Chapter 12 Lecture Notes: Carbohydrates

Chemistry 108Chapter 12 Lecture NotesCarbohydratesChapter 12 Lecture Notes: CarbohydratesEducational Goals1. Given a Fischer projection of a monosaccharide, classify it as either aldoses or ketoses.2. Given a Fischer projection of a monosaccharide, classify it by the number of carbons it contains.3. Given a Fischer projection of a monosaccharide, identify it as a D-sugar or L-sugar.4. Given a Fischer projection of a monosaccharide, identify chiral carbons and determine the numberof stereoisomers that are possible.5. Identify four common types of monosaccharide derivatives.6. Predict the products when a monosaccharide reacts with a reducing agent or with Benedict’s reagent.7. Define the term anomer and explain the difference between α and β anomers.8. Understand and describe mutarotation.9. Given its Haworth projection, identify a monosaccharide either a pyranose or a furanose.10. Identify the anomeric carbon in Haworth structures.11. Compare and contrast monosaccharides, disaccharides, oligosaccharides, and polysaccharides.12. Given the structure of an oligosaccharide or polysaccharide, identify the glycosidic bond(s) andcharacterize the glycosidic linkage by the bonding pattern [for example: β(1 4)].13. Given the Haworth structures of two monosaccharides, be able to draw the disaccharide that isformed when they are connected by a glycosidic bond.14. Understand the difference between homopolysaccharides and heteropolysaccharides.15. Compare and contrast the two components of starch.16. Compare and contrast amylopectin and glycogen.17. Identify acetal and hemiacetal bonding patterns in carbohydrates.1

Chemistry 108Chapter 12 Lecture NotesCarbohydratesIntroduction to CarbohydratesCarbohydrates are also known as .Carbohydrates are an abundant biomolecule. More than 50% of the carbon in organic compounds is found in carbohydrates Plants use photosynthesis to store energy in , a simple sugar6 CO2 6 H2O Energy à C6H12O6 6 O2Carbohydrates are a large class of naturally occurring polyhydroxy and. (also known as simple sugars) are the simplest carbohydrates containing3-7 carbon atoms. A sugar containing:–an aldehyde is known as an–a ketone is known as aClassification of CarbohydratesCarbohydrates are grouped into 3 classes: are the simplest sugars and serve as the building blocks of largermolecules– Example: Glucose contain 2-10 monosaccharides bonded togetherblock residue)– Example: Sucrose contain more than 10 residues– Example: Complex Carbohydrates(building2

Chemistry 108Chapter 12 Lecture NotesCarbohydratesMonosaccharidesMonosaccharides are ketones or aldehydes with 3 or more carbons.Naming MonosaccharidesCarbohydrate nomenclature is unique to“sugar chemistry” — we do not namemonosaccharides using the IUPAC rules. Monosaccharide names end in“ose." Monosaccharides can be classifiedby: Carbonyl group:or ofcarbons: triose, tetrose, etc. Both: aldotriose, ketotriose, and soon 3

Chemistry 108Chapter 12 Lecture NotesCarbohydratesExamples:You try it:Name each of the following monosaccharides as an aldose or ketose & according to its number of C atoms.4

Chemistry 108Chapter 12 Lecture NotesCarbohydratesStereoisomers in CarbohydratesCarbohydrates are molecules since they have carbon atoms carrying fourdifferent groups.The simplest three-carbon sugar is glyceraldehyde. This sugar exists as a pair of . Enantiomers have the same physical properties except they behave differently inthe way they rotate polarized light and the way they are affected by catalysts.Remember: Compounds with n chiral carbon atoms has a maximum of 2n possible stereoisomers and half thatmany pairs of enantiomers (mirror images). The aldotetrose (below), has 2 chiral carbon atoms and a total of 22 4 possible stereoisomers (2 pairsof enantiomers).5

