Unit 1 Carbohydrates PDF

Chapter 7 Carbohydrates Chapter Objectives: Learn how to classify carbohydrates. Learn how to recognize molecules with chiral centers and draw Fischer projections. Learn how to classify the monosaccharides, and learn their chemical and physical properties. Lea...

Chapter 7 Carbohydrates Chapter Objectives: Learn how to classify carbohydrates. Learn how to recognize molecules with chiral centers and draw Fischer projections. Learn how to classify the monosaccharides, and learn their chemical and physical properties. Learn about the disaccharides and oligosaccharides. Learn the major types of polysaccharides and their structural and biological features. Mr. Kevin A. Boudreaux Angelo State University CHEM 2353 Fundamentals of Organic Chemistry Organic and Biochemistry for Today (Seager & Slabaugh) www.angelo.edu/faculty/kboudrea  Biochemistry Biochemistry is the study of the chemistry of biomolecules and living organisms. In organic chemistry, we organized our study of carbon-containing molecules by functional group (alcohol, alkene, ketone, carboxylic acid, etc.). In the first five chapters, we will take a look at several groups of important biological molecules, many of which have more than one functional group: carbohydrates (Ch. 7), lipids (Ch. 8), proteins and enzymes (Ch. 9 and 10), and nucleic acids (Ch. 11). We will also examine the synthesis of proteins (Ch. 11), nutrition (Ch. 12) and metabolism (Ch. 13, 14), and important body fluids (Ch. 15). 2 Classification of Carbohydrates 3  Carbohydrates and Biochemistry Carbohydrates are compounds of tremendous biological importance: – they provide energy through oxidation – they supply carbon for the synthesis of cell components – they serve as a form of stored chemical energy – they form part of the structures of some cells and tissues Carbohydrates, along with lipids, proteins, nucleic acids, and other compounds are known as biomolecules because they are closely associated with living organisms. 4  Carbohydrates Carbohydrates, or saccharides (saccharo is Greek for “sugar”) are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. O H CH2OH C C O CH2OH H C OH OH OH C H O HO C H OH H C OH H C OH OH H C OH H C OH OH CH2OH -D-glucose CH2OH fructose glucose 5  Carbohydrates Carbohydrates include not only sugar, but also the starches that we find in foods, such as bread, pasta, and rice. The term “carbohydrate” comes from the observation that when you heat sugars, you get carbon and water (hence, hydrate of carbon). 6  Classes of Carbohydrates Monosaccharides contain a single polyhydroxy aldehyde or ketone unit (e.g., glucose, fructose). Disaccharides consist of two monosaccharide units linked together by a covalent bond (e.g., sucrose). Oligosaccharides contain from 3 to 10 monosaccharide units (e.g., raffinose). 7  Classes of Carbohydrates Polysaccharides contain very long chains of hundreds or thousands of monosaccharide units, which may be either in straight or branched chains (e.g., cellulose, glycogen, starch). 8 The Stereochemistry of Carbohydrates 9  Two Forms of Glyceraldehyde Glyceraldehyde, the simplest carbohydrate, exists in two isomeric forms that are mirror images of each other: 10  Stereoisomers These forms are stereoisomers of each other. Glyceraldehyde is a chiral molecule — it cannot be superimposed on its mirror image. The two mirror- image forms of glyceraldehyde are enantiomers of each other. CHO CHO HO C H H C OH CH2OH CH2OH L-glyceraldehyde D-glyceraldehyde 11  Chirality and Handedness Chiral molecules have the same relationship to each other that your left and right hands have when reflected in a mirror. 12  Chiral Carbons Chiral objects cannot be superimposed on their mirror images — e.g., hands, gloves, and shoes. Achiral objects can be superimposed on the mirror images — e.g., drinking glasses, spheres, and cubes. Any carbon atom which is connected to four different groups will be chiral, and will have two nonsuperimposable mirror images; it is a chiral carbon or a center of chirality. – If any of the two groups on the carbon are the same, the carbon atom cannot be chiral. Many organic compounds, including carbohydrates, contain more than one chiral carbon. 