In the last article, we have studied monosaccharides. In this article, we shall study disaccharides and polysaccharides.

Disaccharides:

Di-saccharides on hydrolysis give two molecules of monosaccharide. They on hydrolysis with dilute acids or enzymes yield two molecules of either the same or different monosaccharides.

e.g. Cane sugar (Sucrose) (C₁₂H₂₂O₁₁)on hydrolysis gives one molecule of glucose and one molecule of fructose, Maltose (C₁₂H₂₂O₁₁) on hydrolysis gives two molecules of glucose, Lactose (C₁₂H₂₂O₁₁) on hydrolysis gives one molecule of glucose and one molecule of galactose.

• Disaccharides are crystalline, water-soluble, and sweet in taste.
• They have the general formula (C₁₂H₂₂O₁₁).
• The two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule. Such a linkage between two monosaccharide units through oxygen atom is called glycosidic linkage.

Examples of Disaccharides:

Sucrose:

One of the common disaccharides is sucrose which on hydrolysis gives an equimolar mixture of α -D-Glucapyranose and β-D-Fructofuranose. These two monosaccharides are held together by a glycosidic linkage between C1 of α-glucose and C2 of β-fructose.

Since the reducing groups of glucose and fructose are involved in the glycosidic bond formation, sucrose is a non-reducing sugar.

Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. Since the laevorotation of fructose (–92.4°) is more than dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory. Thus, hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo in the sign of rotation, from Dextro (+) to Laevo (–) and the product is named as invert sugar.

Maltose:

Another disaccharides, maltose is composed of two α-D-glucose units in which C1 of one glucose (I) is linked to C4 of another glucose unit (II).

The free aldehyde group can be produced at C1 of second glucose in solution and it shows  reducing properties so it is a reducing sugar

Cellobiose:

Cellobiose is obtained by partial hydrolysis of cellulose, C1 of one β-D-Glucapyranose is linked to C4 of another β-D-Glucapyranose by glucosidic linkage. Thus cellobiose contains 1→ 4 β- glucosic bond.

Cellobiose is reducing sugar because free aldehyde group can be produced at C1 in second glucose molecule

Lactose:

It is more commonly known as milk sugar since this disaccharide is found in milk. It is composed of β-D-galactose (β-D-Galactopyranose) and β-D-glucose (β-D-Glucopyranose). The glucosidic linkage is between C1 of β-D-galactose (b-D-Galactopyranose) and C4 of β-D-glucose (β-D-Glucopyranose). Hence it is also a reducing sugar.

Reducing Sugars:

The saccharides in which aldehydic and ketonic groups are free are called reducing sugars. They reduce Fehling’s solution and Tollen’s reagent. They contain either α-hydroxy aldehyde or α -hydroxy ketone group or contain cyclic hemiacetal or cyclic hemiketal structures.

All monosaccharides are reducing sugars. Diasaccharides like maltose, lactose and cellobiose are reducing sugars.

Nonreducing sugars:

The saccharides in which aldehydic and ketonic groups are not free are called non-reducing sugars. They do not reduce Fehling’s solution and Tollen’s reagent. They contain stable acetal or ketal structures which cannot be opened into the free carbonyl group.

Sucrose, Starch, Cellulose, Glycogen, Dextrin are reducing sugars.

Hemiacetal and Hemiketal Structures:

They are formed when an alcohol oxygen atom adds to the carbonyl carbon of an aldehyde or a ketone. When alcohol adds to an aldehyde, the result is called a hemiacetal; when alcohol adds to a ketone the resulting product is a hemiketal.

This happens through the nucleophilic attack of the hydroxyl group at the electrophilic carbonyl group. Since alcohols are weak nucleophiles, the attack on the carbonyl carbon is usually promoted by protonation of the carbonyl oxygen.

PolySaccharides:

Carbohydrates which on hydrolysis give indefinite or large no. of monosaccharides (more than 10)  are called polysaccharides. They contain a large number of monosaccharide units joined together by glycosidic linkages. These are the most commonly encountered carbohydrates in nature. They mainly act as food storage or structural materials.

• They are natural polymeric carbohydrates. They are insoluble in water, amorphous, and tasteless.
• They are nonsugars.
• They have the general formula (C6H10O5)n.  e.g. Starch, Cellulose, Inulin, Dextrin, etc
• Cellulose is a linear polymer of β-Glucose units while starch is a branched polymer of a-glucose units.

Examples of Polysaccharides:

Starch:

Starch is the main storage polysaccharide of plants. It is the most important dietary source for human beings. High content of starch is found in cereals, roots, tubers, and some vegetables. It is a polymer of α-glucose (α-D-Glucopyranose) and consists of two components Amylose and Amylopectin.

Amylose is a water-soluble component which constitutes about 15-20% of starch. Chemically amylose is a long unbranched chain with 200-1000 α-D-(+)-glucose units held by C1– C4 glycosidic linkage.

Amylopectin is insoluble in water and constitutes about 80-85% of starch. It is a branched chain polymer of α-D-glucose units in which chain is formed by C1–C4 glycosidic linkage whereas branching occurs by C1–C6 glycosidic linkage

Glycogen:

The carbohydrates are stored in the animal body as glycogen. It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched. It is present in the liver, muscles, and brain. When the body needs glucose, enzymes break the glycogen down to glucose (hydrolysis). Glycogen is also found in yeast and fungi.

