Carbohydrates

General molecular formula Cn(H2O)n

Appeared to be hydrates of carbon.

not all carbohydrates have this empirical formula:

deoxysugars, aminosugars

Carbohydrate - polyhydroxy aldehyde, ketones.

General characteristics

Most carbohydrates are found naturally in bound form rather than as simple sugars

Polysaccharides (starch, cellulose, inulin, gums)

Glycoproteins and proteoglycans (hormones, blood group substances, antibodies)

Glycolipids (cerebrosides, gangliosides)

Glycosides

Nucleic acids

Classification of

carbohydrates

Monosaccharides Trioses, tetroses, pentoses, hexoses

DisaccharidesMaltose, sucrose, lactose

Oligosaccharides 3 to 9

Polysaccharides or glycans Homopolysaccharides

Heteropolysaccharides

D-Glucose in Nature

The most abundant carbohydrate is D-glucose.

Cells of organisms oxidize glucose for energy:

In animals excess glucose is converted to a polymer called glycogen.

Disaccharides On hydrolysis give two

molecules of monosaccharides

E.g

Sucrose (Cane sugar)

Lactose (milk sugar)

Maltose (malt sugar)

Polysaccharides

Starch, cellulose, glycogen

On the hydrolysis of each of them, they

yields large number of

monosaccharides.

Monosaccharides

also known as simple sugars

classified by 1. the number of carbons and 2.

whether aldoses or ketoses

most (99%) are straight chain compounds

D-glyceraldehyde is the simplest of the

aldoses (aldotriose)

all other sugars have the ending ose

(glucose, galactose, ribose, lactose, etc…)

Monosaccharides

• General formula (CH2O)n

• Triose: n = 3 (e.g., glyceraldehyde)

• Tetrose: n = 4

• Pentose: n = 5 (e.g., ribose)

• Hexose: n = 6 (e.g., glucose)

• Heptose: n = 7

CONCEPTS OF ISOMERS

Two or more different compounds which contain the

same number and types of atoms and the same

molecular weights.

Stereoisomers: Enantiomers and Diastereomers

Stereoisomers Are not constitutional isomers since they have the constituent atoms connected in the same sequence! They only differ in the arrangement of their atoms in space! Stereoisomers can be subdivided into two categories:

Enantiomers: Are stereoisomers whose molecules are mirror images of each other. (These are like our hands). The molecules of enantiomers are not superimposeable

Diastereomers: Are stereoisomers that

are not mirror images of each other as

indicated in (Fig.).

Monosaccharides

Represented by Fischer projections

Emil Fischer

Nobel Prize 1902

C

CHO

CH2OH

H OH

D-Glyceraldehyde

C

CH2OH

C OH

OHH

D- and L- Notation

Prior to determination of absolute configurations, the 19th century chemists assigned arbitrary designations to structures:

HC O

CH2OH

OHH

HC O

CH2OH

HHO

(R)-(+)-glyceraldehyde (S)-(–)-glyceraldehyde

D-glyceraldehyde L-glyceraldehyde

D- and L- Notation

If the OH group attached to the bottom-most chirality center is on the right, it is a D-sugar:

The D- or L- together with the common name of the monosaccharide

completely describes the structure, since the relative configurations at all

chirality centers is implicit in the common name.

Aldotetroses

Aldotetroses have two chirality centers

hence 22 = 4 stereoisomers:

C

CH2OH

OHH

C O

H

C OHH

C

CH2OH

HOH

C O

H

C HOH

these two aldotetroses are enantiomers.

They are stereoisomers that are mirror

images of each other

C O

H

C HHO

C HHO

CH OH

C

CH2OH

OHH

C O

H

C HHO

C HHO

CHO H

C

CH2OH

OHH

these two aldohexoses are C-4 epimers.

they differ only in the position of the

hydroxyl group on one asymmetric carbon

(carbon 4)

Enantiomers and epimers

Epimers

A pair of diastereomers that differ only in the

configuration about a single carbon atom are said to be

epimers.