13  Examples: Chiral Carbon Atoms Identify the chiral carbon atoms (if any) in each of the following molecules: H Cl Cl Br H C H H C H H C Cl H C Cl H Cl Cl F OH OH OH CH3 C CH3 CH3 C CH2Cl CH3 C CH2CH3 H H H O HO HO O CH3 C CH2CH3 14 Examples: Chiral Carbons in Carbohydrates Identify the chiral carbons (if any) in the following carbohydrates: O H C CHO CH2OH H C OH CH2OH CH OH C O HO C H O H C OH OH CH OH H C OH CH2OH H C OH OH OH H C OH erythrose CH2OH OH CH2OH glucose 15  2n Rule When a molecule has more than one chiral carbon, each carbon can possibly be arranged in either the right-hand or left-hand form, thus if there are n chiral carbons, there are 2n possible stereoisomers. Maximum number of possible stereoisomers = 2n Br H C Cl 21 = 2 possible stereoisomers F OH OH CH3 C C CH2OH 22 = 4 possible stereoisomers H H 16  Examples: Number of Stereoisomers What is the maximum number of possible stereo- isomers of the following compounds? Cl CH2OH O H H C H C O C Cl H C OH H C OH H C OH HO C H OH CH2OH H C OH CH3 C CH2CH3 H CH3 H H C OH CH3 C C CH2OH CH2OH H OH 17  Examples: Number of Stereoisomers What is the maximum number of possible stereo- isomers of the following compound? CH3 CH3 Cholesterol HO 18 28 = 256 possible stereoisomers. (But Nature makes only one!)  Fischer Projections Fischer projections are a convenient way to represent mirror images in two dimensions. Place the carbonyl group at or near the top and the last achiral CH2OH at the bottom. CHO CHO HO C H H C OH CH2OH CH2OH CHO CHO L = Left R = Right HO H H OH CH2OH CH2OH 19 L-glyceraldehyde D-glyceraldehyde  Naming Stereoisomers When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest from the carbonyl group: if the hydroxy group points to right when the carbonyl is “up” it is the D-isomer, and when the hydroxy group points to the left, it is the L-isomer. CHO CHO HO H H OH HO H H OH CH2OH CH2OH L-erythrose D-erythrose 20  Examples: Fischer Projections Draw Fischer projections of D and L lactic acid: Given the structure for D-glucose, draw CO2H the structure of L- glucose: CH3 CH OH CHO H OH Draw Fischer projections HO H of D and L alanine: H OH NH 2 CH3 CH CO 2H H OH CH2OH D-glucose 21  Examples: Fischer Projections Identify the following compounds as D or L isomers, and draw their mirror images. CH2OH CHO C O HO H HO H HO H H OH H OH H OH CH2OH lyxose CH2OH fructose 22 What’s So Great About Chiral Molecules? Molecules which are enantiomers of each other have exactly the same physical properties (melting point, boiling point, index of refraction, etc.) but not their interaction with polarized light. Polarized light vibrates only in one plane; it results from passing light through a polarizing filter. 23  Optical Activity A levorotatory (–) substance rotates polarized light to the left [e.g., l-glucose; (-)-glucose]. A dextrorotatory (+) substance rotates polarized light to the right [e.g., d-glucose; (+)-glucose]. Molecules which rotate the plane of polarized light are optically active. Many biologically important molecules are chiral and optically active. Often, living systems contain only one of the possible stereochemical forms of a compound, or they are found in separate systems. – L-lactic acid is found in living muscles; D-lactic acid is present in sour milk. – In some cases, one form of a molecule is beneficial, and the enantiomer is a poison (e.g., thalidomide). – Humans can metabolize D-monosaccharides but not L-isomers; only L-amino acids are used in protein synthesis 24 Monosaccharides 25  Classification of Monosaccharides The monosaccharides are the simplest of the carbohydrates, since they contain only one polyhydroxy aldehyde or ketone unit. Monosaccharides are classified according to the number of carbon atoms they contain: No. of Class of carbons Monosaccharide 3 triose 4 tetrose 5 pentose 6 hexose The presence of an aldehyde is indicated by the prefix aldo- and a ketone by the prefix keto-. 