Chemistry 108Chapter 12 Lecture NotesCarbohydratesThe D and L Families of Sugars: Drawing Sugar MoleculesFischer Projections represent three-dimensional structures of stereoisomers on a flat page. A chiral carbon atom is represented in the Fisher projection as theof two crossed lines. Bonds that point the page are shown as horizontal lines. Bonds that curve the page are shown as verticallines.In a Fischer projection, the aldehyde or ketone carbonyl group of a monosaccharide is always placedtoward the top of the page. 6

Chemistry 108Chapter 12 Lecture NotesCarbohydratesExample:GlucoseMonosaccharides are divided into families: D form and L form sugars. D: the –OH group on the chiral C furthest from the C O comes out of the plane of paperand points to the . L: the –OH group on the chiral C furthest from the C O comes out of the plane of paperand points to the .7

Chemistry 108Chapter 12 Lecture NotesCarbohydratesMonosaccharidesWe will briefly survey some important pentoses and hexoses, and theirderivatives.D-glucose, also called or, is the most important monosaccharidein human metabolism.D-fructose, or fruit sugar, is most common natural ketose Honey is 40% fructoseMonosaccharide DerivativesIn a hydrogen atom replaces one or more of the -OH groups in amonosaccharide. D-ribose and its derivative D-2-deoxyribose(deoxy minus one oxygen atom) are found in variouscoenzymes and in DNA.8

Chemistry 108Chapter 12 Lecture NotesCarbohydratesMonosaccharide Derivatives (continued)In an -OH group of amonosaccharide has been replaced by an amino (-NH2) group. D-glucosamine is an example. D-glucosamine is an amino derivative in which an amino groupreplaces one hydroxyl groupIn the carbonyl group of a monosaccharide has been reduced to analcohol group.o Sorbitol is an example.Sorbitol and Xylitol are used as sweetenersRibitol is found in the coenzyme FAD9

Chemistry 108Chapter 12 Lecture NotesCarbohydratesMonosaccharide Derivatives(continued) In sugars, analdehyde group of a monosaccharide hasbeen oxidized to form a carboxyl groupD-Gluconic Acid is an example.Reactions of MonosaccharidesReactions of monosaccharides are reactions of carbonyl and hydroxyl groups (chapter 11). Aldehyde and ketone groups can be reduced Aldehyde and alcohol groups can be oxidizedReduction of Monosaccharides The reduction of the C O group inan aldehyde or ketone produces.Oxidation of MonosaccharidesThe oxidation of the aldehyde C O groupproduces.10

Chemistry 108Chapter 12 Lecture NotesCarbohydratesOxidation of Monosaccharides (continued)Benedict’s reagent is a copper compound that will oxidize only(aldoses) and not alcohols.A sugar that reacts withBenedict’s solution is called areducing sugar since it reducesthe ion Cu2 à Cu NOTE: Some ketoses give positive results for Benedict’s test because they toaldehydes in the strongly basic Benedict solution.11

Chemistry 108Chapter 12 Lecture NotesCarbohydratesMonosaccharides: Their Cyclic FormReview:A hydroxyl group in a monosaccharide can react with the carbonyl to form a Hemiacetals are made by the reaction of an with an .A hemiacetal contains a C atom bonded to an and an group.A monosaccharide contains both an alcohol and an aldehyde group. It can react with to form a cyclic hemiacetal.12

Chemistry 108Chapter 12 Lecture NotesCarbohydratesOpen Chain to Cyclic Form Mechanism (MECHANISM NOT ON EXAM)Cyclic forms of monosaccharides are usually drawn with the Haworth Projection in which the ring is viewedfrom the side at an angle. The edge of the ring closest to the viewer is drawn with a bold line for perspective. Substituents on the ring in a Haworth projection are either “up” or “down”The pair of cyclic hemiacetals with the OH on the hemiacetal carbon in different positions are called. For D-sugars, the α-anomer has the OH pointing For D-sugars, the β-anomer has the OH pointing13

