in omega-3 fatty acids (from fish), and the U.S. diet is high in omega-6 fatty acids (from plant oils). An American consumes about double the amount of omega-6 fatty acids and half the amount of omega-3 fatty acids that an Inuit consumes. Several large studies now confirm that benefits can be derived from eating sev- eral servings of fish each week. The choice of fish is important, however. Not all fish are equal in omega-3 fatty acid content. Cold-water fish, also called fatty fi sh because of the extra amounts of fat they have for insulation against the cold, con- tain more omega-3 acids than leaner, warm-water fish. Fatty fish include albacore tuna, salmon, and mackerel (Figure 19.9). Leaner, warm-water fish, which include cod, catfish, halibut, sole, and snapper, do not appear to offer as great a positive effect on heart health as do their “fatter” counterparts. (Note that most of the fish used in fish and chips (e.g., cod, halibut) is on the low end of the omega-3 scale.) Table 19.2 gives the actual omega-3 fatty acid concentrations associated with vari- ous kinds of cold-water fish. Recommendations that the U.S. population increase their consumption of © IFA/Peter Arnold, Inc./ cold-water fish has sparked a demand for omega-3 fish oil supplements. Wild salmon populations are the primary source for such oil. With plunging salmon Photolibrary populations due to overfishing, disease, and pollution, concern is rising about a lack of adequate fish oil supplies. Research is well advanced in getting “fish oil without any fish.” Fish do not make the omega-3 fatty acids they have within themselves. Rather, they obtain these fatty acids from the algae that Figure 19.9 Fish that live in deep, they feed on. Genetic engineering experiments (Section 22.14) are underway in cold water—mackerel, herring, which the genes that allow algae to synthesize omega-3 fatty acids are incor- tuna, and salmon—are better porated into plants. In the future, land-based plants may become sources for sources of omega-3 fatty acids than omega-3 fatty acids. other fish. 666 Chapter 19 Lipids C HE MIC AL CONNECTIONS 19-A The Fat Content of Tree Nuts and Peanuts People who bypass the nut tray at holiday parties usually be- lieve a myth—that nuts are unhealthful high-fat foods. Indeed, nuts are high-fat food. However, the fat is “good fat” rather than “bad fat” (Section 19.5); that is, the fatty acids present © Harris Shiffman/Shutterstock.com are MUFAs and PUFAs rather than SFAs. In most cases, a handful of nuts is better for you than a cookie or bagel. Numerous studies now indicate that eating nuts can have a strong protective effect against coronary heart disease. The most improvement comes from adding small amounts of nuts—an ounce (3–4 teaspoons)—to the diet five or more times a week. Raw, dry-roasted, or lightly salted varieties are best. The recommendation of only one ounce of nuts per day relates to the high calorie content of nuts, which is 160 to 200 calories per ounce. The number of nuts and number of The fat in nuts is “good fat”; the unsatu- calories per ounce for common types of nuts is as follows: rated/saturated fatty acid ratio is higher than Nuts Calories that in most foods. 18 cashews 160 Their low amounts of saturated fatty acids are not the 20 peanuts 160 only reason why nuts help reduce the risk of coronary heart 47 pistachios 160 disease. Nuts also offer valuable antioxidant vitamins, min- 24 almonds 166 erals, and plant fiber protein. The protein content is highest 14 walnut halves 180 (18%–26%) in the cashew, pistachio, almond, and peanut; 8 Brazil nuts 186 here the amount of protein is about the same as in meat, fish, 12 hazelnuts 188 and cheese. The carbohydrate content of nuts is relatively 15 pecan halves 190 low, less than 10% in most cases. 