NPTEL – Biotechnology – Cell Biology

Module 11 Lecture 29 Carbohydrates I

11.1 Introduction Carbohydrates are polyhydroxy or . They are primarily produced by plants and form a very large group of naturally occurring organic substances. Some common examples are cane , , , etc. They have general molecular formulas that make them appear to be hydrates of , Cn(H2O)n, from where the name was derived. Carbohydrates are formed in the plants by from and water in the presence of sunlight (Scheme 1). Oxidation C6H12O6 + 6O2 6CO2 + 6H2O + Energy Photosynthesis ucose Gl Scheme 1 11.2 Classification Carbohydrates are classified into two main classes, and . 11.2.1 Sugars Sugars are sweet crystalline substances that are soluble in water. These are further classified on the basis of their behavior on hydrolysis.

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11.2.1.1 The simplest form of carbohydrates is the . 'Mono' means 'one' and 'saccharide' means 'sugar'. Monosaccharides are polyhydroxy or that cannot be hydrolyzed further to give simpler sugar. They may again be classified on the basis of the nature of . • Polyhydroxy aldehydes are called . Example: Glucose

• Polyhydroxy ketones are called . Example:

1 1 HC O CH2OH 2 2 H OH C O 3 3 HO H HO H 4 4 H OH H OH 5 5 H OH H OH 6 6 CH2OH CH2OH

Glucose Fructose ose e ose (Ald ) (K t )

The aldoses and ketoses are further divided based on the number of present in their molecules, as , , , etc. They are referred to as aldotrioses, aldotetroses, aldopentoses, aldohexoses, ketohexoses etc.

Number of Carbons General term Aldehyde Ketone 3 Aldotriose Ketotriose 4 Aldotetrose Ketotetrose 5 Aldopentose Ketopentose 6 Aldohexose Ketohexose 7 Aldoheptose Ketoheptose

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11.2.1.2 Carbohydrates that produce two to ten monosaccharide units during the hydrolysis are called oligosaccharides. They can be further classified based on the number of monosaccharide units formed on hydrolysis. : They give two monosaccharide units on hydrolysis, which may be the same or different. For example, on hydrolysis gives one molecule each of glucose and fructose, whereas gives two molecules of glucose (Scheme 1).

C12H22O11 + H2O C6H12O6 + C6H12O6 sucrose glucose fructose

C12H22O11 + H2O 2 C6H12O6 ma ose ucose lt gl Scheme 1 : These carbohydrates yield three molecules of monosaccharides units on hydrolysis (Scheme 2).

C18H32O16 + 2 H2O C6H12O6 + C6H12O6 + C6H12O6 ra nose ucose ruc ose a ac ose ffi gl f t g l t

Scheme 3

11.2.1.3 Polysaccharides These carbohydrates give a large number of monosaccharide units on hydrolysis. These monosaccharide units are joined together by oxide bridges. These linkages are called glycosidic linkages. The common and widely distributed polysaccharides correspond to the general formula (C6H10O5)n. Polysaccharides are not sweet in taste, so they are called non-sugars. Some common examples are starch, , , etc (Scheme 3). + H (C6H10O5)n + n H2O n C6H12O6 s arc glucose t h Scheme 3

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11.3 The D and L Notations The notations D and L are used to describe the configurations of carbohydrates and amino acids. has been chosen as arbitrary standard for the D and L notation in sugar . Because, this has an asymmetric carbon and can exist as a pair of .

O HC O HC O H OH HO H HO H H OH HO H CH OH CH OH OH CH2OH CH2OH cera e e - cera e e - cera e e glyceraldehyde D-glyceralldehyde L-glyceraldehyde

In a , the carbonyl group is always placed on the top position for monosaccharide. From its structure, if the –OH group attached to the bottom-most asymmetric center (the carbon that is second from the bottom) is on the right, then, the compound is a D-sugar. If the –OH group is on the left, then, the compound is a L-sugar. Almost all sugars found in nature are D-sugar.

HC O HC O H OH HO H HO H H OH H OH HO H H OH HO H CH OH 2 CH2OH D-series L-series - ucose L- lucose (D gl ) ( g )

Like R and S, D and L indicate the configuration of an asymmetric carbon, but they do not indicate whether the compound rotates polarized light to the right or to the left. For example, D-glyceraldehyde is dextrorotatory, whereas D- is levorotatory. In other words, , like melting or boiling points, is a physical property of a compound, whereas “R, S, D, and L” are conventions humans use to indicate the configuration of a molecule.