Chemistry 108 Chapter 12 Lecture NotesCarbohydratesExample: The open-chain form of D-galactose with its cyclic anomers.In solution, the open chain and cyclic forms of a monosaccharide are in : If we start with a pure open chain or cyclic form in solution, the optical rotation of the solution willchange until equilibrium is achieved and the concentrations of the different forms remain constant. The change in optical rotation observed as the system approaches equilibrium is called.The cyclic forms of monosaccharidesnamed as derivatives of theheterocyclic ethers furan (5 members)pyran (6 members).can beand14

Chemistry 108Chapter 12 Lecture NotesCarbohydratesExample: The aldopentose D-ribose forms a cyclic (the deoxy form is also shown below)15

Chemistry 108Chapter 12 Lecture NotesCarbohydratesOligosaccharidesOligosaccharides are short polymers containing 2-10 monosaccharide .The residues are bonded to each other by.A glycosidic bond is the ether linkage formed when an acetal is made by reacting aof a monosaccharide with a on another sugar.16

Chemistry 108Chapter 12 Lecture NotesCarbohydratesThe glycosidic bond in maltose is referred to as an bond since the monosaccharide onthe left reacts it’s α-anomer hemiacetal at C-1 with a hydroxyl at C-4 on the second monosaccharide.Formation of α and β anomersThe glycosidic bond can be either or .–Example: Cellobiose is a disaccharide formed when the polysaccharide cellulose is broken down.– A disaccharide is composed of two monosaccharide residues.– Cellobiose is made by connecting two glucose molecule by a β(1à4) glycosidic bond.– Cellobiose cannot be used as a source of glucose by humans since we lack the enzyme tohydrolyze the glycosidic bond.17

Chemistry 108Chapter 12 Lecture NotesCarbohydratesExample: Lactose is a disaccharide found in milk Lactose consists of a galactose connected to a glucose residue by a β(1à4) glycosidic bond Sounds like cellobiose! But the OH on C-4 is up in galactose and down in glucoseLactose intolerance is the inability to hydrolyze lactose due to an enzyme deficiency.Sucrose, or table sugar, is a disaccharide with two twists:and . The glycosidic bond in sucrose is formed between the hemiacetal C of α-glucopyranose and thehemiacetal C of β-fructofuranose This is an α,β-(1 2) glycosidic bond18

Chemistry 108Chapter 12 Lecture NotesCarbohydratesNote that, as is the case with monosaccharides, the oligosaccharides can be in equilibrium with their anomersOnly the end, hemiacetal residue can open and close19

Chemistry 108Chapter 12 Lecture NotesCarbohydratesMost common oligosaccharides are disaccharidesThe following are found in peas and beansRaffinose(a trisaccharide composed of three monosaccharide residues)Stachyose(a tetrasaccharide composed of four monosaccharide residues)Verbascose (a pentasaccharide composed of four monosaccharide residues)These oligosaccharides are indigestible since they contain galactopyranose residues involved in α-(1à6)glycosidic bonds that humans lack the enzyme to hydrolyze.are sugar-containing lipids that: Are present in nerve cell membranes. Serve as identifying markers on cell surfaces.The hemiacetal of a sugar residue is connected to alcohol group of a lipid by a bond.20

Chemistry 108Chapter 12 Lecture NotesCarbohydrates“How Sweet It Is!” Sweetness is rated in comparison to sucrose, which is assigned a sweetness 100Sweeteners used in our foods can be divided into two classes: natural and artificial– Natural Sweeteners are sugars or derivatives– Artificial Sweeteners may bear no similarity to sugars!Artificial Sweeteners:21

Chemistry 108Chapter 12 Lecture NotesCarbohydratesPolysaccharidescontain 10 or more residueso In a , all the residues are the same monosaccharideo In a , the residues are built from more than one type ofmonosaccharideo The primary functions of polysaccharides are to: (e.g. cellulose) (e.g. starch and glycogen)Cellulose is a homopolysaccharide consisting of long, linear chains of glucose residues joined by β-(1à4)bonds Cellulose is so strong because the linear chains can form hydrogen bonds withadjacent chains forming sheets of the polymer Wood is about 50% cellulose Bacteria in horses, cows, andtermites have enzyme cellulase to hydrolyzeβ-(1à4) bonds22