26  Classification of Monosaccharides Thus, glucose is an aldohexose (aldehyde + 6 Cs) and ribulose is a ketopentose (ketone + 5 Cs) O H C CH2OH H C OH C O HO C H H C OH H C OH H C OH H C OH CH2OH CH2OH ribulose a ketopentose glucose an aldohexose 27  Examples: Classifying Monosaccharides Classify the following monosaccharides: CH 2OH CHO CH2OH CHO C O HO H C O HO H H OH HO H H OH HO H HO H H OH CH2OH CH2OH CH 2OH CH2OH 28  The Family of D-aldoses (L-forms not shown) CHO H OH Aldotriose CH2OH 21 = 2 D-glyceraldehyde CHO CHO H OH HO H Aldotetroses H OH H OH 22 = 4 CH2OH CH2OH D-erythrose D-threose CHO CHO CHO CHO H OH HO H H OH HO H H OH H OH HO H HO H Aldopentoses H OH H OH H OH H OH 23 = 8 CH2OH CH2OH CH2OH CH2OH D-ribose D-arabinose D-xylose D-lyxose 29  The Family of D-aldoses (L-forms not shown) CHO CHO CHO CHO CHO H OH HO H H OH HO H H OH H OH H OH HO H HO H H OH Aldohexoses H OH H OH H OH H OH HO H 24 = 16 H OH H OH H OH H OH H OH CH2OH CH2OH CH2OH CH2OH CH2OH D-allose D-altrose D-glucose D-mannose D-gulose CHO CHO CHO HO H H OH HO H H OH HO H HO H HO H HO H HO H H OH H OH H OH CH2OH CH2OH CH2OH D-idose D-galactose D-talose 30  The Family of D-ketoses (L-forms not shown) CH2OH O Ketotriose CH2OH 20 = 1 Dihydroxyacetone CH2OH O Ketotetroses H OH 21 = 2 CH2OH D-erythrulose CH2OH CH2OH O O H OH HO H Ketopentoses H OH H OH 22 = 4 CH2OH CH2OH D-ribulose D-xylulose 31  The Family of D-ketoses (L-forms not shown) CH2OH CH2OH CH2OH CH2OH O O O O H OH HO H H OH HO H Ketohexoses HO H H OH H OH HO H 23 = 8 H OH H OH H OH H OH CH2OH CH2OH CH2OH CH2OH D-Psicose D-Fructose D-Sorbose D-Tagatose 32  Physical Properties of Monosaccharides Most monosaccharides have a sweet taste (fructose is sweetest; 73% sweeter than sucrose). They are solids at room temperature. They are extremely soluble in water: – Despite their high molecular weights, the presence of large numbers of OH groups make the monosaccharides much more water soluble than most molecules of similar MW. – Glucose can dissolve in minute amounts of water to make a syrup (1 g / 1 ml H2O). 33 Physical Properties of Monosaccharides Table 7.2 The relative sweetness of sugars (sucrose = 1.00) Sugar Relative Type Sweetness Lactose 0.16 Disaccharide Galactose 0.22 Monosaccharide Maltose 0.32 Disaccharide Xylose 0.40 Monosaccharide Glucose 0.74 Monosaccharide Sucrose 1.00 Disaccharide Invert sugar 1.30 Mixture of glucose and fructose Fructose 1.73 Monosaccharide 34  Chemical Properties of Monosaccharides Monosaccharides do not CH2OH usually exist in solution in H their “open-chain” forms: H C O an alcohol group can add H O C into the carbonyl group in OH H C the same molecule to form HO C C H a pyranose ring containing a stable cyclic hemiacetal H OH or hemiketal. D-glucose CH2OH CH2OH H C O C O OH H H O C H C C H C OH H OH H HO H HO OH C C C C a pyranose ring H OH cyclic H OH -D-glucose hemiacetals -D-glucose 35 -up -down  Glucose Anomers O H CH2OH C CH2OH 1 H C OH OH O O OH HO C H OH 1 1 H C OH OH OH OH H C OH Haworth OH OH structures CH2OH -D-glucose D-glucose -D-glucose 36% 0.02% 64% In the pyranose form of glucose, carbon-1 is chiral, and thus two stereoisomers are possible: one in which the OH group points down (-hydroxy group) and one in which the OH group points up (- hydroxy group). These forms are anomers of each other, and carbon-1 is called the anomeric carbon. 