HC O COOH H OH H OH

CH2OH CH3

- + - cera e e - - - ac c ac D ( ) gly ld hyd D ( ) l ti id

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11.3.1 Configuration of the Aldoses

Aldotetrose: The structural formula of aldotetrose is CH2OH·CHOH·CHOH·CHO. There are four stereoisomers (two pairs of enantiomers). Those stereoisomers correspond to D- and L- and D- and L-. HC O HC O HC O HC O H OH HO H HO H H OH H OH HO H H OH HO H

CH2OH CH2OH CH2OH CH2OH

D-er throse -er rose - reose - reose y L yth D th L th

Aldopentose: The structural formula of aldopentose is

CH2OH·CHOH·CHOH·CHOH·CHO. Since it contains three asymmetric carbons and there are eight stereoisomers (four pairs of enantiomers) possible. Those are the D- and L-forms of , , and .

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Aldohexose: They have four asymmetric center and therefore 16 stereoisomers (eight pairs of enantiomers). The four D-altopentose and eight D-aldohexose are shown below.

Configuration of the D-Aldoses

HC O H OH

CH2OH

D-glyceraldehyde

HC O HC O H OH HO H H OH H OH

CH2OH CH2OH D-erythrose D-threose

HC O HC O HC O HC O H OH HO H H OH HO H H OH H OH HO H HO H H OH H OH H OH H OH

CH2OH CH2OH CH2OH CH2OH D-ribose D-arabinose D-xylose D-lyxose

HC O HC O HC O HC O HC O HC O HC O HC O H OH HO H H OH HO H H OH HO H H OH HO H H OH H OH HO H HO H H OH H OH HO H HO H H OH H OH H OH H OH HO H HO H HO H HO H H OH H OH H OH H OH H OH H OH H OH H OH

CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH

D- D- D-glucose D- D- D- D- D-

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Configuration of the ketoses Ketoses have the keto group in the 2-position with one less chiral carbon compared to aldoses. So, ketoses have a half number of stereoisomers compared to aldoses with same number of carbon atoms. For example, aldopentose has three chiral centers with eight stereoisomers, while ketopentose has two chiral centers with four stereoisomers. The configuration of D-2 ketones is illustrated by the following examples. may not be a sugar, but it is included as the analog of glyceraldehyde. Configuration of the Ketoses

CH2OH C O

CH2OH dihydroxyacetone

CH2OH C O H OH

CH2OH D-

CH OH 2 CH2OH C O C O H OH HO H H OH H OH CH OH 2 CH2OH D- D-

CH2OH CH2OH CH2OH CH2OH C O C O C O C O H OH HO H H OH HO H H OH H OH HO H HO H H OH H OH H OH H OH

CH2OH CH2OH CH2OH CH2OH - s cose - ruc ose -sor ose - a a ose D p i D f t D b D t g t

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Epimers are stereoisomers that differ in configuration of only one asymmetric carbon of enantiomers or . Example, D-glucose and D-mannose are C-2 epimers and D-glucose and D-talose are C-3 epimers. D-fructose and D- are C-4 epimers of ketohexoses.

HC O HC O HC O HC O CH2OH CH2OH H OH HO H HO H HO H C O C O HO H HO H H OH HO H HO H HO H H OH H OH HO H HO H H OH HO H H OH H OH H OH H OH H OH H OH

CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH D-glucose D-mannose D-idose D-talose D-fructose D-tagatose

C-2 epimers C-3 epimers C-4 epimers

11.4 Cyclic Structure of Monosaccharides: Formation Aldoses contain an aldehyde group with several alcohol groups. The cyclic forms of D- glucose are six-membered formed by an intramolecular reaction of the –OH group at C-5 with the aldehyde group. Cyclic structures of monosaccharides are named according to their five- or six-membered rings (Scheme 4-5). A six-membered cyclic hemiacetal is called a , derived from the name of the six-membered cyclic pyran. A five-membered cyclic hemiacetal is called a , derived from the name of the five-membered cyclic ether furan. For example, the six-membered ring of glucose is called glucopyranose and the five- membered ring of fructose is called fructofuranose.

