Structure, Occurence and Reactions of Maltose, Lactose and Sucrose. Qualitative tests for disaacharides. Importance of Glycosides and Amino Sugars. Occurence and Structures of Homopolysaccharides such as Starch and Cellulose. Occurence and Structures of Heteropolysaccharides such as Hyaluronic acid and Chondroitin sulphates.

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• Title
CARBOHYDRATES
PART 2
Dr. S.M.Y. Mohamed Mukthar Ali
Department of Chemistry
Sadakathullah Appa College, Tirunelveli
Email Id: smymukthar@gmail.com

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CARBOHYDRATES PART 2

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Maltose – Occurrence & Structure
• Occurs naturally in malted barley, obtained from the hydrolysis of starch, found in the animal and
human digestive juices and used in beverages and baking food.
• Maltose is a disaccharide with the molecular formula – C12H22O11
• It is disacchride made up of two monomers of -D-Glucose units.
• These two glucose units are connected through -(14) glycosidic linkages.
• -(14) glycosidic linkages – 1st anomeric carbon of the -D-Glucose is linked to the 4th carbon
of the second -D-Glucose molecule through oxygen linkage.
• It is a reducing sugar.

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Formation of -(14) glycosidic linkage in Maltose

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Lactose – Occurrence & Structure
• Occurs naturally in milk and dairy food. Lactose intolerance is a medical condition of accumulation
of lactose in the body. The condition predominantly exist in people who lack the lactase enzyme,
which is essential for the metabolism of lactose.
• Lactose is a disaccharide with the molecular formula – C12H22O11
• It has one monomer of -D-Galactose and one monomer of -D-Glucose.
• These two monosaccharide units are connected through -(14) glycosidic linkages.
• -(14) glycosidic linkages – 1st anomeric carbon of the -D-Galactose is linked to the 4th carbon
of the -D-Glucose molecule.
• It is a reducing sugar.

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Sucrose – Occurrence & Structure
• Occurs naturally in sugarcane and sugarbeets.
• Sucrose is known as table sugar or just sugar.
• It is a disaccharide with the molecular formula – C12H22O11
• It has one monomer of -D-Glucose and one monomer of -D-Fructose.
• These two monosaccharide units are connected through ,-(12) glycosidic linkages.
• ,-(12) glycosidic linkages – 1st anomeric carbon of the -D-Glucose is linked to the 2nd
anomeric carbon of the -D-Fructose.
• The double arrow (⟷) used in this notation indicates that the glycosidic bond is between two
anomeric carbons.
• It is a non-reducing sugar.

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Sucrose – Occurrence & Structure

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Maltose – Reactions
• Since maltose is a carbohydrate, it answers for Molisch Test.
• Since maltose is a reducing sugar, it answers for Benedict’s, Fehling’s and Barfoed Test.
• Maltose reduces the Cu2+ to Cu+ and gives red colour precipitate (Cu2O).
• The free aldehydic group with the anomeric carbon in one of the of -D-glucose unit responds to
these tests.

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Maltose – Reactions
• Since maltose is a reducing sugar, it forms osazone which is a sunflower shaped crystal.
• The free aldehydic group with the anomeric carbon in one of the of -D-glucose unit responds to
this test.

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Maltose – Reactions
• Maltose on hydrolysis produces two molecules of -D-glucose units.
-D-Glucose -D-Glucose

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Lactose – Reactions
• Since lactose is a carbohydrate, it answers for Molisch Test.
• Since lactose is a reducing sugar, it answers for Benedict’s, Fehling’s and Barfoed Test.
• Lactose reduces the Cu2+ to Cu+ and gives red colour precipitate (Cu2O).
• The free aldehydic group with the anomeric carbon the of -D-glucose unit responds to these
tests.

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Lactose – Reactions
• Since Lactose is a reducing sugar, it forms osazone which is a tight balls of needles shaped
crystal.
• The free aldehydic group with the anomeric carbon one of the of -D-glucose unit responds to this
test.

