CarbohydratesCarbohydrates

Carbohydrates [CX(H2O)Y] are usually defined as polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones.

Simple carbohydrates are known as sugars or saccharides (Latin saccharum, sugar) and the ending of the names of most sugars is –ose. For example:

1. Glucose (for the principle sugar in blood)

2. Fructose (for a sugar in fruits and honey)

3. Sucrose (for ordinary table sugar)

4. Maltose (for malt sugar)

Classification of carbohydratesClassification of carbohydrates

MonosaccharideMonosaccharide ( (CC66HH1212OO66))

it is a simple carbohydrate, one that one attempted hydrolysis is not cleaved to smaller carbohydrates. e.g. glucose

DisaccharideDisaccharide on hydrolysis is cleaved to two monosaccharide, which may be the same or different. e.g. sucrose

OligosaccharideOligosaccharide (oligos is a Greek word that means “few”) yields 3 10 monosaccharide unit on hydrolysis. e.g. raffinose

PolysaccharidesPolysaccharides are hydrolyzed to more are hydrolyzed to more than 10 monosaccharide units. than 10 monosaccharide units. CelluloseCellulose is a is a polysaccharide molecule that gives polysaccharide molecule that gives thousands of glucose molecules when thousands of glucose molecules when completely hydrolyzed.completely hydrolyzed.

MonosaccharidesMonosaccharides Over 200 different monosaccharides are known.Over 200 different monosaccharides are known. Monosaccharides are classified according toMonosaccharides are classified according to::1. The number of carbon atoms present in the

molecule.

2. Whether they contain an aldehyde or ketone group. A monosaccharide containing three carbon atoms

is called a triosetriose; one containing four carbon atoms is called a tetrosetetrose.

A monosaccharide containing an aldehyde group is called an aldosealdose; one containing a keto group is called a ketoseketose.

(CHOH)n

CH2OH

CH

O

(CHOH)n

CH2OH

C

CH2OH

An aldose A ketose

O

CH2OH

An aldotetroseC4

CH2OH

C

CH2OH

A ketopentose

O

C5

CHOH

CHOH

CHOH

CHOH

CH

O

D and L Designations of D and L Designations of MonosaccharidesMonosaccharides

The simplest monosaccharide is glyceraldehyde, which contains a stereocenter. Therefore, it exist in two enantiomeric forms.

In 1906, (+)-glyceraldehyde is designated D-(+)- glyceraldehyde and ()- glyceraldehyde is designated L-()- glyceraldehyde.

These two compounds serve as configurational standards for all monosaccharides.

C(R)

CH2OH

COHH

HO

C(S)

CH2OH

CHHO

HO

(+)-Glyceraldehyde

(-)-Glyceraldehyde

A monosaccharide whose highest numbered stereocenter (the penultimate carbon) has the same configuration as D-(+)- glyceraldehyde is designated as a D sugar; one whose highest numbered stereocenter has the same configuration as L-()- glyceraldehyde is designated as an L sugar.

D and L designations are not related to the optical rotations of the sugars to which they are applied.

One may encounter other sugars that are D-(+)- or D-()- and ones that are L-(+)- or L-()-.

Structural formulas for Structural formulas for MonosaccharidesMonosaccharides

Fisher projectionFisher projection Haworth formulasHaworth formulas anomer or anomer or anomer anomer

CHO

OHH

HHO

OHH

OHH

CH2OH

H

H OH

HO H

O

H

HOHO

O

H

HO

H

HO

H

OHH

OH

OH

OH

indicates or

Not all carbohydrates exist in equilibrium with six-membered hemiacetal rings, in several instances the ring is five membered.

If the monosaccharide ring is six membered, the compound is called a pyranose (e.g. -D-glucopyranose ); if the ring is five membered, the compound is designated as a furanose. (e.g. -D-ribofuranose).