1 CH2OH 2 O 3 H 6 1 HO H HOH2C CH2OH HOH C CH OH 4 O 2 O 2 H OH H 5 H 2 O OH 5 H H OH H 4 3 6 OH H H CH2OH OH D-fructose D-fructofuranose c c c orm y li f Scheme 4

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H O

H OH HO H H OH H OH

CH2OH Fischer projjecttion

H CH OH H CH2OH OH H OH H HO O H OH

H CH2OH HOH2C CH2OH H O H H O H O OH H H H H OH H OH H OH H HO OH HO O HO H H OH H OH H OH

α-D-gllucopyranose C-6 rotated up β -D-glucopyranose

Haworth projjection Haworth projjection Scheme 5

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Haworth projection is "flattened" diagrams used to represent the stereochemistry carbohydrates. The six-membered ring of D-pyranose is represented as flat and is viewed edge on. The ring is always placed in the back right-hand corner of the ring, with the anomeric carbon (C-1) on the right-hand side and the primary alcohol group drawn up from the back left-hand corner (C-5).

• Groups on the right in a Fischer projection are down (below the ring) in a (Figure 1).

• Groups on the left in a Fischer projection are up (above the ring) in a Haworth projection.

Fisher projections

1 H OH HO H 1 2 H OH HO H H OH H OH 2 3 H OH H OH HO H HO H 3 O O 4 O O H OH H OH H OH H OH 4 5 H H H H 5 6 CH2OH CH2OH CH2OH CH2OH

α-D-gluocopyranose β-D-gluocopyranose α-D-ribofuranose β-D-ribofuranose

HO 6 HO HO 5 HO 5 OH OH O O O 1 O 4 OH 1 OH 4 2 3 OH OH 3 OH OH 2 OH OH OH OH OH OH

Haworth projections Figure 1

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D-Furanose is viewed edge on, with the ring oxygen away from the viewer. The anomeric carbon is on the right-hand side of the molecule, and the primary alcohol group is drawn up from the back left-hand corner (Figure 2).

Fisher projections

1 1 2 2 HO CH OH HOH2C OH HO CH2OH HOH2C OH 2 3 3 HO H HO H HO H HO H O O 4 4 H H H OH H OH H OH O O O 5 H 5 OH H OH H H CH OH H2C H2C 6CH2OH 2 6

α-D-fructopyranose β-D-fructopyranose α-D-fructofuranose β-D-fructofuranose

OH HO 6 1 OH HO 6 1 OH OH O O O O 5 HO 2 HO 5 HO 2 H 4 O OH OH OH 4 OH 3 OH 3 OH OH OH OH OH

Haworth projections Figure 2

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Module 11 Carbohydrates Lecture 30 Carbohydrates II

11.5 Acylation and Alkylation of Monosaccharides The –OH group of monosaccharide reacts with acetic anhydride or acetyl chloride to form esters. Similarly, ether can be prepared using methyl iodide/silver oxide. The nucleophilic character of –OH group is relatively poor, so silver oxide is used to increase the leaving tendency of the iodide ion in the SN2 reaction (Scheme 1). O

OH OCCH3 O O O O pyridine O O HO + H3CCO HO OH H3C O CH3 H3CCO OCCH3 OH O OCCH3 O β-D- lucose en a- -ace - - - ucose g p t O tyl β D gl

OH OCH3 CH I O HO O 3 H3CO H CO H HO OH Ag2O 3 OC 3 OH OCH3 - - ucose me e ra- -me -β- - ucos e β D gl thyl t t O thyl D gl id

H δ+ δ+ δ- δ+ + H -H O C I Ag O Ag +O CH O R H H 3 R CH3 H R suger hydroxyl group polarized CH I ether 3

Scheme 1

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11.6 Glycosides are cyclic acetal form of sugars and the bond between the anomeric carbon and the alkoxy oxygen is called a . They are prepared by the acid- catalyzed reaction of an alcohol with a pyranose or furanose. Naming of glycosides is done by replacing the “e” of the sugar with “ide”. Example, a of glucose is glucoside and if pyranose or furanose name is used, the acetal is called pyranoside or furanoside. Both α- and β-glycoside obtained from the reaction of a single with an alcohol (Scheme 2). OH OH OH

H O CH3CH2OH O O O HO + HO HO OH HO OCH2CH3 HO OH HCl OH OH OCH2CH3 β-D-glucose ethyl β-D-glucoside ethyl α-D-glucoside α β-D-glucopyranose ethyl β-D-glucopyranoside ethyl -D-glucopyranoside

Scheme 2

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The reason for the formation of both glycosides is shown in Scheme 3. The protonation of the anomeric carbon –OH group followed by elimination of water gives a planar sp2 hybridized oxocarbenium ion. This can react with alcohol from both faces to give the β- glycoside and the α-glycoside.

