Carbohydrates. Poly hydroxy aldehydes or poly hydroxy-ketones of formula (CH 2 O) n And are usually classified according to their structure: Monosaccharides.

Carbohydrates

Carbohydrates• Poly hydroxy aldehydes or poly hydroxy-ketones of formula

(CH2O)n

And are usually classified according to their structure: Monosaccharides - simple sugars with multiple OH groups.

Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose, cannot be hydrolyzed to simpler carbohydrates; eg. Glucose or fructose.

Disaccharides - 2 monosaccharides covalently linked can be hydrolyzed into two monosaccharide units; eg. Sucrose, which is hydrolyzed into glucose and fructose.

Oligosaccharides - a few monosaccharides covalently linked. Polysaccharides - polymers consisting of chains of

monosaccharide or disaccharide units. eg Starch or cellulose.

Monosaccharides

Aldoses (e.g., glucose) have an aldehyde group at one end.

Ketoses (e.g., fructose) have a keto group, usually at C2 .

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

D vs L Designation

D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde.

The lower representations are Fischer Projections.

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

L-glyceraldehydeD-glyceraldehyde

L-glyceraldehydeD-glyceraldehyde

Sugar Nomenclature

For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.

Most naturally occurring sugars are D isomers.

O H O H C C H – C – OH HO – C – H

HO – C – H H – C – OH

H – C – OH HO – C – H

H – C – OH HO – C – H

CH2OH CH2OH

D-glucose L-glucose

Hemiacetal & hemiketal formation

An aldehyde can react with an alcohol to form a hemiacetal.

A ketone can react with an alcohol to form a hemiketal.

O C

H

R

OH

O C

R

R'

OHC

R

R'

O

aldehyde alcohol hemiacetal

ketone alcohol hemiketal

C

H

R

O R'R' OH

"R OH "R

+

+

Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH.Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring.

These representations of the cyclic sugars are called Haworth projections.

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose (linear form)

Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, a & b.

Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1:

a (OH below the ring) b (OH above the ring).

H O

OH

H

OHH

OH

CH2OH

H

-D-glucose

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose

23

4

5

6

1 1

6

5

4

3 2

Glycosidic BondsThe anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond:

R-OH + HO-R' R-O-R' + H2O

E.g., methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl-glucopyranose).

O

H

HO

H

HO

H

OH

OHHH

OH

-D-glucopyranose

O

H

HO

H

HO

H

OCH3

OHHH

OH

methyl- -D-glucopyranose

CH 3-O H+

methanol

H2O

Cellobiose, a product of cellulose breakdown, is the otherwise equivalent b anomer (O on C1 points up). The b(1® 4) glycosidic linkage is represented as a zig-zag.

H O

O H

H

O HH

O H

CH 2O H

H

O H

O H

H

O HH

O H

CH 2O H

H

O

HH

1

23

5

4

6

1

23

4

5

6

m altose

H O

O H

H

O HH

O H

CH 2O H

H

O O H

H

H

O HH

O H

CH 2O H

H

H

H

O1

23

4

5

6

1

23

4

5

6

cellobiose

Disaccharides:Maltose, a cleavage product of starch (e.g., amylose), is a disaccharide with an a(1® 4) glycosidic link between C1 - C4 OH of 2 glucoses.

Other disaccharides include: Sucrose, common table sugar, has a glycosidic bond

linking the anomeric hydroxyls of glucose & fructose.

Because the configuration at the anomeric C of glucose is a (O points down from ring), the linkage is a(12).

Lactose, milk sugar, is composed of galactose & glucose, with b(14) linkage from the anomeric OH of galactose.

Polysaccharides:

Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch.

Glucose storage in polymeric form minimizes osmotic effects.

Amylose is a glucose polymer with a(14) linkages.

The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end.

H O

OH

H

OHH

OH

CH 2 OH

HO H

H

OHH

OH

CH 2 OH

H

O

HH H O

OH

OHH

OH

CH 2 OH

HH H O

H

OHH

OH

CH 2 OH

H

OH

HH O

OH

OHH

OH

CH 2 OH

H

O

H

1

6

5

4

3

1

2

a m y lo s e

Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more a(16) branches. The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants.

H O

OH

H

OHH

OH

CH 2OH

HO H

H

OHH

OH

CH 2OH

H

O

HH H O

OH

OHH

OH

CH 2

HH H O

H

OHH

OH

CH 2OH

H

OH

HH O

OH

OHH

OH

CH 2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH 2OH

HH H O

H

OHH

OH

CH 2OH

HH

O1

OH

3

4

5

2

glycogen

Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with b(1®4) linkages.Every other glucose is flipped over, due to b linkages. This promotes intra-chain and inter-chain H-bonds and

c e l lu lo s e

H O

OH

H

OHH

OH

CH 2 OH

HO

H

OHH

OH

CH 2 OH

HO

H H O

O H

OHH

OH

CH 2 OH

HH O

H

OHH

OH

CH 2 OH

H

H

OHH O

O H

OHH

OH

CH 2 OH

HO

H H H H

1

6

5

4

3

1

2

van der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles - microfibrils.

Schematic of arrangement of cellulose chains in a microfibril.