O

O

Pyran

Furan

MutarotationMutarotation The spontaneous change that takes place in the

optical rotation of and anomers of a sugar when they are dissolved in water. The optical rotations of the sugars change until they reach the same value.

the explanation for this mutarotation lies in the existence of an equilibrium between the open-chain form of D-(+)-glucose and the and forms of the cyclic hemiacetals.

the concentration of open-chain D-(+)-glucose in the concentration of open-chain D-(+)-glucose in solution at equilibrium is very small.solution at equilibrium is very small.

R C OR'

OH

R''

HCl(g)

R' OHR C OR'

OR'

R''

Hemiacetal (R'' may be H) An acetal (R'' may be H)

H2O

HemiacetalHemiacetal: a functional group, consisting of a : a functional group, consisting of a carbon atom bonded to an alkoxy group and to a carbon atom bonded to an alkoxy group and to a hydroxyl group. Hemiacetals are synthesized by hydroxyl group. Hemiacetals are synthesized by adding one molar equivalent of an alcohol to an adding one molar equivalent of an alcohol to an aldehyde or a ketone.aldehyde or a ketone.

Glycoside FormationGlycoside Formation Carbohydrate acetals, generally, are called

glycosides, and an acetal of glucose is called a glucoside.

The methyl D-glucosides have been shown to have six-membered ring, so they are properly named methyl -D-glucopyranoside and -D-glucopyranoside.

Glycosides are stable in basic solutions because they are acetals.

In acidic solutions, glycosides undergo hydrolysis to produce a sugar and an alcohol (aglycone).

Sugars that contain nitrogenSugars that contain nitrogen

1. Glycosylamines1. Glycosylamines

A sugar in which an amino group replaces the anomeric OH group.

Adenosine is an example of a glycosylamine that is also called a nucleoside.

O

H

HO

H

HO

H

H

OHHNH2

OH

-D-Glucopyranosylamine

2. Amino sugars2. Amino sugars

A sugar in which an amino group replaces a nonanomeric OH group. e.g. D-glucosamine.

D-glucosamine can be obtained by hydrolysis of chitin, a polysaccharide found in the shells of lobsters and crabs and in the external skeletons of insects and spiders.

O

H

HO

H

HO

H

H

NH2

HOH

OH

-D-Glucosamine

Other Reactions of MonosaccharidesOther Reactions of Monosaccharides

1. Enolization, Tautomerization, and Isomerization1. Enolization, Tautomerization, and Isomerization

• Dissolving monosaccharides in aqueous base Dissolving monosaccharides in aqueous base causes them to undergo enolizations and a series causes them to undergo enolizations and a series of keto-enol tautomerizations that lead to of keto-enol tautomerizations that lead to isomerizations. isomerizations.

RC

OH

CR2

RC

O

CR2 RC

O

CR2

Enol

Enolate ion

EnolsEnols are in equilibrium with an isomeric aldehyde or are in equilibrium with an isomeric aldehyde or ketone, but are normally much less stable than aldehydes ketone, but are normally much less stable than aldehydes and ketones. and ketones.

Enolate ionEnolate ion is the conjugate base of an enol. Enolate ions is the conjugate base of an enol. Enolate ions are stabilized by electron delocalization. are stabilized by electron delocalization.

TautomerizmTautomerizm is a process by which two isomers are is a process by which two isomers are interconverted by a movement of an atom or a group. interconverted by a movement of an atom or a group. Enolization is a form of tautomerism. Enolization is a form of tautomerism.

2. Formation of Ethers2. Formation of Ethers

O

H

HO

H

HO

H

H

OH

OH OCH3

Mthyl glucoside

O

H

HO

H

HO

H

H

OH

O OCH3

O

H

HO

H

HO

H

H

OH

OCH3 OCH3

H3C OSO3CH3

OH

The hydroxyl groups of monosaccharides are more acidic than The hydroxyl groups of monosaccharides are more acidic than those of ordinary alcohols those of ordinary alcohols becausebecause the monosaccharide contains the monosaccharide contains so many electronegative oxygen atoms, all of which exert electron-so many electronegative oxygen atoms, all of which exert electron-withdrawing inductive effects on nearby hydroxyl groups.withdrawing inductive effects on nearby hydroxyl groups.