OH OH O + HO H B HO O H + + B HO OH HO OH OH OH

OH + top O HO + H O HO H 2 OH bottem

CH3CH2OH attacks oxocarbenium ion CH3CH2OH attacks from top from bottem

OH OH

HO O H HO O + + HO OCH3CH3 HO OH OH +OCH3CH3 H HB+ B HB+ B

OH OH

HO O HO O + HO OCH3CH3 HO OH OH OCH3CH3 β-glycoside α-glycoside major product Scheme 3 11.7 Anomeric Effect When a pyranose or furanose ring closes, the hemiacetal carbon atom is converted from a flat carbonyl group to an asymmetric carbon. Depending on which face of the (protonated) carbonyl group is attacked, the hemiacetal –OH group can be directed either up or down. These two orientations of the hemiacetal –OH group give diastereomeric products called , and the hemiacetal or acetal carbon atom is called the anomeric carbon atom. The preference of certain substituents bonded to the anomeric carbon for the axial position is called the anomeric effect. Ano is Greek for “upper”; thus, anomers

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differ in configuration at the upper-most asymmetric carbon. The anomeric carbon is the only carbon in the molecule that is bonded to two oxygen atoms. The anomer with the anomeric –OH group down (axial) is called the α-anomer, and the one with the anomeric –OH group up (equatorial) is called the β-anomer (Scheme 4). HOH HOH HOH H O H OH H O HO HO HO HO H HO O HO OH H OH H OH H OH H OH H H H H

α-D-glucopyranose open chain form β-D-glucopyranose α-anomer β-anomer Scheme 4 In fructose, the α-anomer has the anomeric - OH group down, trans to the terminal –

CH2OH group, while the β-anomer has it up, cis to the terminal –CH2OH group (Scheme 5).

CH OH O OH O 2 OH CH2OH HOH2C HOH2C HOH2C OH H OH H OH H O H H H H CH2OH O OH H OH H OH H α-D-fructofuranose β-D-fructofuranose trans = α cis = β Scheme 5

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11.8 Normally D-(+)-glucose has a melting point of 146 °C. However, when D-(+)-glucose is crystallized by evaporating an aqueous solution kept above 98 °C, a second form of D- (+)-glucose with a melting point of 150 °C can be obtained. When the optical rotations of these two forms are measured, they are found to be significantly different, but when an aqueous solution of either form is allowed to stand, its rotation changes. The of one form decreases and the other increases, until both solutions show the same value. For example, a solution of -D-(+)-glucose (mp 146 °C) specific rotation gradually decreases from an initial value of + 112.2° to + 52.7°, while The -D-(+)- glucose (mp 150 °C) specific rotation gradually increases from an initial value of + 18.7° to + 52.7°. The three forms of glucose reach equilibrium concentrations with the specific rotation of +52.7. This change ("mutation") in the specific rotation toward equilibrium is called mutarotation (Scheme 6). HOH HOH HOH H O H OH H O HO HO HO HO H HO O HO OH H OH H OH H OH H OH H H H H α-anomer equilibrium mixture of α and β β-anomer

crystalize crystalize below 98 °C above 98 °C

CHO HOH H OH HOH H O H2O HO H H2O H O HO HO H H H OH H OH O H O H OH H OH H OH OH H H CH2OH α -D-(+)-glucopyranose open chain form β-D-(+)-glucopyranose ° α ° D- + - lucose ° α ° mp 146 C; [ ]D = + 112.2 ( ) g mp 150 C; [ ]D = + 18.7 α ° [ ]D = + 52.7 Scheme 6

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11.9 Reducing and Non-reducing Sugars The carbohydrates may also be classified as either reducing or non-reducing sugars. Cyclic acetals or ketals are not in equilibrium with their open chain carbonyl group containing forms in neutral or basic aqueous solutions. They cannot be oxidized by + - reagents such as Tollen’s reagent (Ag , NH3, OH ) or Br2. So, these are referred as non- reducing sugars. Whereas hemiacetals or hemiketals are in equilibrium with the open- chain sugars in aqueous solution. These compounds can reduce an oxidizing agent (eg.

