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Introduction

Classification of Carbohydrates

Carbohydrates have the general formula C_x(H₂O)_y
Carbohydrates are defined as polyhydroxy aldehydes or ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones
Monosaccharides are carbohydrates that cannot be hydrolyzed to simpler carbohydrates

Disaccharides can be hydrolyzed to two monosaccharides
Oligosaccharides yield 2 to 10 monosaccharides
Polysacccharides yield >10 monosaccharides

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Photosynthesis and Carbohydrate Metabolism

Carbohydrates are synthesized in plants by photosynthesis

Light from the sun is absorbed by chlorophyll and this is converted to the energy necessary to biosynthesize carbohydrates

Carbohydrates act as a repository of solar energy

The energy is released when animals or plants metabolize carbohydrates

Much of the energy released by oxidation of glucose is trapped in the molecule adenosine triphosphate (ATP)

The phosphoric anhydride bond formed when adenosine triphosphate (ADP) is phosphorylated to make ATP is the repository of this energy
This chemical energy is released when ATP is hydrolyzed or a new anhydride linkage is created

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Chapter 22

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Monosaccharides

Classification of Monosaccharides

Monosaccharides are classified according to:

(1) The number of carbon atoms present in the molecule and
(2) whether they contain an aldehyde or ketone group

D and L Designations of Monosaccharides

The simplest carbohydrates are glyceraldehyde, which is chiral, and dihydroxyacetone, which is achiral

Glyceraldehyde exists as two enantiomers

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In the early 20th century (+)-glyceraldehyde was given the stereochemical designation (D) and (-)-glyceraldehyde was given the designation (L)
A monosaccharide whose highest numbered stereogenic center has the same configuration as D-(+)-glyceraldehyde is a D sugar
A monosaccharide whose highest numbered stereogenic center has the same configuration as L-(-)-glyceraldehyde is an L sugar

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Structural Formulas for Monosaccharides

Fischer projections are used to represent stereochemistry in carbohydrates
In Fischer projections horizontal lines are understood to project out of the plane toward the reader and vertical lines are understood to project behind the plane

A Fischer projection cannot be removed from the plane of the paper or turned 90^o and still represent the molecule accurately

Glucose exists primarily in two cyclic hemiacetal forms that are diastereomers of each other

The cyclic hemiacetal forms interconvert via the open-chain form
The cyclic hemiacetals differ only in configuration at C1 and are called anomers
The carbon at which their configurations differ is called the anomeric carbon
The α-anomer has the C1 hydroxyl trans to the -CH₂OH group
The β-anomer has the C1 hydroxyl cis to the -CH₂OH group

The flat cyclic representation of carbohydrates is called a Haworth formula

Cyclic glucose actually exists in the chair form

Chapter 22

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Chapter 22

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In the β-anomer of glucose all groups around the ring are equatorial
The configuration at the anomeric carbon of a cyclic carbohydrate can be left unspecified using a wavy bond

Fischer projections and Haworth formulas can easily be interconverted (next slide)
A six-membered ring monosaccharide is designated a pyranose, e.g., glucose in this form is glucopyranose
A five-membered ring monosaccharide is designated a furanose,

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Chapter 22

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Mutarotation

The α- and β-forms of glucose can be isolated separately

Pure α-glucose has a specific rotation of +112^o
Pure β-glucose has a specific rotation of +18.7^o

When either form of glucose is allowed to stand in aqueous solution, the specific rotation of the solution slowly changes to +52.7^o

It does not matter whether one starts with pure α- or β-glucose

Mutarotation is the change in optical rotation as an equilibrium mixture of anomers forms

Mutarotation of glucose results in an equilibrium mixture of 36% α-glucose and 64% β-glucose

The more stable β-glucose form predominates
A very small amount of the open-chain form exists in this equilibrium