Importance of Carbohydrates:

• Carbohydrates are essential for life in both plants and animals.
• They form a major portion of our food. Honey has been used for a long time as an instant source of energy by ‘Vaids’ in Ayurvedic system of medicine.
• Carbohydrates are used as storage molecules as starch in plants and glycogen in animals.
• The cell wall of bacteria and plants is made up of cellulose. We build furniture, etc. from cellulose in the form of wood and clothe ourselves with cellulose in the form of cotton fibre.
• They provide raw materials for many important industries like textiles, paper, lacquers and breweries.
• Two aldopentoses viz. D-ribose and 2-deoxy D-ribose (Section are present in nucleic acids. Carbohydrates are found in biosystem in the combination with many proteins and lipids.

Science > Chemistry > Biomolecules > Disaccharides and Polysaccharides

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In the last article, we have seen what are carbohydrates and how are they classified. Based on hydrolysis behaviour the carbohydrates are classified into three types. a) Mono-Saccharides b) Oligo-Saccharides and c) Poly-Saccharides. In this article, we shall study monosaccharides in detail particularly glucose and fructose.

Monosaccharides:

Carbohydrates which are basic units or which cannot be hydrolyzed further are called monosaccharides.

Characteristics of mono-saccharides:

• Carbohydrates which are basic units or which cannot be hydrolysed further are called monosaccharides.
• They are basic units of carbohydrates.
• They cannot be hydrolysed further into small units.
• They contain six carbon atoms in a molecule.
• They are water-soluble and sweat in taste.
• Depending upon the presence of an aldehydic group or a ketonic group they are further subclassified into aldoses and ketoses respectively.

Aldoses: The monosaccharides containing the aldehydic group are called aldoses. Examples: Aldopentose C₅H₁₀O₅ – Arbaniose, Xylose, Ribose. Aldohexose C₆H₁₂O₆ – Glucose, Galactose

Ketoses: The monosaccharides containing the ketonic group are called ketoses. Examples: Ketopentose C₅H₁₀O₅ – Ribulose.Ketohexose C₆H₁₂O₆ – Fructose.

Preparation of Glucose:

From Sucrose (Cane sugar): (Laboratory Method):

When powdered cane sugar is heated with a concentrated alcoholic solution of HCl on a water bath for about 2 hours at about 323 K, glucose is formed.

It is insoluble in alcohol while fructose is soluble in alcohol. Hence glucose crystallizes out first leaving fructose in the solution.  A few crystals of glucose may be added to the solution for quicker crystallization. This is known as seeding. Purification is done by recrystallizing it from methanol.

From Starch: (Commercial Method):

Starch on hydrolysis with dilute sulphuric acid by heating under 3 to 5 atmospheric pressure gives glucose.

When hydrolysis is complete the excess of unreacted sulphuric acid is neutralised with calcium carbonate and filtered to remove the precipitate of calcium sulphate.

CaCO₃ + H₂SO₄   →   CaSO₄   + H₂O + CO₂ ­

The filtrate which contains glucose and one molecule of water is decolorized using animal charcoal. The clear solution is then evaporated in a vacuum to get a thick syrup on cooling crystallizes to give glucose-monohydrate. It is recrystallized from methanol.

Structure of Glucose:

Open Chain Structure:

Following chemical reactions of glucose confirm its open chain structure

• On prolonged heating with HI it gives n-hexane suggesting that all the six carbons are linked in a straight chain.

• Hydroxylamine condenses with an aldehydic group to form glucose-oxime.

• Hydrogen cyanide adds to an aldehydic group to form cyanohydrin.

• On oxidation by a mild oxidizing agent like bromine water, it gives gluconic acid, which shows that the carbonyl group is the aldehyde group.

• Glucose, as well as gluconic acid on oxidation by dilute nitric acid, gives dicarboxylic acid, saccharic acid, which shows the presence of the alcoholic group.

• On acetylation by acetic anhydride it gives glucose-pentaacetate., which confirms the presence of five hydroxyl group. As glucose is a stable compound these five hydroxyl group must be on five different carbon atoms.

Challenges to open chain structure or the need of cyclic structure:

The following points indicate the absence of a free aldehyde group in glucose.

• In spite of having an aldehyde group it does not give a condensation reaction with 2,4 dinitro-phenyl hydrazine
• Glucose-pentaacetate does not condense with hydroxylamine
• It is found to exist in two different crystalline forms α and β called anomers.

Haworth Projection of Glucose:

The cyclic structure in Haworth projection depicts the ring as being flat. The substituents that are to the right in a Fischer projection formula are down and those to the left are up in the corresponding Haworth projection formula. Orient the Haworth projection formula with the ring oxygen at the back and the anomeric carbon at the right.

For carbohydrates of D series a) If hydroxyl is down, the configuration of anomeric carbon is α and b) If hydroxyl is up, the configuration of anomeric carbon is β.

For carbohydrates of L series a) If hydroxyl is up, the configuration of anomeric carbon is α and b) If hydroxyl is down, the configuration of anomeric carbon is β.

Note: A ring containing five carbons and one oxygen is referred pyran

Physical Properties of Glucose:

• It is a white crystalline solid.
• It is soluble in water but sparingly soluble in alcohol.
• As it contains 4 asymmetric carbon atoms, it is an optically active compound.
• It is dextrorotatory. It has a specific rotation of  + 52.5°.

Fructose::

Fructose also has the molecular formula C₆H₁₂O₆ and on the basis of its reactions, it was found to contain a ketonic functional group at carbon number 2 and six carbons in the straight chain as in the case of glucose.

It belongs to D-series and is a laevorotatory compound. It is appropriately written as D-(–)-fructose. Its open chain structure is as shown.

Science > Chemistry > Biomolecules > Glucose and Fructose

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