Br2), thus, they are classified as a . 11.10 Determination of Ring Size The anomeric carbon can be found via methylation of the –OH groups, followed by

hydrolysis. In the first step, all the –OH groups are transformed to –OCH3 groups with excess methyl iodide and silver oxide. The hydrolysis of the acetal then forms a hemiacetal in presence of acid. This pyranose structure is in equilibrium with its open- chain form. From the open-chain form we can determine the size of the ring because the anomeric carbon attached –OH group is the one that forms the cyclic hemiacetal (Scheme 7).

OH OCH3 O CH I O HO 3 H3CO HO OH H3CO OCH3 Ag2O OH OCH3 acetol

HCl H2O

CHO OCH3 H OCH3 O H CO H H3CO 3 OH H3CO H OCH3 OCH 3 H OH hemiacetol CH2OCH3

Scheme 7

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A monosaccharide’s ring size can be determined by the oxidation of an acetal of the monosaccharide with excess periodic acid. The products obtained from periodate cleavage of a six-membered ring acetal are different from those obtained from cleavage of a five-membered ring acetal (Scheme 8-9).

OH H OCH3 CHO H O O 2 HIO4 O HCl HO O H H OH + OCH OCH3 HO 3 HO C H2O OH CH O CH2OH H O O six-membered ring formic acid D-glyceraldehyde Scheme 8 H H CHO H O HO O 2 HIO O HO 4 O OHC OCH3 HCl CHOH + H CHO H2O H O OCH3 O O HO OH

five-membered ring Scheme 9 11.11 Disaccharides If the glycoside or acetol is formed by reaction of the anomeric carbon of a monosaccharide with OH group of another monosaccharide molecule, then the glycoside product is a (Scheme 10). OH OH O R OH O HO HO OH + OR HO HO OH H OH

monosaccharide a glycoside

+ glycosidic bond OH OH OH O O HO HO O OH O HO OH OH OH OH OH OH second monosachharide sac ar e di hh id Scheme 10

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The anomeric carbon can react with any of the hydroxyl groups of another monosaccharide unit to form a disaccharide. Disaccharides can be categorized by the position of the hydroxyl group of another monosaccharide making up the glycoside.

Disaccharides have three naturally occurring glycosidic linkages

• 1-4’ link: The anomeric carbon is bonded to oxygen on C-4 of second monosaccharide. • 1-6’ link: The anomeric carbon is bonded to oxygen on C-6 of second monosaccharide. • 1-2’ link: The anomeric carbons of the two monosaccharide unit are bonded through an oxygen.

The “prime” superscript indicates that –OH group bonded carbon position of the second monosaccharide unit, α- and β-configuration given by based on the configuration at the anomeric carbon of the first monosaccharide unit.

1-4’ Glycosides: These represent the most common naturally occurring disaccharides. The linkage is between C-1 of one sugar subunit and C-4 of the other. For example, maltose is a disaccharide with two D-glucose units bearing 1,4’-glycosidic linkage. The stereochemistry of this linkage is α. So, the glycosidic linkage is called α-1,4’-glycosidic linkage. OH O HO 1 OH HO 4' OH O O HO OH OH α-1,4'-linkage stereochemistry not specified

maltose - - α- - uco ranos - - uco ranose 4 O ( D gl py yl) D gl py

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Cellobiose also contains two D-glucose subunits. The only difference from maltose is that the two glucose subunits are joined through a β-1,4’-glycosidic linkage.

β-1,4'-linkage H O OH O 4' H O O 1 O H O H HO O OH OH

- - - - uco ranos - - uco ranose 4 O (β D gl py yl) D gl py , a disaccharide present in milk, contains D-galactose (non-reducing) and D- glucose (reducing) monosaccharide units. These units are hooked together by a β-1,4’- glycosidic linkage.