12 macadamias 200 An unexpected discovery involving the anticancer drug Taxol and hazelnuts was made in the year 2000. The ac- The amount of fat present in nuts ranges from 74% in tive chemical component in this drug, paclitaxel, was found the macadamia nut, 68% in pecans, and 63% in hazelnuts to in hazelnuts. It was the first report of this potent chemical around 50% in nuts such as the almond, cashew, peanut, and being found in a plant other than in the bark of the Pacific pistachio, as is shown in the table below. yew tree, a slow-growing plant found in limited quantities in The different fatty acid fractions (SFAs, MUFAs, and the Pacific Northwest. Although the amount of the chemi- PUFAs) present in nuts also vary, but with definite trends. cal found in a hazelnut tree is about one-tenth that found in Unsaturated fatty acids always significantly dominate satu- yew bark, the effort required to extract paclitaxel from these rated fatty acids. The unsaturation/saturation ratio is high- sources is comparable. Because hazelnut trees are more com- est for hazelnuts (11.9), pecans (10.9), walnuts (9.0), and mon, this finding could reduce the cost of the commercial almonds (9.0) and is lowest for cashews (3.9). drug and make it more readily available. Fat and Fatty Acid Composition of Selected Nuts Total Fat (percentage SFA MUFA PUFA of weight) (percentage of total fat) UFA/SFA Ratio almonds 52 10 68 22 9.0 cashews 46 20 62 18 3.9 hazelnuts 63 8 82 10 11.9 macadamias 74 16 82 2 5.4 peanuts 49 15 51 34 5.7 pecans 68 8 66 26 10.9 pistachios 48 13 72 15 6.6 walnuts 62 10 24 66 9.0  19.5 Dietary Considerations and Triacylglycerols 667 Table 19.2 Omega-3 Fatty Acid Amounts Associated with Various Kinds of Cold-Water Fish Per 3.5-oz. Serving (raw) Omega-3s (grams)* mackerel 2.3 albacore tuna 2.1 herring, Atlantic 1.6 anchovy 1.5 salmon, wild king (Chinook) 1.4 salmon, wild sockeye (red) 1.2 tuna, bluefin 1.2 salmon, wild pink 1.0 salmon, wild Coho (silver) 0.8 oysters, Pacific 0.7 salmon, farm-raised Atlantic 0.6 swordfish 0.6 trout, rainbow 0.6 *Omega-3 content of fish can vary depending on harvest location and time of year. Essential Fatty Acids An essential fatty acid is a fatty acid needed in the human body that must be obtained from dietary sources because it cannot be synthesized within the body, in adequate amounts, from other substances. There are two essential fatty acids: linoleic acid and linolenic acid. Linoleic acid (18:2) is the primary member of the omega-6 acid family, and linolenic acid (18:3) is the primary member of the omega-3 acid family. Their structures are given in Table 19.1. These two acids (1) are needed for proper membrane structure and (2) serve In 2001, the FDA gave approval for as starting materials for the production of several nutritionally important longer- manufacturers of baby formula to chain omega-6 and omega-3 acids. When these two acids are missing from the diet, add the fatty acids DHA (docosa- hexaenoic acid) and AA (arachidonic the skin reddens and becomes irritated, infections and dehydration are likely to acid) to infant formulas. Human occur, and the liver may develop abnormalities. If the fatty acids are restored, then breast milk naturally contains these the conditions reverse themselves. Infants are especially in need of these acids for acids, which are important in brain their growth. Human breast milk has a much higher percentage of the essential and vision development. Because fatty acids than cow’s milk. not all mothers can breast-feed, health officials regulate the ingre- Linoleic acid is the starting material for the biosynthesis of arachidonic acid. dients in infant formula so that formula-fed babies get the next Linoleic acid (18:2) h arachidonic acid (20:4) best thing to mother’s milk. Omega-6 fatty acids Arachidonic acid is the major starting material for eicosanoids (Section 19.13), substances that help regulate blood pressure, clotting, and several other important body functions. Linolenic acid is the starting material for the biosynthesis of two additional omega-3 fatty acids. Linolenic acid (18:3) h EPA (20:5) h DHA (22:6) Omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are important con- stituents of the communication membranes of the brain and are necessary for nor- mal brain development. EPA and DHA are also active in the retina of the eye. 668 Chapter 19 Lipids C HE MIC AL CONNECTIONS 19-B Fat Substitutes Sugar substitutes (artificial sweeteners) have been an ac- heating cottonseed and/or soybean oil with sucrose in the cepted part of the diet of most people for many years. New presence of methyl alcohol. Chemically, olestra has a struc- since the 1990s are fat substitutes—substances that create ture somewhat similar to that of a triacylglycerol; sucrose the sensations of “richness” of taste and “creaminess” of takes the place of the glycerol molecule, and six to eight fatty texture in food without the negative effects associated with acids are attached by ester linkages to it rather than the three dietary fats (heart disease and obesity). Today, in most gro- fatty acids in a