36  Fructose Anomers Fructose closes on itself to form a furanose ring: H CH2OH O O O OH CH2OH a furanose ring OH D-fructose -hydroxy CH2OH CH2OH CH2OH OH O O HO HO OH -hydroxy CH2OH OH OH -D-fructose -D-fructose 37  Drawing Furanose and Pyranose Rings Monosaccharides are often represented using the Haworth structures shown below for furanose and pyranose rings. The remaining OH groups on the ring point up or down depending on the identity of the sugar. always above ring for D-saccharides CH2OH CH2OH O O Furanose Pyranose ring ring 38  Examples: Anomers With Friends Like These, Who Needs Anomers? Identify the structures below as being the - or - forms, and draw the structure of their anomers: CH2OH OH CH2OH O OH O OH OH OH OH ribose OH galactose 39  Oxidation of Monosaccharides Aldehydes and ketones that have an OH group on the carbon next to the carbonyl group react with a basic solution of Cu2+ (Benedict’s reagent) to form a red-orange precipitate of copper(I) oxide (Cu2O). Sugars that undergo this reaction are called reducing sugars. (All of the monosaccharides are reducing sugars.) oxidation Reducing sugar + Cu2+ product + Cu2O deep blue red-orange solution ppt Benedict’s Reagent Copper(I) oxide (blue) (red-orange ppt) 40  Formation of Phosphate Esters O O – O P O CH2 – O P O CH2 OH O– O O OH O– HO CH2OH OH OH OH OH fructose 6-phosphate glucose 6-phosphate Phosphate esters can form at the 6-carbon of aldohexoses and aldoketoses. Phosphate esters of monosaccharides are found in the sugar-phosphate backbone of DNA and RNA, in ATP, and as intermediates in the metabolism of carbohydrates in the body. 41  Glycoside Formation The hemiacetal and hemiketal forms of monosaccharides can react with CH2OH alcohols to form acetal and ketal structures called glycosides. The O new carbon-oxygen bond is called OH the glycosidic linkage. OH OCH3 CH2OH OH O methyl -D-glycopyranoside OH + CH3OH H+ + CH2OH OH OH OCH3 O OH OH -D-glucose OH OH 42 methyl -D-glycopyranoside  Glycoside Formation Once the glycoside is formed, the ring can no longer open up to the open-chain form. Glycosides, therefore, are not reducing sugars. Glycoside + Cu2+ NR 43  Examples: Glycoside Formation Identify the glycosidic linkage in each of the following molecules: CH2OH CH2OH OCH 3 OH O O OH CH2OH OCH2CH3 OH OH OH 44  Important Monosaccharides CH2OH OH CH2OH OH O O OH OH OH -D-ribose CH2OH -D-deoxyribose Forms the sugar backbone of Forms the sugar backbone of ribonucleic acid (RNA) OH O OH deoxyribonucleic acid (DNA) OH OH -D-galactose Incorporated with glucose into lactose (milk sugar) 45  Important Monosaccharides CH2OH CH2OH OH O OH O HO OH CH2OH OH OH -D-fructose OH Also known as levulose and fruit -D-glucose sugar. Fructose is the sweetest of Also known as dextrose and blood the monosaccharides. It is present sugar; present in honey and fruits. in honey (1:1 ratio with glucose), Glucose is metabolized in the body fruits, and corn syrup. It is often for energy. Other sugars absorbed used to sweeten foods, since less into the body must be converted to fructose is needed to achieve the glucose by the liver. same degree of sweetness. 46 Disaccharides and Oligosaccharides 47  Disaccharides Two monosaccharides can be linked together through a glycosidic linkage to form a disaccharide. CH2OH CH2OH O O OH OH + OH OH OH OH OH OH -D-glucose -D-glucose cyclic hemiacetal CH2OH CH2OH O O OH OH + H2 O O OH OH OH OH (1→4) glycosidic linkage Maltose 48  Disaccharides Disaccharides can be hydrolyzed into their mono- saccharide building blocks by boiling them with dilute acids or reacting them with the appropriate enzymes. Disaccharides that contain hemiacetal groups are reducing sugars. H+ maltose + H2O D-glucose + D-glucose CH2OH CH2OH CH2OH CH2OH O O O OH H OH OH OH OH O O O OH OH OH OH OH OH OH Maltose reacts with Benedict's reagent 49  Important Disaccharides CH2OH (1→4) glycosidic linkage O CH2OH OH OH O O OH OH CH2OH CH2OH OH -D-glucose -D-galactose O O OH OH OH Lactose Also known as milk sugar. Lactose O constitutes 5% of cow's milk and OH OH -D-glucose 7% of human milk. It is digested by -D-glucoseOH OH the enzyme lactase. Pure lactose is Maltose found in whey, the watery Also known as malt sugar. It is byproduct of cheese production. produced in germinating grain (such as barley) as starch is broken down during malting, and is formed during the hydrolysis of starch to glucose during digestion. 