β-1,4'-linkage H HO O OH 4' O O 1 O H O H HO O OH OH

lactose - - - - a ac o ranos - - uco ranose 4 O (β D g l t py yl) D gl py

1-6’ Glycosides: The anomeric carbon of one unit hooked by the oxygen of the terminal carbon (C-6) of another monosaccharide unit. Example, gentiobiose is a sugar with two glucose units joined by a β-1,6’-glucosidic linkage. OH β-1,6'-linkage HO O 1 O HO 6' OH O HO HO OH OH gentiobiose - - - - uco ranos - - uco ranose 6 O (β D gl py yl) D gl py

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1-2’ Glycosides: The glycosidic bond is hooked between the two anomeric carbon of the monosaccharide units. For example, sucrose contains a D-glucose subunit and a D- fructose subunit, which have been joined by a glycosidic bond between C-1 of glucose (in the α-position) and C-2 of fructose (in the β-position). OH

HO O 1 HO 1,2'-linkage OH HO O O HO 2'

HO OH sucrose α- - uco ranos - - - ruc o uranos e D gl py yl β D f t f id 11.12 Polysaccharides Polysaccharides are carbohydrates that contain many monosaccharide units joined by glycosidic bonds. All the anomeric carbon atoms of polysaccharides are involved in acetal formation. So, polysaccharides do not react with Tollen’s reagent, and they do not mutarotate. Polysaccharides that are polymers of a single monosaccharide are called homopolysaccharides. If they made by more than one type of monosaccharide are called heteropolysaccharides. Example, a is made by glucose units and , which is made by galactose units. There are three important polysaccharides, which are starch, glycogen and cellulose.

Starch is a glucose polymer that is the principal food storage carbohydrate in plants. It is a mixture of two components that can be separated on the basis of water solubility. + + soluble H H Maltose Glucose H H 20% 2O 2O

H2O Starch

+ + Maltose H H + Glucose insoluble H2O H2O 80%

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Amylose is a linear polymer of D-glucose units joined by α-1,4’-glycosidic linkages. OH O O OH HO OH O O H 1 O HO 4' OH O O OH HO OH O O α-1,4'- l cosidic linka e g y g HO OH O ar a s ruc ure o am ose p ti l t t f yl

Amylopectin is a branched polymer of D-glucose units hooked by α-1,4’-glycosidic linkages and the branches are created by α-1,6’-glycosidic linkages.

OH O O OH HO OH O O 1 HO α- - cos c n a e OH 1,6' gly idi li k g OH O O O 6' HO OH O O OH HO OH O O α-1,4'- l cosidic linka e g y g HO OH O ar a s ruc ure o am o ec n p ti l t t f yl p ti

Glycogen functions as a carbohydrate storage form for animals. Like amylopectin, it is non-liner polymer of D-glucose units joined by α-1,4’-glycosidic linkages and α-1,6' - glycosidic linkages at branches. The structure of glycogen is similar to that amylopectin, but it has more branches. The highly branched structure of glycogen provides many available glucose end groups for immediate hydrolysis to provide glucose needed for .

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Cellulose serves as structural material in plants, providing structural strength and rigidity to plants. It is a linear polymer of D-glucose units joined by β-1,4’-glycoside bonds. Humans and other mammals do not have the β-glucosidase enzyme needed to hydrolyze cellulose, so they cannot obtain glucose directly from cellulose.

β-1,4'-glycosidic linkage H O OH OH O OH O 4' O O O 1 O O H O H HO O O O OH H O H HO O OH ar a s ruc ure o ce u ose p ti l t t f ll l

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Module 11 Carbohydrates Lecture 31 Carbohydrates III

11.13 Reactions Monosaccharides contain carbonyl and alcohol functional groups, so it can be oxidized or reduced and can react with nucleophiles to form corresponding products. 11.13.1 Epimerization In the presence of base, D-glucose may be converted into D-mannose via the removal of at C-2 carbon followed by protonation of the enolate (Scheme 1).

H O H O H O H O

H C OH C OH C OH OH C H HO OH HO H HO H O HO H HO H H H H OH H OH H OH H OH H OH H OH H OH H OH CH2OH CH2OH CH2OH CH2OH

D-glucose enolate D-mannose Scheme 1

11.13.2 Enediol Rearrangement The position of carbonyl group may shift via enediol intermediate under basic condition. For example, rearrangement of D-glucose gives D-fructose (Scheme 2).