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Chapter 22

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Glycoside Formation

Glycosides are acetals at the anomeric carbon of carbohydrates

When glucose reacts with methanol in the presence of catalytic acid, the methyl glycoside is obtained
A glycoside made from glucose is called a glucoside

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Chapter 22

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Glycosides can be hydrolyzed in aqueous acid

The alcohol obtained after hydrolysis of a glycoside is called an aglycone

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Other Reactions of Monosaccharides

Enolization, Tautomerization and Isomerization

Monosaccharides dissolved in aqueous base will isomerize by a series of keto-enol tautomerizations

A solution of D-glucose containing calcium hydroxide will form several products, including D-fructose and D-mannose

A monosaccharide can be protected from keto-enol tautomerization by conversion to a glycoside

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Formation of Ethers

The hydroxyl groups of carbohydrates can be converted to ethers by the Williamson ether synthesis
Benzyl ethers are commonly used to protect or block carbohydrate hydroxyl groups

Benzyl ethers can be easily removed by hydrogenolysis

Exhaustive methylation of methyl glucoside can be carried out using dimethylsulfate

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Exhaustive methylation can be used to prove that glucose exists in the pyranose (6-membered ring) form

Hydrolysis of the pentamethyl derivative results in a free C5 hydroxyl

Silyl ethers are also used as protecting groups for the hydroxyls of carbohydrates

Sterically bulky tert-butyldiphenylsilyl chloride (TBDPSCl) reacts selectively at the C6 primary hydroxyl of glucose

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Conversion to Esters

Carbohydrates react with acetic anhydride in the presence of weak base to convert all hydroxyl groups to acetate esters

Conversion to Cyclic Acetals

Carbohydrates form cyclic acetals with acetone selectively between cis-vicinal hydroxyl groups

Cyclic acetals from reaction with acetone are called acetonides

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Oxidation Reactions of Monosaccharides

Benedict’s or Tollens’ Reagents: Reducing Sugars

Aldoses and ketoses give positive tests when treated with Tollens’ solution or Benedict’s reagent

Tollens’ reagent [Ag(NH₃)₂OH] gives a silver mirror when Ag⁺ is reduced to Ag⁰
Benedict’s reagent (an alkaline solution of cupric citrate complex) gives a brick red precipitate of Cu₂O
In basic solution a ketose can be converted to an aldose that can then react with Tollens’ or Benedicts’s reagent

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Carbohydrates with hemiacetal linkages are reducing sugars because they react with Tollens’ and Benedict’s reagents

The hemiacetal form is in equilibrium with a small amount of the aldehyde or ketone form, which can react with Tollens’ and Benedict’s reagents

Carbohydrates with only acetal groups (glycosidic linkages) do not react with these reagents and are called non-reducing sugars

Acetals are not in equilibrium with the aldehyde or ketone and so cannot react with these reagents

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Bromine Water: The Synthesis of Aldonic Acids

Bromine in water selectively oxidizes the aldehyde group of an aldose to the corresponding carboxylic acid

An aldose becomes an aldonic acid

Nitric Acid Oxidation: Aldaric Acids

Dilute nitric acid oxidizes both the aldehyde and primary hydroxyl groups of an aldose to an aldaric acid

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Periodate Oxidations: Oxidative Cleavage of Polyhydroxy Compounds

Compounds with hydroxyl groups on adjacent carbons undergo cleavage of carbon-carbon bonds between the hydroxyl groups

The products are aldehydes, ketones or carboxylic acids

With three or more contiguous hydroxyl groups, the internal carbons become formic acid

Cleavage also takes place when a hydroxyl group is adjacent to an aldehyde or ketone group

An aldehyde is oxidized to formic acid; a ketone is oxidized to carbon dioxide

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No cleavage results if there are intervening carbons that do not bear hydroxyl or carbonyl groups

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Reduction of Monosaccharides: Alditols

Aldoses and ketoses can be reduced to alditols

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Reactions of Monosaccharides with Phenylhydrazine: Osazones