H OH

OH

HO H

HO H

H OH

CH2OH

H OH

OH

HO H

H OH

H OH

CH2OH

HO H

OH

HO H

H OH

H OH

CH2OH

D(+)-Galactose D(+)-MannoseD(+)-Glucose

EpimersEpimers

Diastereomers

POLARIMETER

Dextrorotatory -plane polarized light rotated to clockwise (or to the

right)

Levoratatory - plane polarized light rotated to counterclockwise.

POLARIMETRY

Measurement of optical activity in chiral or

asymmetric molecules using plane polarized light

Molecules may be chiral because of certain atoms

or because of chiral axes or chiral planes

Measurement uses an instrument called a

polarimeter

polarimetry

Magnitude of rotation depends upon:

1. the nature of the compound

2. the length of the tube (cell or sample container) usually

expressed in decimeters (dm)

3. the wavelength of the light source employed; usually

either sodium D line at 589.3 nm or mercury vapor lamp

at 546.1 nm

4. temperature of sample

5. concentration of analyte in grams per 100 ml

[]DT

l x c

observed x 100 =

D = Na D line

T = temperature oC

obs : observed rotation in degree (specify solvent)

l = length of tube in decimeter

c = concentration in grams/100ml

[] = specific rotation

Specific rotation of various

carbohydrates at 20oC

D-glucose +52.7

D-fructose -92.4

D-galactose +80.2

L-arabinose +104.5

D-mannose +14.2

D-arabinose -105.0

D-xylose +18.8

Lactose +55.4

Sucrose +66.5

Maltose+ +130.4

Invert sugar -19.8

Dextrin +195

Most common monosaccharide.

Commercially from starch.

Mutarotation ---The optical changes of glucose in water solution to constant value

20D = +520

- D - glucose -> D - glucose <- b - D - glucose

20D = 113 20

D = 52 20D = = 19

At equilibrium = 35% of - form and 65% of b - form.

Glucose (dextrose)

MONOSACCHARIDE

Hexoses

1. Glucose (dextrose) --- rotate the polarized light to the

right.

OH

OH

CH

H C OH

C H

H

HO

H C

CH2OH

O

C

OH

O

OH

OHHO

CH2 OH

1

23

45

6

3. Fructose (levulsoe) --- Rotation in polarimeter is left

D-Fructose b-D-Fructose -D-Fructose

CH2OH

O

CH2OH

C

HO HC

OHCH

H C

OH

O

CH2OH

C

HO HC

OHCH

H C

CH2OHCH2OH

CH

HO

H C OH

C HHO

C

OH

CH2OH

O

or

b - D - Fructofuranose - D - Fructofuranose

O

HO

OH

CH2 OH

HOCH2 OH

O

HO

OH

HOCH2CH2OH

OH

Fructose (levulsoe)

Hexoses C6H12O6

C

C

C

CHO

C

CH2OH

OHH

H OH

HO H

H OH

D- Glucose

Oxidation reactions

Aldoses may be oxidized to acids

Aldonic acids: aldehyde group is converted to a carboxyl group ( glucose – gluconic acid)

Saccharic acids (glycaric acids) – oxidation at both ends of monosaccharide) Glucose ---- saccharic acid

Galactose --- mucic acid

Mannose --- mannaric acid

Oxidation Reactions: 1- Nitric acid,

HNO3: Nitric acid is a potent oxidizing reagent and

will convert both aldehydes into carboxylic acids. This

usually results in the conversion of an aldose into a

dicarboxylic acid derivative:ose/aric

b- Conc. HNO3

C

C

C

CHO

C

CH2OH

OHH

H OH

HO H

H OH

glucose

conc. HNO3

C

C

C

COOH

C

COOH

OHH

H OH

HO H

H OH

saccharic acid

C

C

C

CH2OH

C

CH2OH

OHH

H OH

HO H

O

D- fructose

COOH

C

C

COOH

OHH

H OH

meso-tartaric acid

Conc. HNO3

CH2OH

COOH+

glycollic acid

Bromine water

Ose/onic

H OH

OH

HO H

H OH

H OH

CH2OH

D(+)-Glucose

Br2/H2OH OH

OHO

HO H

H OH

H OH

CH2OH

Gluconic acid

Monosaccharides are further classified as reducing or non-reducing sugars according to their behaviour toward Ag(I) (Tollens’ solution) or Cu(II) (Benedict’s solution). Both Tollens’ and Benedict’s tests are visual tests for aldehyde groups which, when oxidized to carboxylic acids, reduce the silver or copper ions yielding metallic silver metal or red copper(I)oxide precipitates (Fig.).