H O H H H H OH H O O H OH H C OH C OH O C OH C O O HO OH H H OH H H C O HO H H H H H HO H O O HO H H OH H H H H H OH O O H OH H OH H H H H H OH O O H OH CH OH 2 CH2OH CH2OH CH2OH CH2OH D-glucose enolate enediol enolate D-fructose

Scheme 2

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11.13.3 Reduction The monosaccharide contains carbonyl group which can be reduced by the reducing agents such as NaBH4. Reduction of forms one alditol and ketose forms two alditols (Scheme 3).

CHO CH2OH CH2OH CH2OH HO H HO H O H OH

HO H NaBH4 HO H NaBH4 HO H NaBH4 HO H H OH + H OH + H OH + H OH H3O H3O H3O H OH H OH H OH H OH

CH2OH CH2OH CH2OH CH2OH

D-mannose D-mannitol D-fructose D-glucitol

(an alditol) (an alditol) Scheme 3 11.13.4 Oxidation • Bromine water oxidizes aldehyde functional group, but it cannot oxidize ketones or alcohols. Therefore, aldose can be distinguished from ketose by observing reddish-brown colour of bromine. The oxidized product is an (Scheme 4).

COOH CHO H OH H OH HO H HO H H O + r 2 H OH + r H OH B 2 2 B H OH H OH red colorless CH2OH CH2OH - ucon c ac D-glucose D gl i id an a on c ac ( ld i id) Scheme 4

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• Tollen’s reagent can oxidize both aldose and ketose to aldonic acids. For example, the of both D-fructose and D-glucose, as well as the enol of D- mannose are same (Scheme 5).

CH2OH HC OH CHO CHO C O C OH H OH HO H HO H HO H HO H HO H OH, H2O OH, H2O H OH H OH H OH + H OH

H OH OH, H2O H OH OH, H2O H OH H OH

CH2OH CH2OH CH2OH CH2OH

D-fructose enol of D-fructose, D-glucose D-mannose D-glucose and (ketose) -mannose (aldose) D

CHO COO NH4 H OH H OH HO H Tollens' Reagent HO H + Ag H OH H OH H OH H OH

CH2OH CH2OH

D-glucose

Scheme 5

Both aldehyde and primary alcohol groups of an aldose are oxidized by strong oxidizing agent such as HNO3. The oxidized product called an . Ketose also reacts with

HNO3 to give more complex product mixtures (Scheme 6).

CHO COOH HO H HO H HO H HNO3 HO H H OH H OH H OH H OH

CH2OH COOH

D-mannose D-mannaric acid a ar c ac ( ld i id) Scheme 6

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11.13.5 Osazone Formation Aldose and ketose react with one equiv of phenylhydrazine to produce phenylhydrazones. In contrast, both C-1 and C-2 react with three equivalent of phenylhydrazine to form a bis-hydrazone known as an osazone (Scheme 7).

CHO CH2OH HC N NHPh C O + HC OH H C N NHPh or CHOH + 3 Ph NH NH2 + PhNH + + 2 H O (CHOH)n ( )n (CHOH)n 2 NH3 2 CH OH CH2OH 2 CH2OH aldose ketose osazone Scheme 7 The configuration at C-1 or C-2 is lost in the formation of osazone, C-2 epimers form identical osazones. For example, D-gluose and D-idose are C-2 epimers; both form the same osazone (Scheme 8).

CHO HC NHNHPh CHO H OH C NHNHPh HO H

H OH 3 PhNHNH2 H OH 3 PhNHNH2 H OH HO H HO H HO H catalytic catalytic H OH H OH H OH H+ H+ CH2OH CH2OH CH2OH D- ulose osazone D-idose g

Scheme 8

Ketose reacts with phenylhydrazine at C-1 and C-2 position to form osazone. D-Glucose and D-fructose form the same osazone (Scheme 9).

CHO HC NHNHPh CH2OH H OH C NHNHPh C O

HO H 3 PhNHNH2 HO H 3 PhNHNH2 HO H H OH H OH H OH catalytic catalytic H OH H OH H OH H+ H+ CH2OH CH2OH CH2OH D- lucose osazone D-fructose g Scheme 9

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11.13.6 The Ruff Degradation Aldose chain is shortened by oxidizing the aldehyde to –COOH, then decarboxylation. In the Ruff degradation, the calcium salt of an aldonic acid is oxidized with hydrogen peroxide. Ferric ion catalyzes the oxidation reaction, which cleaves the bond between C-1 and C-2, forming an aldehyde. The calcium salt of the aldonic acid prepared from oxidation of an aldose with an aqueous solution of bromine and then adding calcium hydroxide to the reaction mixture (Scheme 10).