In aqueous NaOH, the hydroxyl groups are converted to alkoxide In aqueous NaOH, the hydroxyl groups are converted to alkoxide ions and each of these in turn, reacts with dimethyl sulfate to yield a ions and each of these in turn, reacts with dimethyl sulfate to yield a methyl ether. methyl ether.

33 . .Conversion to EstersConversion to Esters

O

H

H3CCO

H

H3CCO

H

H

OCCH3

OCCH3 OCCH3

O

O

O

O

O

O

H

HO

H

HO

H

H

OH

OH OH

(CH3CO)2O°Pyridine,0 C

Treating a monosaccharide with excess acetic Treating a monosaccharide with excess acetic anhydride and a weak base (such as pyridine or anhydride and a weak base (such as pyridine or sodium acetate) converts all of the hydroxyl groups, sodium acetate) converts all of the hydroxyl groups, including the anomeric hydroxyl, to ester groups.including the anomeric hydroxyl, to ester groups.

4. Oxidation Reactions of Monosaccharides4. Oxidation Reactions of Monosaccharides

A. Fehling’s solution or Benedict’s reagentA. Fehling’s solution or Benedict’s reagent A characteristic property of an aldehyde function is its

sensitivity to oxidation. Carbohydrates that give positive tests with Fehling’s or

Benedict’s reagents are termed Reducing Sugars. Ketoses are also reducing sugars, since under the

conditions of the test, ketoses equilibrate with aldoses by way of enediol intermediates, and the aldoses are oxidized by the reagents.

CH

O

2Cu2+ R CO

O

Cu2O 3H2O

Aldehyde From copper(II)sulfate

Hydroxide ion

Carboxylateanion

Copper(I)oxide

Water

R 5HO

B. Bromine Water: the synthesis of aldonic acidsB. Bromine Water: the synthesis of aldonic acids

Bromine water is a general reagent that selectively Bromine water is a general reagent that selectively oxidizes the oxidizes the CHO group to a CHO group to a COCO22H group.H group.

(CHOH)n

CHO

CH2OH

Br2

H2O(CHOH)n

CO2H

CH2OH

Aldose Aldonic acid

C. Nitric Acid Oxidation: Aldaric AcidsC. Nitric Acid Oxidation: Aldaric Acids

(CHOH)n

CHO

CH2OH

(CHOH)n

CO2H

CO2H

Aldose Aldaric acid

HNO3

Dilute nitric acid – a stronger oxidizing agent than Dilute nitric acid – a stronger oxidizing agent than bromine water – oxidizes both the –CHO group and bromine water – oxidizes both the –CHO group and the terminal –CHthe terminal –CH22OH group of an aldose to –COOH group of an aldose to –CO22H H group. These dicarboxylic acids are known as group. These dicarboxylic acids are known as aldaric acidsaldaric acids..

D. Periodate oxidations: oxidative cleavage of D. Periodate oxidations: oxidative cleavage of polyhydroxy compounds polyhydroxy compounds

Compounds that have hydroxyl groups on adjacent Compounds that have hydroxyl groups on adjacent atoms undergo oxidative cleavage when they are atoms undergo oxidative cleavage when they are treated with aqueous periodic acid (HIOtreated with aqueous periodic acid (HIO44).).

In these periodate oxidations that In these periodate oxidations that for every Cfor every CC C bond broken, a Cbond broken, a CO bond is formed at each carbonO bond is formed at each carbon..

C

C

OH

OH

HIO4 2 C

O

HIO4 H2O

When three or more –CHOH groups are When three or more –CHOH groups are contiguous, the internal ones are obtained as contiguous, the internal ones are obtained as formic acidformic acid. For example, glycerol. For example, glycerol

H

C

C

C

H

H

H

H OH

OH

OH

Glycerol

2 IO4

C

H H

O

C

H OH

O

C

H H

O

(formaldehyde)

(formaldehyde)

(formic acid)