50  Important Disaccharides C H 2 O H Sucrose  -D -g lu c o s e Also known as table sugar. Both anomeric O carbons of glucose and fructose are tied together O H in the glycosidic linkage; thus neither ring can open, and sucrose is not a reducing sugar.  −  →  −  O H g l y c o s i d i c Sucrose is found in fruits, nectar, sugar cane, and O C H 2O H O H lin k a g e sugar beets; maple syrup contains about 65% O sucrose, with glucose and fructose present as H O well. Caramel is the solid residue formed from heating sucrose. A flavoring agent called invert C H 2O H sugar is produced by the hydrolysis of sucrose  - D - f r u c t o s e under acidic conditions, which breaks it apart into O H glucose and fructose; invert sugar is sweeter than OH sucrose because of the fructose. Some of the sugar found in honey is formed in this fashion; O HO OH invert sugar is also produced in jams and jellies O OH HO prepared from acid-containing fruits. OH O OH OH 51  Oligosaccharides Oligosaccharides contain from 3 to 10 monosaccharide units. CH2OH HOCH2 O O CH2OH OH HO OH O O O OH CH2OH OH OH -D-fructose -D-glucose -D-galactose OH Raffinose An oligosaccharide found in peas and beans; largely undigested until reaching the intestinal flora in the large intestine, releasing hydrogen, carbon dioxide, and methane) 52 Polysaccharides 53  Polysaccharides Polysaccharides contain hundreds or thousands of carbohydrate units. Polysaccharides are not reducing sugars, since the anomeric carbons are connected through glycosidic linkages. We will consider three kinds of polysaccharides, all of which are polymers of glucose: starch, glycogen, and cellulose. 54  Starch Starch is a polymer consisting of D-glucose units. Starches (and other glucose polymers) are usually insoluble in water because of the high molecular weight. – Because they contain large numbers of OH groups, some starches can form thick colloidal dispersions when heated in water (e.g., flour or starch used as a thickening agent in gravies or sauces). There are two forms of starch: amylose and amylopectin. 55  Starch — Amylose Amylose consists of long, unbranched chains of glucose (from 1000 to 2000 molecules) connected by (1→4) glycosidic linkages. CH2OH CH2OH CH2OH O O O OH OH OH O O O O OH OH OH (1→4) glycosidic linkage Amylose 10%-20% of the starch in plants is in this form. The amylose chain is flexible enough to allow the molecules to twist into the shape of a helix. Because it packs more tightly, it is slower to digest than other starches. 56  Starch — Amylose Amylose helices can trap molecules of iodine, forming a characteristic deep blue-purple color. (Iodine is often used as a test for the presence of starch.) 57  Starch — Amylopectin Amylopectin consists of long chains of glucose (up to 105 molecules) connected by (1→4) glycosidic linkages, with (1→6) branches every 24 to 30 glucose units along the chain. 80%-90% of the starch in plants is in this form. CH2OH CH2OH O O OH OH (1→6) branch point O O O OH OH CH2OH CH2OH CH2 CH2OH O O O O OH OH OH OH O O O O O OH OH OH OH (1→4) glycosidic linkage 58 Amylopectin  Glycogen Glycogen, also known as animal starch, is structurally similar to amylopectin, containing both (1→4) glycosidic linkages and (1→6) branch points. Glycogen is even more highly branched, with branches occurring every 8 to 12 glucose units. Glycogen is abundant in the liver and muscles; on hydrolysis it forms D-glucose, which maintains normal blood sugar level and provides energy. Starches (plants) Amylose – unbranched Amylopectin – branched Glycogen (animals) – highly branched 59  Cellulose Cellulose is a polymer consisting of long, unbranched chains of D-glucose connected by (1→4) glycosidic linkages; it may contain from 300 to 3000 glucose units in one molecule. CH2OH O CH2OH OH O O O CH2OH OH OH O O (1→4) glycosidic linkage OH OH O Cellulose OH 60  Cellulose Because of the -linkages, cellulose has a different overall shape from amylose, forming extended straight chains which hydrogen bond to each other, resulting in a very rigid