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1 Chem 352 - Lecture 7 Carbohydrates Question for the Day: Unlike amino acids, which owe their diversity to a diverse array of functional groups, monosaccharides feature primarily two functional groups, hydroxyl groups and either a ketone or aldehyde group. What, then, do monosaccharides owe their diversity to? Introduction to Carbohydrates 2 Carbohydrates are one of the four major classes of biological molecules, which include, ✦ Proteins ✦ Nucleic acids ✦ Carbohydrates ✦ Lipids Chem 352, Lecture 7 - Carbohydrates X Introduction to Carbohydrates 3 ✦ Carbohydrates represent a major source of energy for living organisms. ✦ They also play major structural, protective and communication roles. glucose Chem 352, Lecture 7 - Carbohydrates X Introduction to Carbohydrates 4 Carbohydrates are chemically simple, but can be structurally complex ✦ The simple sugars have the basis stoichiometric empirical formula(CH2O)n, where n = 3 to 7 Like amino acid, simple sugars (monosaccharides) can combine to form polymers. ✦ monosaccharides (monomer) ✦ oligosaccharides (several monomers linked together) ✦ polysaccharides (many monomers linked together Chem 352, Lecture 7 - Carbohydrates X 5 9.1 Monosaccharides Monosaccharides 6 Question: Monosaccharides are What is the name ending that is ✦ either Aldoses typically used to designate a carbohydrate? • polyhydroxylaldehydes A. -ase ✦ or Ketoses • polyhydroxylketones B. -ose Classes based on number of carbonsC. -ol ✦ triose (3) D. -al ✦ tetrose (4) E. -amine ✦ pentose (5) ✦ hexose (6) ✦ heptose (7) Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 7 Trioses ✦ L and D Glyceraldehyde • Contains a chiral carbon • Fischer projections ✦ Dihydroxyacetone Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 8 There are different ways to designate the stereochemistry of a chiral center: ✦ R vs S • Rectus versus Sinister - based on the atomic mass of the substituents ✦ d (+) vs l (-) • dextrorotatory versus levorotatory - based on the bending of plane polarized light ✦ D vs L • Based on how glyceraldehyde bends plane polarized light Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 9 Trioses ✦ L and D Glyceraldehyde enantiomers • Contains a chiral carbon • Fischer projections ✦ Dihydroxyacetone Chem 352, Lecture 7 - Carbohydrates X These are FischerMonosaccharides 10 Clicker Question:Projections Aldohexoses•Aldoses contain 4 chiral carbons. Therefore, how many different stereoisomers are members of this group of ✦ Trioses through hexoses monosaccharides? A. 4 B. 16 C. 8 D. 12 E. 32 Chem 352, Lecture 7 - Carbohydrates X 11 L-enantiomersMonosaccharides Trioses through hexoses ✦ This figure shows only the D–enantantiomers ✦ The L-enantiomers are mirror images of the D- enantiomers. ✦ Members of an enantiomeric pair are are distinguished using the chirial carbon that is furthest from the carbonyl group. ✦ Most of the monosaccharides that we will encounter are the D-enantiomers. L D D L L D L D Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 12 Question:Question:•Types of optical isomers WhatWhich is the of relationshipfollowing monosaccharides between D-glucose is an and epimer D-mannose? of glucose: ✦ Enantiomers are stereoisomers that are mirror images A. Enantiomersof one another B. Epimers✦ Epimer are stereoisomers having more than one chiral carbon that differ from one another at just one chiral C. Diastereomerscarbon. D. Not stereoisomers A.✦ DiastereomersB. are stereoisomersC. havingD. more than one chiral carbon that differ from one another at multiple chiral carbons. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 13 Ketoses ✦ Trioses through hexoses Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 14 •The ones to remember ✦ Aldoses • triose ✦ D-glyceraldehyde • tetrose ‣ D-erythrose • pentoses ✦ D-ribose • hexoses ✦ D-glucose ✦ D-mannose ✦ D-galactose Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 15 •The ones to remember ✦ Ketoses • trisoes ✦ dihydroxyacetone • pentoses ✦ D-ribulose ✦ D-xylulose • hexoses ✦ D-fructose Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 16 •Cyclization of aldoses and ketoses ✦ An aldehyde can react spontaneously with an alcohol to form a hemiacetal. ✦ A ketone can react spontaneously with an alcohol to form a hemiketal. ✦ Both of these reactions are addition reactions that lead to the formation of a new chiral carbon. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 17 •Cyclization of aldoses and ketoses ✦ For aldoses and ketoses, this reaction occurs intramolecularly and leads to a cyclic molecule. ✦ The chiral hemiacetal or hemiketal carbon is called the anomeric carbon. ≈≈ 0% 0% • The new stereoisomers are designated α (-OH down) and β anomers (-OH up). ✦ Haworth projections are used to represent the cyclic 21.5% 58.5% 6.5% 13.5% form of monosaccharides.36%36% 64%64% Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 18 •Cyclization of aldoses and ketoses ✦ The six-member rings are called pyranose rings ✦ The five-member rings are called furanose rings. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 19 •Cyclization of aldoses and ketoses ✦ pyranose rings • D-glucopyranose (aldohexose) • D-mannopyranose (aldohexose) • D-galactopyranose (aldohexose) ✦ furanose rings • D-fructofuranose (ketohexose) • D-ribofuranose (aldopentose) Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 20 Clicker Question: •Conformations of Monosaccharides Which of following conformations for β-D-glucosopyranose is predicted✦ Monosaccharides to be more stable: can have different conformations. A. B. Chem 352, Lecture 7 - Carbohydrates X 21 9.2 Derivatives of the Monosaccharides Monosaccharides 22 Derivatives of monosaccharides are produced by chemical modifications. ✦ Phosphate esters ✦ Deoxy sugars also called sialic acid • One of the hydroxyl groups is replaced with a hydrogen ✦ Amino sugars • One of the hydroxyl groups is replaced with an amino group. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 23 Sugar alcohols ✦ The aldehyde or ketone group is reduced to an alcohol. ✦ Some are used as natural artificial sweeteners. erythritol xylitol Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 24 Sugar acids ✦ The aldehyde can be oxidized to an acid to produce aldonic acids. • Which in turn can cyclize to form lactones. ✦ The 6-hydroxyl group on aldohexoses can also be oxidized to an acid to produce uronic acids . Chem 352, Lecture 7 - Carbohydrates X Glycosides 25 The hemiacetal or hemiketal carbon can go on to react with the hydroxyl group from another molecule to form an acetal or ketal. ✦ The bond formed is called a glycosidic bond. ✦ Glycosidic bonds are used to connect one monosaccharide to another. Chem 352, Lecture 7 - Carbohydrates X Glycosides 26 ✦ Unlike hemiacetals and hemiketals, acetals and ketals cannot open and close dynamically, • The glycosidic bond blocks a pyranose or furanose ring from reopening again. ✦ In cells, glycosidic bond formation is enzyme catalyzed and requires a source of free energy. Chem 352, Lecture 7 - Carbohydrates X Monosaccharides 27 Cyclized aldoses and ketoses tend to get locked into the following ring configurations ✦ pyranose rings • 21.5%D-glucopyranose58.5% (aldohexose)6.5% 13.5% • D-mannopyranose (aldohexose) • D-galactopyranose (aldohexose) ✦ furanose rings • D-fructofuranose (ketohexose) • D-ribofuranose (aldopentose) Chem 352, Lecture 7 - Carbohydrates X 28 9.3 Oligosaccharides Disaccharides 29 The glycosidic bond can be used to connect two monosaccharides together to form disaccharides. Important disaccharides include: ✦ Maltose (obtained from starch) α(1-4) glycosidic bond β-anonomer Chem 352, Lecture 7 - Carbohydrates 3 Disaccharides 30 TheClickerQuestion: glycosidic Question: bond can be used to connect two monosacchridesWhichWhat typeanomer of glycosidicof togetherlactose bond is shown?to does form lactose a disaccharides. have? ImportantA. α (1-4) disaccharides include: • B.Lactoseβ(1-6) C. β(1-4) (milk sugar) (1-4) glycosidic bond D. α(1-5) β α-anonomer Chem 352, Lecture 7 - Carbohydrates 4 Disaccharides 31 The glycosidic bond can be used to connect two monosaccharides together to form disaccharides. Important disaccharides include: • Cellobiose (obtained from cellulose) Chem 352, Lecture 7 - Carbohydrates 5 Disaccharides 32 The glycosidic bond can be used to connect two monosaccharides together to form disaccharides. Important disaccharides include: • Sucrose (table sugar) α1-β2 glycosidic bond Chem 352, Lecture 7 - Carbohydrates 6 Disaccharides 33 Question: Because a hemiacetal or hemiketal can easily open to expose Whicheither anend aldehyde of the disaccharideor α-hydroxy maltose ketone, isthey the can reducing still serve end? as reducing agents for the Cu2+ ions found in Fehling’s and Benedict’s reagents. A. B. O O R C H + Cu2+ R C OH + Cu+ CuSO4(aq) Cu2O(s) clear blue red ppt. ‣ This reaction is used to distinguish the two monosaccharides in a disaccharide as the reducing and the nonreducing ends. Chem 352, Lecture 7 - Carbohydrates 7 Disaccharides 34 Not all disaccharides have a reducing end ✦ For example, the disaccharide sucrose contains both and acetal and a ketal, but no hemiacetal or hemiketal. Chem 352, Lecture 7 - Carbohydrates 8 Glycosides 35 Monosaccharides also form glycosidic bonds to non- sugar. ✦ For example, nucleotides, such as • ATP • UDP-glucose • NAD and NADP • FMN and FAD ✦ And sterols, such as Adenosine triphosphate • Ouabain Flavin mononucleotide (FMN) Flavin adenine dinucleotide (FAD) Chem 352, Lecture 7 - Carbohydrates 9 Glycosides 36 Ribitol
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  • Structural Features

    Structural Features

    1 Structural features As defined by the International Union of Pure and Applied Chemistry gly- cans are structures of multiple monosaccharides linked through glycosidic bonds. The terms sugar and saccharide are synonyms, depending on your preference for Arabic (“sukkar”) or Greek (“sakkēaron”). Saccharide is the root for monosaccha- rides (a single carbohydrate unit), oligosaccharides (3 to 20 units) and polysac- charides (large polymers of more than 20 units). Carbohydrates follow the basic formula (CH2O)N>2. Glycolaldehyde (CH2O)2 would be the simplest member of the family if molecules of two C-atoms were not excluded from the biochemical repertoire. Glycolaldehyde has been found in space in cosmic dust surrounding star-forming regions of the Milky Way galaxy. Glycolaldehyde is a precursor of several organic molecules. For example, reaction of glycolaldehyde with propenal, another interstellar molecule, yields ribose, a carbohydrate that is also the backbone of nucleic acids. Figure 1 – The Rho Ophiuchi star-forming region is shown in infrared light as captured by NASA’s Wide-field Infrared Explorer. Glycolaldehyde was identified in the gas surrounding the star-forming region IRAS 16293-2422, which is is the red object in the centre of the marked square. This star-forming region is 26’000 light-years away from Earth. Glycolaldehyde can react with propenal to form ribose. Image source: www.eso.org/public/images/eso1234a/ Beginning the count at three carbon atoms, glyceraldehyde and dihydroxy- acetone share the common chemical formula (CH2O)3 and represent the smallest carbohydrates. As their names imply, glyceraldehyde has an aldehyde group (at C1) and dihydoxyacetone a carbonyl group (at C2).