triacylglycerol. cery stores, sitting next to almost all high-fat foods on the CH2OOOC(O)EHEHEHEHE shelf are lower-fat counterparts (see accompanying photo). O In most cases, the lower-fat products contain fat substitutes. HEHEHEHEH(O)COO HEHEHEHEH(O)COO OOC(O)EHEHEHEHE O HEHEHEHEH(O)COOOCH2 O © Kristin Piljay/Alamy OOC(O)EHEHEHEHE CH2OOOC(O)EHEHEHEHE HEHEHEHEH(O)COO Olestra Foods that contain fat substitutes have less Unlike triacylglycerols, however, olestra cannot be hydrolyzed fat but not necessarily fewer calories. by the body’s digestive enzymes nor processed by colonic bacte- ria, and therefore passes through the digestive tract undigested. Food scientists have been trying to develop fat substitutes Olestra looks, feels, and tastes like dietary fat and can since the 1960s. Now available for consumer use are two substitute for fats and oils in foods such as shortenings, oils, types of fat substitutes: calorie-reduced fat substitutes and margarines, snacks, ice creams, and other desserts. It has the calorie-free substitutes. They differ in their chemical struc- same cooking properties as fats and oils. tures and therefore in how the body handles them. In the digestive tract, Olestra interferes with the absorption Simplesse, the best-known calorie-reduced fat substitute, of both dietary and body-produced cholesterol; thus it may received FDA marketing approval in 1990. It is made from lower total cholesterol levels. A problem with its use is that it the protein of fresh egg whites and milk by a procedure called also reduces the absorption of the fat-soluble vitamins A, D, E, microparticulation. This procedure produces tiny, round pro- and K. To avoid such depletion, Olestra is fortified with these tein particles so fine that the tongue perceives them as a fluid vitamins. Another problem with Olestra use is that in some rather than as the solid they are. Their fineness creates a sensa- individuals it can cause gastrointestinal irritation and/or diar- tion of smoothness, richness, and creaminess on the tongue. rhea. All products containing Olestra must carry the follow- In the body, Simplesse is digested and absorbed, contributing ing label: “Olestra may cause abdominal cramping and loose to energy intake. But 1 g of Simplesse provides 1.3 cal, compared stools. Olestra inhibits the absorption of some vitamins and with the 9 cal provided by 1 g of fat. Simplesse is used only to other nutrients. Vitamins A, D, E, and K have been added.” replace fats in formulated foods such as salad dressings, cheeses, The terminology used to describe products that contain sour creams, and other dairy products. Simplesse is unsuitable fat substitutes can be confusing. Fat-free means less than for frying or baking because it turns rubbery or rigid (gels) when 0.5 g of fat per serving. Low-fat means 3 g or less fat per 50 g heated. Consequently, it is not available for home use. serving. Reduced-fat or less-fat means at least 25% less fat Olestra, the best-known calorie-free fat substitute, re- per serving than the “regular” food. Calorie-free means less ceived FDA marketing approval in 1996. It is produced by than 0.5 kilocalories per serving. Table 19.3 gives pronunciation guidelines for the names of the two essential fatty acids and of the other acids mentioned that are biosynthesized from them. Fat Substitutes (Artificial Fats) In response to consumer demand for low-fat, low-calorie foods, food scientists have developed several types of “artificial fats.” Such substances replicate the taste, texture, and cooking properties of fats but are themselves not lipids. The focus on relevancy feature Chemical Connections 19-B above considers further the topic of fat substitutes (“artificial fats”).  19.6 Chemical Reactions of Triacylglycerols 669 Table 19.3 Biochemically Important Omega-3 and Omega-6 Fatty Acids Omega-3 Acids Omega-6-Acids linolenic acid (18:3) linoleic acid (18:2) (lin-oh-LEN-ic) (lin-oh-LAY-ic) eicosapentaenoic acid (20:5) arachidonic acid (20:4) (EYE-cossa-PENTA-ee-NO-ic) (a-RACK-ih-DON-ic) docosahexaenoic acid (20:6) (DOE-cossa-HEXA-ee-NO-ic) 19.6 Chemical Reactions of Triacylglycerols The chemical properties of triacylglycerols (fats and oils) are typical of esters and alkenes because these are the two functional groups present in triacylglycerols. Four important triacylglycerol reactions are hydrolysis, saponification, hydrogena- tion, and oxidation. Hydrolysis Hydrolysis of a triacylglycerol is the reverse