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-D-Galactose -D-Glucose
Lactose – Reactions
• Lactose on hydrolysis produces one molecule of -D-galactose and one molecule of -D-glucose
units.

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Sucrose – Reactions
• Since sucrose is a carbohydrate, it answers for Molisch Test.
• Since sucrose is a non-reducing sugar, it will not answer for Benedict’s, Fehling’s and Barfoed
Test.
• Both of its aldehydic and ketonic groups are not free for answering the test.
• Similarly, there is no reaction with phenyl hydrazine.

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Sucrose – Reactions
• Sucrose on hydrolysis produces one molecule of -D-glucose and one molecule of -D-fructose
units.
-D-Fructose
-D-Glucose

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Qualitative Tests for Disaccharides – Lactose, Sucrose and Maltose

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Qualitative Tests for Disaccharides – Lactose, Sucrose and Maltose
• The following tests are needed to be done consequently to confirm the above disaccharides.
i. Molisch test – Positive answer confirms the carbohydrates.
ii. I2/KI test – Positive answer confirms starch/polysaccharide. Negative answer confirms
the simple oligosaccharide or monosaccharide.
iii. Barfoed’s test – Quick response (2 – 3 min) confirms the reducing monosaccharide.
Slow response confirms the reducing disaccharides. Negative answer confirms the non-
reducing disaccharides.
iv. Seliwanoff’s test – Blue-green colour confirms the aldehyde functionality in the
disaccharide while cherry-red colour confirms the keto functionality in the disaccharide.
v. Benedict’s or Fehling’s Test – Positive response for reducing disaccharides while
negative response is for non-reducing disaccharide.
• The hydrolysis of disaccharides produce two monosaccharide molecules. Qualitative analysis of
those monosaccharides after separation provide the idea about disaccharides.

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Qualitative Tests for Disaccharides – Lactose, Sucrose and Maltose
1. Molisch Test:
• To the aqueous solution of carbohydrates, conc. H2SO4 is added to get the dehydrated
hydroxymethyl furfural.
• Alcoholic -naphthol with 5-hydroxymethyl furfural forms the furfural derivatives. This compound
forms a reddish-violet coloured or purple ring at the junction of the two liquids. Molisch’s reagent
is 5% solution of alpha naphthol in alcohol.
• This confirms the presence of carbohydrates.
2. Barfoed Test:
• It is also copper reduction test but in the presence of acid medium.
• Barfoed reagent is CuSO4.5H2O in acetic acid medium.
• To the sugar solution, Barfoed solution is added and heated in water bath.
• Quick response (2 – 3 min) confirms the reducing monosaccharide. Slow response confirms the
reducing disaccharides. Negative answer confirms the non-reducing disaccharides.

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Qualitative Tests for Disaccharides – Lactose, Sucrose and Maltose
3. Seliwanoff’s Test:
• It distinguishes aldoses and ketoses.
• Seliwanoff reagent is 0.5% resorcinol in con. HCl.
• To the sugar solution, 2 ml of Seliwanoff reagent is added and the mixture is heated.
• Strong colour change (cherry red) indicates ketoses while slower colour change (bluish green)
indicates aldoses.
4. Benedict’s or Fehling’s Test:
• It distinguishes between reducing or non-reducing sugars.
• Benedict solution is sodium citrate and sodium carbonate and CuSO4 (17.3 g in 100 ml) solution
mixture.
• Fehling Solution A – CuSO4.5H2O. Fehling Solution B – KOH and sodium potassium tartarate.
• To the sugar solution, either Benedict or Fehling solution is added and the mixture is heated.
• Appearance of yellow or red precipitate confirms the reducing sugar.