2+ HC O COO (Ca )1/2 HC O H OH H OH i Br , H O HO H HO H ) 2 2 HO H Fe2(SO4)3 + H2O2 H OH + CO H OH a H OH 2 ii) C (OH)2 H OH H OH H OH CH2OH CH2OH CH2OH D-glucose ca c um - ucona e D-arabinose l i D gl t

Scheme 10 11.13.7 The Kiliani–Fischer Synthesis An aldose carbon chain can be increased by one carbon in a Kiliani–Fischer synthesis (Scheme 11). It is the opposite of Ruff Degradation reaction. This synthesis leads to formation of a pair of C-2 epimers.

CN HC NH CHO CHO HCN CHOH H2 CHOH H3O CHOH (CHOH)n KCN (CHOH)n Pd/BaSO4 (CHOH)n (CHOH)n CH2OH CH2OH CH2OH CH2OH aldose cyanohydrin chain-lenghenedimine chain-lengthened a ose ld Scheme 11

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D-Erythrose gives the corresponding chain lengthened products D-ribose and D- arabinose (Scheme 12). C N HC NH HC O H OH H OH H HCl H OH H OH 2 H OH + HCl H OH + NH4 H OH Pd/BaSO H O 4 H OH 2 H OH CH OH HC O 2 CH2OH CH2OH H OH + C N D-ribose H OH CH OH C N 2 HC NH HC O HCl HO H HO H D-erythrose H2 HO H HCl + H OH H OH + NH4 Pd Ba H OH H OH / SO4 H2O H OH H OH CH2OH CH2OH CH2OH D-arabinose Scheme 12

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11.14 Synthetic Sweeteners Some molecules bind to a receptor on a taste bud cell of the tongue and give sweet taste. When this molecule binds, a nerve impulse passes from the taste bud to the brain, where the molecule is interpreted as being sweet. The degree of sweetness differs for different sugars. The relative sweetness of glucose is 1.00 and that of sucrose is 1.45. Fructose is the sweetest of all sugars, which is 1.65. Toxicity, stability and cost must be considered, while developing a synthetic sweetener. Saccharin is the first synthetic sweetener, was discovered by Ira Remsen and his student Constantine Fahlberg at Johns Hopkins University in 1878. It is 300 times sweeter than glucose.

CH3 CH3 CH3 2 ClSO2OH SO2Cl +

SO2Cl

CH3 CH3 SO2Cl SO2NH2 + (NH4)2CO3

O OH O CH3 NH SO2NH2 KMnO4 SO2NH2 SO2

acc ar n S h i

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Aspartame was discovered in 1965 by a group of scientists working for G.D. Searle, that later become a Monsanto company, to develop a new anti-ulcer drug based on a tetrapeptide. It is 200 times sweeter than sucrose. It undergoes slow hydrolysis in solution and it also decomposes with heat. So, we cannot use for soft drinks and cooking purposes.

O

O O O O H O + N OH OH H N O + 3 NH2 OH NH2 O

as ar ame p t

Sucralose is a trichloro derivative of sucrose that is an artificial sweetener. Sucralose is 600 times sweeter than sucrose. It tastes like sugar and used for cooking and baking. ClOH O Cl HO O OH HO O

HO Cl sucra ose l Alitame is related to compound aspartame, with improved properties. It is 2000 times as sweet as sucrose and more stable than aspartame. Sucronic acid is designed on the basis of sweetness receptor sites, which is reported to be 200,000 times sweeter than sucrose. O H N COOH H NH2

H CH3 H N H N CH3 H3C HO O S N N CN H3C H CH3 O ame sucron c ac Alit i id

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Acesulfame-K is 200 times sweeter than sucrose. It is highly used for packed goods and baking due to stable nature under heat. O O H Cl NH2 O S S N CH3 O Na O O S O O O H C O O O O SO Cl 2 2 2 NH3 O -NaCl Cl Cl Cl Cl 2 KOH

O

N K S O O O acesu ame- lf K Dulcin and sodium cyclamate are widely used synthetic sweeteners and later taken off the market due to its toxicity. O NH2 + O NH NHSO3 Na H3C u c n sodium c clamate d l i y

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