of the esterification reaction by which Naturally occurring mono- and it was formed (see Figure 19.5). Triacylglyercol hydrolysis, when carried out in a diacylglycerols are seldom encoun- tered. Synthetic mono- and diacyl- laboratory setting, requires the presence of an acid or a base. Under acidic condi- glycerols are used as emulsifiers in tions, the hydrolysis products are glycerol and fatty acids. Under basic conditions, many food products. Emulsifiers the hydrolysis products are glycerols and fatty acid salts. prevent suspended particles in Within the human body, triacylglycerol hydrolysis occurs during the process colloidal solutions (Section 8.7) from of digestion. Such hydrolysis requires the help of enzymes (protein catalysts; Sec- coalescing and settling. Emulsifiers are usually present in so-called fat-free tion 21.1) produced by the pancreas. These enzymes cause the triacylglycerol to be cakes and other fat-free products. hydrolyzed in a stepwise fashion. First, one of the outer fatty acids is removed, then the other outer one, leaving a monoacylglycerol. In most cases, this is the end prod- uct of the initial digestion (hydrolysis) of the triacylglycerol. Sometimes, enzymes remove all three fatty acids, leaving a free molecule of glycerol. In situations where all three fatty acids are removed, the hydrolysis process is referred to as complete hydrolysis, which is depicted in Figure 19.10a. If one or more of the fatty acid residues remains attached to the glycerol, the hydrolysis pro- cess is called partial hydrolysis (Figure 19.10b). EX AM PLE 19. 3 Writing a Structural Equation for the Hydrolysis of a Triacylglycerol Write an equation for the acid-catalyzed hydrolysis of the following triacylglycerol. O CH2 O C O CH O C O CH2 O C (continued) 670 Chapter 19 Lipids Solution Three water molecules are required for the hydrolysis, one to interact with each of the ester linkages present in the triacylglycerol. Breaking of the three ester linkages pro- duces four product molecules: glycerol and three fatty acids. O O CH2 O C CH2 OH HO C O H+ O CH O C +3H O H CH OH + HO C Water O O CH2 O C CH2 OH HO C Triacylglycerol Glycerol Fatty acids Practice Exercise 19.3 Write a structural equation for the acid-catalyzed hydrolysis of the following tri- acylglycerol. O CH2 O C O CH O C O CH2 O C Answer: O O CH2 O C CH2 OH HO C O H+ O CH O C + 3H2O CH OH + HO C O O CH2 O C CH2 OH HO C Saponification Recall from Section 16.8 the Saponification (Section 16.15) is a reaction carried out in an alkaline (basic) solu- structural difference between a tion. For fats and oils, the products of saponification are glycerol and fatty acid salts. carboxylic acid and a carboxylic The overall reaction of triacylglycerol saponification can be thought of as acid salt. occurring in two steps. The first step is the hydrolysis of the ester linkages to pro- O O duce glycerol and three fatty acid molecules: B B R O CO OH R O CO O2 Na1 Fat or oil 1 3H2O 4 3 fatty acids 1 glycerol Carboxylic Carboxylic acid acid salt The second step involves a reaction between the fatty acid molecules and the base (usually NaOH) in the alkaline solution. This is an acid–base reaction that pro- duces water plus salts: 3 fatty acids 1 3NaOH 4 3 fatty acid salts 1 3H2O  19.6 Chemical Reactions of Triacylglycerols 671 Complete hydrolysis Figure 19.10 Complete Water H O H and partial hydrolysis of a H O H O triacylglycerol. H C O C H C O H HO C Water H O H Steam O O H C O C H+ H C O H HO C Water H O H O O H C O C H C O H HO C H H a Complete hydrolysis of a triacylglycerol produces glycerol and three fatty acid molecules. Partial hydrolysis Water H O H H O H O H C O C H C O H HO C Enzymes O O H C O C H C O C Water H O H O O H C O C H C O H HO C H H b Partial hydrolysis (during digestion) of a triacylglycerol produces a monoacylglycerol and two fatty acid molecules. Saponification of animal fat is the process by which soap was made in pioneer times. Soap making involved heating lard (fat) with lye (ashes of wood, an impure form of KOH). Today most soap is prepared by hydrolyzing fats and oils (animal fat and coconut oil) under high pressure and high temperature. Sodium carbonate is used as the base. The cleansing action of soap is related to the structure of the carboxylate ions present in the fatty acid salts of soap and the fact that these ions readily participate in micelle formation. A micelle is a spherical cluster of molecules in which the