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Qualitative Tests for Disaccharides – Lactose, Sucrose and Maltose
S.No. Qualitative Tests Lactose Maltose Sucrose
Observation Inference Observation Inference Observation Inference
1. Molisch Test Reddish
violet
Carbohydrate Reddish
violet
Carbohydrate Reddish
violet
Carbohydrate
2. I2/KI Test No
response
No
polysaccharide
No
response
No
polysaccharide
No
response
No
polysaccharide
3. Barfoed’s Test Slow
response
reducing
disaccharide
Slow
response
reducing
disaccharide
No
response
Non-reducing
disaccharide
4. Seliwanoff’s Test Blue-green Aldoses Blue-green Aldoses Red Aldoses and
ketoses
5. Benedict’s or
Fehling Test
Red ppt Reducing
disaccharide
Red ppt Reducing
disaccharide
No
response
Non-reducing
disaccharide
https://www.onlinebiologynotes.com/tests-for-specific-
carbohydrates-seliwanoffs-test-bials-test-and-iodine-test/

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Glycosides
• Glycosides are formed when the hemiacetal or hemiketal group of (of anomeric carbon) of a
carbohydrate reacts with a hydroxyl group of another carbohydrate or a non-carbohydrate
(methanol, phenol, glycerol) is known as glycosidic bond and the non-carbohydrate moiety (if
present) is known as aglycone.
• What is hemiacetal or hemiketal?
• Nomenclature of glycosidic linkages between the carbon atoms based on the anomeric carbon
atoms. E.x. Lactose - (14) glycosidic linkages.

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Physiological significance of glycosides
• Vanilla flavour extracted from plants contain Glucovanillin (Vanillin-D-glucoside), is a glycoside.
• Cardiac glycosides (steroidal glycosides): Digoxin and digitoxin are the cardiac glycosides
used to stimulate the muscle contraction. They contain steroids as aglycone.
• Streptomycin, an important antibiotic used to treat tuberculosis (TB) is a glycoside.
• Ouabain is a glycoside, which inhibits Na+-K+ ATPase and blocks the active transport of Na+.
Digitoxin Streptomycin
Glucovanillin Ouabain

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Amino sugars and their importance
• When one or more hydroxyl groups of the monsaccharides are replaced by amino groups, the
products are called Amino sugars. Ex. D-Glucosamine and D-Galactosamine.
• Amino group of the amino sugars are sometimes acetylated. Ex. N-acetyl D-glucosamine.
• N-acetyl neuraminic acid (NANA) is a derivative of N-acetylmannose and pyruvic acid. It is an
constituent of glycoproteins and glycolipids.
• Certain antibiotics with amino sugars involved in antibiotic activity. Ex. Erythromycin.
N-acetyl D-Glucosamine NANA

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Amino sugars and their importance
• Glucosamine molecules link up to form polymers such as glycosaminoglycans (GAGs) and PGs.
These together with protein micro-fibers, collagen and elastin, form the extra-cellular matrix.
• The production of amino group of the amino sugars is important because they also form part of
the glycocalix, a thin layer of tissue which lines and protects the digestive, respiratory and genito-
urinary tracts.
• Amino sugars also produce GAGs which make up the lubricating fluids that cushion our joints, fill
the eyeball and plump the skin.

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Polysaccharides
• Polysaccharides or glycans consist of repeating units of monosaccharides or their derivatives
linked together by glycosidic linkages.
• On hydrolysis, they may produce multiple untis of monosaccharides and oligosaachrides.
• They are very important for structural and energy storage functions.
• They exist in both linear and branched polymers based on the glycosdic bond linkages.
• There are two types of polysaccharides namely 1. Homopolysaccharides and 2.
Heteropolysaccharides

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Polysaccharides types and definition
Homopolysaccharides:
• Homopolysaccharides on hydrolysis yield only a single type of monosaccharides.
• They are named base on the nature of monosaccharide unit.
• Glucans are the polymers of glucose while fructosans are the polymers of fructose.
Heteropolysaccharides:
• On hydrolysis yield only a mixture of a few monosaccharides or their derivatives.