Effect of Ba(OH)2 or Ca(OH)2

The alkaline reaction conditions facilitate the a

tautomeric equilibrium between the enol and keto

forms of the open-chain structure. Enolization may

result in the formation of glucose from fructose,

mannose or vice versa.

H OH

OH

OH H

H OH

H OH

CH2OH

OH H

OH

OH H

H OH

H OH

CH2OH

D-MannoseD-Glucose

C

CH OH

OH H

H OH

H OH

CH2OH

H OH

C O

OH H

H OH

H OH

CH2OH

H

H

OH

D-Fructose

1,2-Enediol

Acylation of monosaccharides Reaction with acetic anhydride: the alcohol groups of

sugars react with acetic anhydride to make ester

derivatives.

Reduction

either done catalytically (hydrogen and a catalyst) or enzymatically

the resultant product is a polyol or sugar alcohol (alditol)

glucose form sorbitol (glucitol)

mannose forms mannitol

fructose forms a mixture of mannitol and sorbitol

glyceraldehyde gives glycerol

Reaction of carbonyl groups

C

C

C

CHO

C

CH2OH

OHH

H OH

HO H

H OH

D- Glucose

C

C

C

CH2OH

C

CH2OH

OHH

H OH

HO H

H OH

sorbitol

Na/Hg

C

C

C

CH2OH

C

CH2OH

OHH

H OH

HO H

O

D- fructose

C

C

C

CH2OH

C

CH2OH

OHH

H OH

HO H

H OH

sorbitol

Na/Hg

C

C

C

CH2OH

C

CH2OH

OHH

H OH

HO H

HO H

+

mannitol

Reaction with HCN give cyanohydrin

C

C

C

CHO

C

CH2OH

OHH

H OH

HO H

H OH

D- Glucose

C

C

C

C

CH2OH

OHH

H OH

HO H

H OH

Glucose cyanide

HCN

CN

OHH

Reaction with hydroxylamine

C

C

C

CHO

C

CH2OH

OHH

H OH

HO H

H OH

D- Glucose

NH2OH. HCl

C

C

C

CH

C

CH2OH

OHH

H OH

HO H

H OH

Glucose oxime

N OH

Formation of osazones

once used for the identification of sugars

consists of reacting the monosaccharide with phenylhydrazine

a crystalline compound with a sharp melting point will be obtained

D-fructose and D-mannose give the same osazone as D-glucose

seldom used for identification; we now use HPLC or mass spectrometry

Formation of Osazone

C

(CHOH)3

CHO

H OH

D- Glucose

CH2OH

PhNHNH2C

(CHOH)3

CH=NHNHPh

H OH

CH2OH

PhNHNH2C

(CHOH)3

CH=NHNHPh

O

CH2OH

PhNHNH2

C

(CHOH)3

CH=NHNHPh

NHNHPh

CH2OH

OsazoneC

(CHOH)3

CH2OH

O

D- fructose

CH2OH

PhNHNH2C

(CHOH)3

CH2OH

NHNHPh

CH2OH

PhNHNH2

C

(CHOH)3

CHO

NHNHPh

CH2OH

PhNHNH2

Formation (Glycosides). Acetal derivatives formed when a monosaccharide

reacts with an alcohol in the presence of an acid

catalyst are called glycosides. "ose" suffix of the

sugar name is replaced by "oside", and the alcohol

group name is placed first

Sucrose

-D-glucopyranosido-b-D-fructofuranoside

b-D-fructofuranosido--D-glucopyranoside

sugar cane or sugar beet

hydrolysis yield glucose and fructose (invert sugar) ( sucrose: +66.5o ; glucose +52.5o; fructose –92o)

used pharmaceutically to make syrups, troches

Sugar cane

Sugar beet

Sucrose

2-0--D-Glucopyranosyl b-D-Fructofuranoside

O

OH

OHHO

CH2 OH

CH2OH

OCH2OH

O

HO

OH

H1

2

3 4

5

6

Lactose

b-D-galactose joined to -D-glucose via b(1,4) linkage

milk contains the and b-anomers in a 2:3 ratio

b-lactose is sweeter and more soluble than ordinary - lactose

used in infant formulations, medium for penicillin production and as a diluent in pharmaceuticals