•All monosaccharides•Maltose, LactoseReducing

sugars

•Sucrose•All polysaccharidesNon-reducing

sugars

Classification upon reducing end

Common Oxidizing Agents

Benedict’s sol. Fehling’s sol. Tollen’s reagent

Oxidizing Reagent Benedict's Solution Fehling's Solution Tollen's Reagent

Composition copper sulfate in

alkaline citrate

copper sulfate in alkaline

tartrate

silver nitrate in aqueous

ammonia

Color of Solution deep blue deep blue colorless

Color After Reaction with a

Reducing Sugar

brick red precipitate

Cu2O(s)

brick red precipitate

Cu2O(s)

silver mirror forms

Ag(s)

Species Being Reduced

(the oxidant)

Cu2+

Cu2+ + e ---> Cu+

Cu2+

Cu2+ + e ---> Cu+

Ag+

Ag+ + e ---> Ag(s)

Species Being Oxidized

(the reductant)

reducing sugar

oxidized to carboxylate

reducing sugar

oxidized to carboxylate

reducing sugar

oxidized to carboxylate

In-lab ExperimentsBe

nedi

ct’s

Test

For

re

duci

ng s

ugar

s

Barf

oed’

s Te

st

for m

onos

acch

arid

es

Bial

’s (O

rcin

ol) T

est f

or

pent

oses

Seliw

anoff

’s Te

st

for K

etos

es

Picr

ic A

cid

Test

For

re

duci

ng s

ugar

s

1 .Benedict's Test(positive for reducing sugars)

• Principle:Benedict's reagent contains cupric ions, which in an alkaline environment, oxidize the aldehyde group to a carboxylic acid. Cupric ions are reduced to cuprous oxide, which forms a red precipitateRCHO + 2Cu2+ + 4OH- ----> RCOOH + Cu2O + 2H2O

Procedure

1. Place 1mL of the following 1% carbohydrate solutions in separate, labeled test tubes: glucose, fructose, sucrose, lactose, maltose, and starch.

2. Also place 1 ml of distilled water in another tube to serve as a control.

3. To each tube, add 1 ml of Benedict's reagent and heat the tubes in a boiling water bath for 5 minutes.

4. Remove the tubes from water bath. Note and record the results.

In the presence of a reducing sugar a precipitate which may be red, yellow or green will form.

2 .Barfoed's Test(Used to distinguish between mono- & di-saccharides)

• PrincipleBarfoed's reagent reacts with mono-saccharides to produce cuprous oxide at a faster rate than disaccharides do:

• RCHO + 2Cu2+ + 2H2O -----> RCOOH + Cu2O + 4H+

Procedure

• Procedure: 1. Place 1 mL of the following 1% carbohydrate

solutions in separate, labeled test tubes: glucose, fructose, sucrose, lactose, and maltose.

2. To each tube, add 1 ml of Barfoed's reagent, and heat in a boiling water bath for 10 minutes.

3. Remove the tubes from water bath. Note and record your observations.

A red precipitate will form if the test is positive.

3 .Bial's (Orcinol) Test for pentoses( for the detection of pentoses)

Principle• Pentoses are converted to furfural by this

reagent, which forms a blue green color with orcinol.

Procedure

1. Add about 1 ml of 1% xylose, glucose, fructose, maltose, arabinose, and xylose solution to their respective labeled test tubes.

2. Add 1.5 ml of Bial's reagent to each tube and mix well.3. Carefully heat each tube (with some agitation) directly over the

burner flame. Hold the tube at a diagonal and heat along the sides of the tube rather than at the bottom to prevent eruption of the liquid from the tube. Move the tube diagonally in and out of the flame, until the mixture just begins to boil. Stop heating when the mixture begins to boil.

A blue-green color indicates a positive result. Prolonged heating of some hexoses yields hydroxymethyl furfural which also reacts with orcinol to give colored complexes.

Bial's reagent (0.1 % orcinol in concentrated HCl containing 0.1 % FeCl3.6H2O).

4 .Seliwanoff's (Resorcinol) Test(used for detection of Ketoses)

• PrincipleKetohexoses (such as fructose) and disaccharides containing a ketohexose (such as sucrose) form a cherry-red condensation product. Other sugars (e.g. aldose) may produce yellow to faint pink colors.

Procedure

1. Add about 3 ml of Seliwanoff's reagent to each labeled test tube.

2. Add 1 drop of the respective sugar solution to the appropriate test tubes, and mix well.

3. Place all the test tubes in the boiling water bath at the same time and heat for 3 min after the water begins to boil again. Record your observations.

A positive result is indicated by the formation of a red color with or without the separation of a brown-red precipitate.

Seliwanoff's reagent (0.5 % resorcinol in 3N HCl).

5 .Picric Acid Test(for reducing sugars)

• PrinciplePicric acid (2,4,6-trinitrophenol) or TNP reacts

with reducing sugars to give a red colored picramic acid C6H2.OH.NH2(NO2)2

Procedure

1. Into a test tube add 1 ml of maltose solution, into the second tube, 1ml of sucrose solution.

2. Add into each tube 1 ml of a saturated solution of picric acid, and then add into each tube 0.5 ml of sodium hydroxide solution.

3. Heat both samples in a boiling water bath. In the presence of reducing sugars, the

solution stains red; a sodium salt of picric acid is formed.