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Occurrence and structure of starch
Occurrence of starch:
• Starch is carbohydrate reserve of plants which is the most important dietary source for higher
animals.
• High content of starch if found in cereals, roots, tubers, vegetables etc.
Structure of starch:
• Starch is a homopolymer consist of D-glucose units linked through -glycosidic linkage. Starch is
also known as glucosan or glucan.
• Starch is composed of two polysaccharide components namely water-soluble amylose (15-20%)
and water-insoluble amylopectin (80-85%).
• Amylose is a linear chain with 200-1000 D-glucose units with (14)-glycosidic linkages.

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Structure of starch

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Structure of starch:
• Amylopectin is a branched chain consists of D-glucose with (16)-glycosidic linkages at the
branching points while has (14)-glycosidic linkages inside the branches.
• Amylopectin has a few thousand molecules of D-glucose while each of its branches has about
20-30 molecules of D-glucose.
• Starch is hydrolysed by amylase enzyme in saliva or in pancreas to liberate dextrins, and finally
maltose and glucose units.
• Amylase specifically hydrolyse on (14)-glycosidic linkages.
• The various intermediate dextrins products can be identified by Iodine test – amylodextrin
(violet), eryhtrodextrin (red) and achrodextrin (no colour).
Structure of starch

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Reactions of starch
Hydrolysis of Starch:
• Hydrolysis of starch using acid or base produces maltodextrin, maltose, and glucose.
• Based on the duration of hydrolysis, the product distribution varies.
Acetylation of Starch:
• Acetylation of starch using acetic anhydride in the presence of base gives acetylated starch

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Reactions of starch
Succinylation of Starch:
• Succinylation of starch using succinic acid in the presence of pyridine lead to the formation of
starch succinate.
Phosphorylation of Starch:
• It is done using pyrophosphoric acid to get phosphorylated starch.

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Occurrence and structure of cellulose
Occurrence of cellulose:
• Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant
kingdom.
• Cell wall predominantly present in plant cell wall.
• It is completely absent in animal body.
Structure of cellulose:
• Cellulose is a homopolysaccharide consist of -D-glucose units linked through (14)-glycosidic
linkage.
• Cellulose cannot be digested in mammals due to the absence of -glycosidic linkage cleaving
enzymes.
• Hydrolysis of cellulose yields a disaccharide cellobiose, followed by -D-glucose units.

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Heteropolysaccharides
• Polysaccharides composed of different types of sugars or their derivatives is known as
heteropolysaccharides or heteropolygans.
• Ex: Hyaluronic acid, chondroitin sulphate and mucopolysaccharides i.e. glycosaminoglycans
(CAG), mucoproteins or mucoids or proteogycans
Occurrence of Hyaluronic acid:
• Found in the synovial fluid of joints and vitreous humor of eyes.
• Also found in the connective tissues and forms a gel around the ovum.
• Serves as lubricant and shock absorbant in joints.

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Structure of Hyaluronic acid
• Hyaluronic acid is composed of alternate units of D-glucuronic acid and N-acetyl D-glucosamine.
• D-glucuronic acid and N-acetyl D-glucosamine units are linked through (13) glycosidic bond.
• N-acetyl D-glucosamine and D-glucuronic acid units are linked through (14) linkage.
• Hyaluronic acids contains about 250 – 25000 disaccharide units (D-glucuronic acid and N-acetyl
D-glucosamine).
• Hyaluronidase is an enzyme that breaks (14) linkages of hyaluronic acid.

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Chondroitin sulphates
Occurrence of Chondroitin sulphates:
• Found in the various mammalian tissues (bone, cartilage, tendons, heart, valves, skin, cornea
etc.)
Structure of Chondroitin sulphates: There are two types based on sulphate group substitution.
• Chondroitin 4-sulphate:
• It has D-glucuronic acid and N-acetyl D-galactosamine 4-sulphate units, which are linked
through (13) bond. These disaccharide units are linked through (14) linkage.
• Chondroitin 6-sulphate:
• It has D-glucuronic acid and N-acetyl D-galactosamine 6-sulphate units, which are linked
through (13) bond. These disaccharide units are linked through (14) linkage.
Chondroitin 6-sulphate
Chondroitin 4-sulphate

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Thank You