structure. Cellulose is the most important structural polysaccharide, and is the single most abundant organic compound on earth. It is the material in plant cell walls that provides strength and rigidity; wood is 50% cellulose. Plant cell wall 61  Cellulose Most animals lack the enzymes needed to digest cellulose, but it does provide roughage (dietary fiber) to stimulate contraction of the intestines and help pass food through the digestive system. – Some animals, such as cows, sheep, and goats (ruminants), process cellulose using colonies of bacteria in the digestive system which are capable of breaking down cellulose, and a series of stomachs to give cellulose a longer time to digest. – Some other animals have a longer intestinal tract (e.g., horses), and others reprocess digested food (e.g., rabbits) to allow more time for the breakdown of cellulose to occur. Cellulose is important industrially, from its presence in wood, paper, cotton, cellophane, rayon, linen, nitrocellulose (guncotton), photographic films (cellulose acetate), etc. 62  Nitrocellulose, Celluloid, and Rayon Guncotton (German, schiessbaumwolle) is cotton which has been treated with a mixture of nitric and sulfuric acids. It was discovered by Christian Friedrich Schönbein in 1845, when he used his wife’s cotton apron to wipe up a mixture of nitric and sulfuric acids in his kitchen, which vanished in a flash of flame when it dried out over a fire. Schönbein attempted to market it as a smokeless powder, but it combusted so readily it was dangerous to handle. Eventually its use was replaced by cordite (James Dewar and Frederick Abel, 1891), a mixture of nitrocellulose, nitroglycerine, and petroleum jelly, which could be extruded into cords. Celluloid (John Hyatt, 1869) was the first synthetic plastic, made by combining partially nitrated cellulose with alcohol and ether and adding camphor to make it softer and more malleable. It was used in manufacturing synthetic billiard balls (as a replacement for ivory), photographic film, etc.; it was eventually replaced by less flammable plastics. Rayon (Louis Marie Chardonnet, 1884) consists of partially nitrated cellulose mixed with solvents and extruded through small holes, allowing the solvent to evaporate; rayon was a sensation when introduced since it was a good substitute for silk, but it was still highly flammable. 63  Dietary Fiber Dietary fiber consists of complex carbohydrates, such as cellulose, and other substances that make up the cell walls and structural parts of plants. Good sources of dietary fiber include cereal grains, oatmeal, fresh fruits and vegetables, and grain products. Soluble fiber, such as pectin, has a lower molecular weight, and is more water soluble. Soluble fiber traps carbohydrates and slows their digestion and absorption, thereby leveling out blood sugar levels during the day. Soluble fiber also helps to lower cholesterol levels by binding dietary cholesterol. Insoluble fiber, such as cellulose, provides bulk to the stool, which helps the body to eliminate solid wastes. 64  Chitin Chitin is a polymer of N-acetylglucosamine, an amide derivative of the amino sugar glucosamine, in which one of the OH groups is converted to an amine (NH2) group. The polymer is extremely strong because of the increased hydrogen bonding provided by the amide groups. CH OH 2 CH2OH O OH O OH CH2OH OH O OH O O NH2 CH2OH OH Glucosamine HN O CH2OH O O OH OH O HN O CH3 OH O OH HN O HN O CH3 CH3 N-Acetylglucosamine CH3 Chitin 65  Chitin Chitin is the main component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans and insects, and the beaks of cephalopods. The chitin is often embedded in either a protein matrix, or in calcium carbonate crystals. Since this matrix cannot expand easily, it must be shed by molting as the animal grows. 