An aldose or ketose will react with three equivalents of phenylhydrazine to yield a phenylosazone

The mechanism for phenylosazone formation involves the following steps:

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Reaction of D-Glucose or D-Mannose with excess phenylhydrazine yields the same phenylosazone

Therefore, D-glucose and D-mannose differ in configuration only at C2
Compounds that differ in configuration at only one stereogenic center are called epimers

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Synthesis and Degradation of Monosaccharides

Kiliani-Fischer Synthesis

The carbon chain of an aldose can be extended by:

Addition of cyanide to epimeric cyanohydrins
Hydrolysis to a mixture of epimeric aldonic acids or aldonolactones
Reduction of the epimeric aldonic acids or aldonolactones to the corresponding aldoses

For example, D-glyceraldehyde can be converted to D-erythrose and D-threose by the Kiliani-Fischer synthesis

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Chapter 22

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The Ruff Degradation

An aldose can be shortened by one carbon via:

Oxidation with bromine water to the aldonic acid
Oxidative decarboxylation with hydrogen peroxide and ferric sulfate

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The D Family of Aldoses

Most biologically important aldoses are of the D family

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Disaccharides

Sucrose (Table sugar)

Sucrose is a disaccharide formed from D-glucose and D-fructose

The glycosidic linkage is between C1 of glucose and C2 of fructose
Sucrose is a nonreducing sugar because of its acetal linkage

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Maltose

Maltose is the disaccharide of D- glucose having an α-linkage
Maltose results from hydrolysis of starch by the enzyme diastase

Maltose has a hemiacetal group in one glucose moiety; it is a reducing sugar
The two glucose units of maltose are joined by an α-glucosidic linkage

Cellobiose

Cellobiose is the disaccharide of D-glucose having a β-linkage
Cellobiose results from partial hydrolysis of cellulose

Cellobiose has a hemiacetal group in one glucose moiety; it is a reducing sugar
The two glucose units are connected by a β-glucosidic linkage

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Polysaccharides

Homopolysaccharides are polymers of a single monosaccharide whereas heteropolysaccharides contain more than one type of monosaccharide

A polysaccharide made up of only glucose units is called a glucan
Three important glucans are starch, glycogen and cellulose

Starch

The storage form of glucose in plants is called starch
The two forms of starch are amylose and amylopectin
Amylose consists typically of more than 1000 D-glucopyranoside units connected by α linkages between C1 of one unit and C4 of the next

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Amylose adopts a very compact helical arrangement

Amylopectin is similar to amylose but has branching points every 20-25 glucose units

Branches occur between C1 of one glucose unit and C6 of another

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Glycogen

Glycogen is the major carbohydrate storage molecule in animals
Glycogen is similar to amylopectin except that glycogen has far more branching

Branching occurs ever 10-12 glucose units in glycogen

Glycogen is a very large polysaccharide

The large size of glycogen prevents if from leaving the storage cell
The storage of tens of thousands of glucose molecules into one molecule greatly relieves the osmotic problem for the storage cell (this would be caused by the attempted storage of many individual glucose molecules)
The highly branched nature of glycogen allows hydrolytic enzymes to have many chain ends from which glucose molecules can be hydrolyzed

Glucose is the source of “ready energy” for the body

Long chain fatty acids of triacylglycerols are used for long term energy storage

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Cellulose

In cellulose, glucose units are joined by β-glycosidic linkages
Cellulose chains are relatively straight
The linear chains of cellulose hydrogen bond with each other to give the rigid, insoluble fibers found in plant cell walls

The resulting sheets then stack on top of each other

Humans lack enzymes to cleave the β linkages in cellulose and so cannot use cellulose as a source of glucose

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Glycolipids and Glycoproteins of the Cell Surface

Glycolipids and Glycoproteins are important for cell signaling and recognition
A, B, and O human blood groups are determined by glycoprotein antigens designated A, B, and H

Chapter 22