pharmacognosy- 1 phg 222 · • a monosaccharide containing three ... carbohydrates that give...

Prof. Dr. Amani S. Awaad

Professor of PharmacognosyPharmacognosy Department,

College of Pharmacy Salman Bin Abdulaziz

University,

Al-Kharj. KSA.

Email: [email protected]

Pharmacognosy- 1

PHG 222

Carbohydrates

To know what is carbohydrates

To Recognize Classification of

carbohydrates

To Identify types Monosaccharides

To know Structural formulas for

Monosaccharides

To understand Reactions of

Monosaccharides

To know Polysaccharides

To understand and identefiy Carbohydrate

Antibiotics

• 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)

Carbohydrates

• Monosaccharide (C6H12O6)

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

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

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

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

Classification of carbohydrates

Carbohydrates

• Over 200 different monosaccharidesare known.

• Monosaccharides 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 threecarbon atoms is called a triose; one containing four carbon atoms is called a tetrose.

• A monosaccharide containing an aldehyde group is called an aldose; one containing a keto group is called a ketose.

Monosaccharaides

Carbohydrates


Monosaccharaides

Carbohydrates


• 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.

Monosaccharaides

Carbohydrates


• 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-()-.

Monosaccharaides

Carbohydrates

Structural formulas for Monosaccharides

• Fisher projection

• Haworth formulas

• anomer or anome

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

• 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 mutarotationlies 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 solution at equilibrium is very small.

Mutarotation

Monosaccharaides

Carbohydrates


H2OH2O

Glycoside Formation

Carbohydrates

Monosaccharaides


Hemiacetal: a functional group, consisting of a carbon atom bonded to an alkoxy group

and to a hydroxyl group. Hemiacetals are synthesized by adding one molar equivalent of

an alcohol to an aldehyde or a ketone.

The reactions of aldehydes and ketones with alcohols parallel their reactions

with water.

• 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).

Glycoside Formation

Carbohydrates

Monosaccharaides


Carbohydrates

Sugars that contain nitrogen

1. Glycosylamines

Glycosylamines are a class of biochemical compounds consisting of an amine with a β-N-glycosidic bond to a carbohydrate, forming a cyclic hemiaminal ether bond (α-aminoether).

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

2. 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.

Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the

biochemical synthesis of glycosylated proteins and lipids.

http://en.wikipedia.org/wiki/Amine

http://en.wikipedia.org/wiki/Glycosidic_bond

http://en.wikipedia.org/wiki/Carbohydrate

http://en.wikipedia.org/wiki/Hemiaminal_ether

Reactions of MonosaccharidesCarbohydrates

1. Enolization, Tautomerization, and Isomerization

Dissolving monosaccharides in aqueous base

causes them to undergo enolizations and a

series of keto-enol tautomerizations that lead to

isomerizations.

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

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

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

RC

OH

CR2

RC

O

CR2 RC

O

CR2

Enol

Enolate ion

Tautomerization creates structural isomers

D-Glucose to D- Fructose

Note: Formation of a glycoside prevents enolization

Reactions of Monosaccharides

Carbohydrates

2. Formation of Ethers

The hydroxyl groups of

monosaccharides are more acidic than

those of ordinary alcohols because the

monosaccharide contains so many

electronegative oxygen atoms, all of

which exert electron-withdrawing

inductive effects on nearby hydroxyl

groups.

In aqueous NaOH, the hydroxyl

groups are converted to alkoxide ions

and each of these in turn, reacts with

dimethyl sulfate to yield a methyl

ether.

Formation of Ethers

– Exhaustive Methylation

• SN2 Mechanism

• Base-catalyzed


Carbohydrates

3. Conversion to Esters

Treating a monosaccharide with excess acetic anhydride and a weak base

(such as pyridine or sodium acetate) converts all of the hydroxyl groups,

including the anomeric hydroxyl, to ester groups

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


Carbohydrates

4. Oxidation Reactions of Monosaccharides

A. 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

Fehling’s

Benedict’s


Carbohydrates

B. Bromine Water: the synthesis of aldonic acids


Bromine water is a general reagent

that selectively oxidizes the - CHO

group to α-CO2H group

Bromine Water:

The Synthesis of Aldonic Acid

Selectively oxidizes Aldehydes,

Gives a positive Test for Aldose,

and

Gives a negative test for Ketose.


Carbohydrates


C. Nitric Acid Oxidation: Aldaric Acids

Dilute nitric acid – a stronger oxidizing agent

than bromine water – oxidizes both the –CHO

group and the terminal –CH2OH group of an

aldose to –CO2H group. These dicarboxylic

acids are known as aldaric acids.

(CHOH)n

CHO

CH2OH

(CHOH)n

CO2H

CO2H

Aldose Aldaric acid

HNO3


Carbohydrates


D. Periodate oxidations: oxidative cleavage of polyhydroxy compounds

Compounds that have hydroxyl groups on adjacent atoms undergo

oxidative cleavage when they are treated with aqueous periodic acid

(HIO4).

In these periodate oxidations that for every C-C bond broken, a C-O

bond is formed at each carbon.

C

C

OH

OH

HIO42 C

O

HIO4 H2O

Stoichiometric cleavage

When three or more –CHOH groups are contiguous,

the internal ones are obtained as formic acid. For

example, glycerol

Oxidative cleavage also takes place when an –OH

group is adjacent to the carbonyl group of an

aldehyde or ketone (but not that of an acid or an

ester). For example, glyceraldehyde

Periodic acid does not cleave compounds in which the

hydroxyl groups are separated by an intervening –CH2 –

group, nor those in which a hydroxyl group is adjacent

to an ether or acetal function.

Reactions of MonosaccharidesCarbohydrates

Aldoses (and ketoses) can be reduced with sodium borohydride to

compounds called alditols. For example, D-glucitol (or D-sorbitol)

5. Reduction of Monosaccharides: Alditols

(CHOH)n

CHO

CH2OH

(CHOH)n

CH2OH

CH2OH

Aldose Alditol

NaBH4

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

e.g. 1)sucrose,

2)lactose,

3)maltose

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

DisaccharidesCarbohydrates

• 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).

Disaccharides

Carbohydrates

Sucrose

• 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.

Polysaccharides

Carbohydrates

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

• Starch is the principle food reserve of plant.

• Glycogen functions as a carbohydrate reserve for animals.

• Cellulose serves as structural material in plants.

Polysaccharides Carbohydrates

Carbohydrate 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 = NHCH3

R' = CH2OH

O

O

Streptomycin

Streptidine

L-Streptose

2-Deoxy-2-methylamino-L-glucopyranose

Carbohydrates

Carbohydrate antibiotics refers to microbial metabolites (mainly from

Actinomyces species) having antibacterial and antifungal properties and which

are carbohydrates or contain a carbohydrate component in their chemical

structure. Types include the aminoglycosides, macrolides, nucleoside analogs,

and glycosylated aromatic structures

pharmacognosy- 1 phg 222 · • a monosaccharide containing three ... carbohydrates that give...

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