66 How Sweet It Is! Sugar Substitutes 67  Saccharin O NH S O O Saccharin The first of the artificial sweeteners, saccharin is noncaloric and about 500 times sweeter than sugar. It was discovered in 1879 by Constantine Fahlberg, a chemistry student at Johns Hopkins University working for Ira Remsen; he noticed that the bread he was eating was unusually sweet, and went back to his lab bench and tasted all of the compounds he had been working with that day to find the compound responsible. It was marketed commercially as a non-nutritive sweetener very quickly, especially for use by diabetics. It was banned in some areas for some time because it was a suspected carcinogen. 68  Aspartame (NutraSweet) O OCH3 O O N Aspartame (NutraSweet) H NH2 OH Aspartame (NutraSweet) is about 160 times sweeter than sugar; it is composed of the amino acids aspartic acid and phenylalanine, neither of which has a sweet taste. It was discovered at Searle by James Schlatter in 1965, who was preparing intermediates for the synthesis of a tetrapedtide for an anti-ulcer project. Schlatter had spilled some of the dipeptide intermediate on his hands, and noticed later that there was a strong, sweet taste; he went back to his bench and tasted the dipeptide and found that it indeed was extremely sweet. Aspartame is sensitive to heat, so it cannot be used in cooked foods, and it decomposes slowly in liquids, reducing their shelf life. 69  O H O N N– K+ S O- Na+ O S O CH3 O Cyclamate O (Sodium N-cyclohexylsulfamate) Acesulfame-K (Sunette, Sweet One) Discovered by Michael Sveda in Discovered accidentally at Hoechst 1937, who noticed that a cigar that AG when a chemist accidentally he was smoking in the lab tasted dipped his fingers into the chemicals especially sweet. It is about 30-50 that he was working with, and later times sweeter than sugar, and is licked his finger to pick up a piece of often used in combination with paper. It is 200 times sweeter than other artificial sweeteners. It is sugar, noncaloric, and is heat-stable heat-stable. Cyclamate was banned and can be used during cooking. It is by the FDA in 1970, although it is often used in combination with other still used in other countries. artificial sweetners, so that each one masks the others' aftertaste. 70  CH2OH H OH CH2OH CH2Cl Cl O O HO H OH HO O H OH "galactose" CH2Cl OH OH "fructose" H OH Sucralose (Splenda) A non-caloric artificial sweetener CH2OH approved by the FDA in 1998. Sorbitol The glucose in sucrose is replaced A sugar alcohol; incompletely by a galactose, and three of the absorbed during digestion, and OH groups are replaced by Cl contribute fewer calories than atoms. It is about 600 times carbohydrates; found naturally in sweeter than sucrose, and is non- fruits; used commercially in caloric and heat-stable. sugar-free candies, cookies, chewing gum, etc. 71  Steviol (Truvia, PureVia) Stevia rebaudiana (sweetleaf) is a plant in the sunflower family widely grown for its Stevia sweet leaves. The leaves contain a compound called steviol, which is attached via glycoside linkages to glucose OH molecules to form stevioside (250-300 HO times sweeter than sugar) and OH rebaudioside A (aka rebiana, contains an additional glucose group, 350-450 times O sweeter than sugar). The FDA put HO OH rebaudioside A on the GRAS list in 2009. O OH OH O O OH OH HO O HO Steviol HO O Stevioside O (a steviol glycoside) 72 OH O  The Maillard Reaction The Maillard Reaction occurs between foods containing carbohydrates and proteins under heating; it results in a complex mixture of products and the development of a brown color. In addition, many complex flavors are developed. It is observed in the grilling or browning of meats, the formation of crust on baked breads, toast and pretzels, the toasting of marshmallows, the frying of potatoes and onions, the boiling of maple syrup, the brewing of beer (malting of barley), the roasting of coffee and cocoa beans, the roasting of nuts, and other sources. There are hundreds of compounds produced during these reactions, not many of which are well- characterized. O O O N O O N H NH2 Furfural 2-A cetylpyrroline 6-A cetyl-2,3,4,5- A crylam ide tetrahydropyridine 73 The End 74