polar portions of the molecules are on the surface, and the nonpolar portions are located in the interior. The focus on relevancy feature Chemical Connections 19-C on the next page further discusses micelle formation as it relates to the cleansing action of soap. Hydrogenation Hydrogenation is a chemical reaction first encountered in Section 13.9. It involves hydrogen addition across carbon–carbon multiple bonds, which increases the de- gree of saturation as some double bonds are converted to single bonds. With this change, there is a corresponding increase in the melting point of the substance. Hydrogenation involving just one carbon–carbon bond within a fatty acid res- idue of a triacylglycerol can be diagrammed as follows: , CH2 O CH2 O CHP CHO CH2 O CH2 , 1 H2 , CH2 O CH2 O CH2 O CH2 O CH2 O CH2 , Portion of an unsaturated fatty acid The double bond has been converted to residue in a triacylglycerol containing a single bond; the degree of saturation one double bond has increased 672 Chapter 19 Lipids C HE MIC AL CONNECTIONS 19-C The Cleansing Action of Soap and Detergents Soaps are carboxylic acid salts (Section 16.8). They are thus Nonpolar substances, such as fats, oils, and greases, are ionic compounds, as are all salts. A representative structural insoluble in water. Soap or detergent affects the solubility of formula for a carboxylic acid salt is such substances in water. The nonpolar “tail” of the soap or O B CH3OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCOO2Na1 Detergents are also acid salts. They are, however, salts of detergent molecule interacts with (dissolves in) the insoluble sulfonic acids rather than carboxylic acids. Detergents were nonpolar substance, while the polar “head” of the soap or initially developed during World War II, a time when car- detergent molecule interacts with polar water molecules. The boxylic acid supplies available for soap making were very soap or detergent thus overcomes the nonpolar-polar solu- limited. Sulfonic acids were used as substitutes for carboxylic bility barrier. acids. The general structures for a sulfonic acid and a car- Soaps and detergents solubilize oily and greasy materi- boxylic acid (for comparision purposes) are als in the following manner: The nonpolar portion of the carboxylate or sulfonate ion dissolves in the nonpolar oil or O O B B grease, and the polar portion maintains its solubility in the ROSOOH ROCOOH polar water. B The penetration of the oil or grease by the nonpolar end O of the carboxylate or sulfonate ion is followed by the for- Sulfonic acid Carboxylic acid mation of micelles (see the accompanying diagram). The Detergents are synthetic structural analogs of soaps, as carboxyl sulfonyl groups (the micelle exterior) and water the following representative structural formula for a deter- molecules are attracted to each other, causing the solubiliz- gent molecule shows. ing of the micelle. O B CH3OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OCH2OSOO2Na1 B O Structurally, both soaps and detergents contain a very The micelles do not combine into larger drops because small positive ion (usually Na1 or K1) and a negative ion their surfaces are all negatively charged, and like charges that contains a very long carbon chain. The negative ion is repel each other. The water-soluble micelles are subsequently the “active ingredient” in both soaps and detergents. In aque- rinsed away, leaving a material devoid of oil and grease. ous solution, salt dissociation occurs, which releases the salt’s – COO– COO COO– constituent ions. This allows the carboxylate ions (soaps) COO– COO– and sulfonate ions (detergents) present to exert their effects. COO– COO– The cleansing action of soaps and detergents relates to Grease COO– COO– the “dual polarity” that carboxylate and sulfonate ions pos- COO– COO– sess. The long carbon chain present, which is called the “tail” of the ion, is nonpolar, whereas the small oxygen-containing COO– COO– group present, which is called the “head” of the ion, is polar. COO– COO– COO– COO– COO– COO– COO– oxygen-containing Long carbon chain group Nonpolar tail Polar head micelle The structural equation for the complete hydrogenation of a triacylglycerol in which all three fatty acid residues are oleic acid (18:1) is shown in Figure 19.11. Many food products are produced via partial hydrogenation. In partial hydroge- nation, some, but not all, of the double bonds present are converted into single