Oxidative cleavage also takes place when an –OH Oxidative cleavage also takes place when an –OH group is adjacent to the carbonyl group of an group is adjacent to the carbonyl group of an aldehyde or ketone (but not that of an acid or an aldehyde or ketone (but not that of an acid or an ester). For example, glyceraldehydeester). For example, glyceraldehyde

O

C

C

C

H

H

H OH

OH

H

Glyceraldehyde

2 IO4

C

H OH

O

C

H OH

O

C

H H

O

(formic acid)

(formaldehyde)

(formic acid)

Periodic acid does not cleave compounds in which Periodic acid does not cleave compounds in which the hydroxyl groups are separated by an the hydroxyl groups are separated by an intervening –CHintervening –CH2 2 – group, nor those in which a – group, nor those in which a hydroxyl group is adjacent to an ether or acetal hydroxyl group is adjacent to an ether or acetal function.function.

H2C

CH2

H2C OH

OH

IO4 no cleavage

H2C

HC

H2C R

OH

OCH3

IO4 no cleavage

5. Reduction of Monosaccharides: Alditols5. Reduction of Monosaccharides: Alditols

(CHOH)n

CHO

CH2OH

(CHOH)n

CH2OH

CH2OH

Aldose Alditol

NaBH4

Aldoses (and ketoses) can be reduced with sodium Aldoses (and ketoses) can be reduced with sodium borohydride to compounds called borohydride to compounds called alditolsalditols. For . For example, D-glucitol (or D-sorbitol)example, D-glucitol (or D-sorbitol)

DisaccharidesDisaccharides

Disaccharides are carbohydrates that yield Disaccharides are carbohydrates that yield two monosaccharide molecules on hydrolysis.two monosaccharide molecules on hydrolysis.

e.g. sucrose, lactose, maltosee.g. sucrose, lactose, maltose

Structurally, disaccharides are Structurally, disaccharides are glycosidesglycosides in in which the alkoxy group attached to the which the alkoxy group attached to the anomeric carbon is derived from a second anomeric carbon is derived from a second sugar molecule. sugar molecule.

SucroseSucroseHHOH2C

HO H

H OH

O

CH2OHO

Glucosidiclinkage

Fructosidiclinkage

1

2

3 4

56

O

H

HO

H

HO

H

H

OH

OH

H12

3

45

6

Ordinary table sugar (C12H22O11) Acid hydrolysis yields D-glucose and D-fructose. Sucrose is a nonreducing sugar; it gives negative

tests with Fehling’s solution because neither the glucose nor the fructose portion of sucrose has a hemiacetal group (both carbonyl groups are present as full acetals (i.e. as glycosides).

PolysaccharidesPolysaccharides Polysaccharides, also known as glycans, consist of

monosaccharides joined together by glycosidic linkages.

Polysaccharides that are polymers of a single monosaccharide are called homopolysaccharides; those made up of more than one type of monosaccharide are called heteropolysaccharides.

Homopolysaccharides are also classified on the basis of their monosaccharide units. A homopolysaccharide consisting of glucose monomeric unit is called a glucan, one consisting of galactose units is a galactan, and so on.

PolysaccharidesPolysaccharides

Three important polysaccharides, all of which Three important polysaccharides, all of which are glucans, are starch, glycogen, and are glucans, are starch, glycogen, and cellulose.cellulose.

StarchStarch is the principle food reserve of plant. is the principle food reserve of plant. GlycogenGlycogen functions as a carbohydrate reserve functions as a carbohydrate reserve

for animals.for animals. CelluloseCellulose serves as structural material in serves as structural material in

plants.plants.

Carbohydrate AntibioticsCarbohydrate Antibiotics One of the important

discoveries in carbohydrate chemistry was the isolation of the carbohydrate antibiotic called streptomycin.

Streptomycin is made up of three unusual components.

The glycosidic linkage is

nearly always .

OH

HN

NH

OH

NH2

NH

NH

H2N

OH

O

CHO

HO

H3C

O

R'R

HO

HO

R = NHCH3R' = CH2OH

O

O

Streptomycin

Streptidine

L-Streptose

2-Deoxy-2-methylamino- L-glucopyranose