Lactose

Principal sugar in milk

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

Maltose

2-glucose molecules joined via (1,4)

linkage

known as malt sugar

produced by the partial hydrolysis of

starch (either salivary amylase or

pancreatic amylase)

Disaccharides (anydrides of 2 monosaccharides):

Maltose: 4-0--D-Glucopyranosyl (1->4) -D-

Glucopyranose

O

OH

OHHO

CH2 OH

O

OH

OH

CH2 OH

OOH

Cellobiose

4-0-b-D-Glucopyranosyl (1->4)-b-D-Glucopyranose

O

OH

OHHO

CH2 OH

O

OH

OH

CH2 OH

O

OH

Sucralfate (Carafate)

Polysaccharides

homoglycans (starch, cellulose, glycogen,

inulin)

heteroglycans (gums, mucopolysaccharides)

characteristics: polymers (MW from 200,000)

White and amorphous products

not sweet

not reducing; do not give the typical aldose or ketose

reactions)

form colloidal solutions or suspensions

Starch

most common storage polysaccharide in plants

composed of 10 – 30% -amylose and 70-90% amylopectin depending on the source

the chains are of varying length, having molecular weights from several thousands to half a million

The reserve carbohydrate of plants. Occurs as granules

in the cell. Made of amylose and amylopectin.

Amylose --- ploymer of -D- Glucose (1->4) linkage-

straight-chain.

STARCH

O

OH

OH

CH2 OH

OHO OO

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

Amylose and amylopectin are the 2 forms of starch. Amylopectin

is a highly branched structure, with branches occurring every 12

to 30 residues

suspensions of amylose

in water adopt a helical

conformation

iodine (I2) can insert in

the middle of the amylose

helix to give a blue color

that is characteristic and

diagnostic for starch

(in starch)

(in cellulose)

Cellulose

Polymer of b-D-glucose attached by b(1,4) linkages

Yields glucose upon complete hydrolysis

Partial hydrolysis yields cellobiose

Most abundant of all carbohydrates Cotton flax: 97-99% cellulose

Wood: ~ 50% cellulose

Gives no color with iodine

Held together with lignin in woody plant tissues

POLYSACCHARIDE

Cellulose --- polymer of b-D-Glucose (1, 4) linkage.

Repeating cellobiose moiety.

O

OH

OH

CH2 OH

OH

O OO

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

O

OH

OH

CH2 OH

n

Structure of cellulose

Glycogen

also known as animal starch

stored in muscle and liver

present in cells as granules (high MW)

contains both (1,4) links and (1,6) branches at every 8 to 12 glucose unit

complete hydrolysis yields glucose

glycogen and iodine gives a red-violet color

hydrolyzed by both and b-amylases and by glycogen phosphorylase

GLYCOGEN

Animal starch.

- (1 -> 4) linkage and - (1 -> 6) linkage

12 : 1

RELATIVE SWEETNESS OF DIFFERENT SUGARS

Sucrose 100

Glucose 74

Fructose 174

Lactose 16

Invert Sugar 126

Maltose 32

Galactose 32

CARBOHYDRATE DETERMINATION

1. Monosaccharides and Oligosaccharides

A. Enzymatic Method

1. Glucose oxidase

2. Hexokinase

B. Chromatography Method

1. Paper or thin layer chromatography

2. Gas chromatography

3. Liquid column chromatography

2. Polysaccharides

Glucose Oxidase System

Glucose Oxidase

D-Glucose + O2 Gluconic Acid + H2O2

Peroxidase

H2O2+ 0 - Dianisidine 2 H2O + Oxidized 0-Dianisidine

(Colorless) (Brown)

OCH3H3CO

H2N NH2

H3CO OCH3

HN NH

Synthetic Sweeteners