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Oligosaccharides

They contain 2-9 (<10) monomers (or their derivatives) and their names end in suffix, -ose. Upon acid hydrolysis, they yield the constituent monosaccharides.

 

The monosaccharides are linked by glycosidic bonds that is formed between a -OH of Cl of one monomer and other -OH of second monomer by the condensation reaction by elimination of a molecule of H20 (dehydration synthesis). There are many types of glycosidic bonds:

 

(i)                 1,4-Jinkage, where C1 of one monomer and C4 of other monomer are linked   together (between two -OH groups in a-position), found in unbranched chains of polysaccharides, maltose, etc.

(ii)               β-1,4-linkage formed between C1 of one monomer and C4 of other monomer (two –OH groups in β-position), e.g., lactose.

(iii)             1,6-linkage formed between C1 and C6 at the points of branching of a chain, e.g., isomaltose. [Polymerization reactions in proteins, carbohydrates, nucleic acids are also condensation reactions and their hydrolysis needs incorporation of H20 molecules]

 

Like monosaccharides, they can also be reducing or non-reducing in nature:

 

(i)                 Reducing sugars: They have a free -CHO or >C=O group (e.g., maltose, lactose).

(ii)               Non-reducing sugars: In these sugars, the reducing groups are involved in glycosidic linkage, e.g., sucrose.

 

On the basis of number of monomeric units, oligosaccharides are classified into the following:

 

1. Disaccharides

They are the most important oligosaccharides, which are soluble in water and, like monosaccharides, are sweet in taste, e.g.,

 

(i) Sucrose: Having formula C12H22O11, and commonly known as table sugar, cane sugar or beet sugar. It is also found in honey, pineapple, banana and other fruits. Commercially, it is obtained from sugar cane and sugar beet. It is optically active, dextrorotatory, with specific rotation +66.5, but does not show mutarotation [no α- and β-sucrose is found, as it does not contain any free -CHO or >C=O group. It is non-reducing (no free -CHO or >C=O group). α.-1, β 2 glycosidic bond is found between the monomeric units, glucose and fructose.

 

 

c12H22O 11  +

Sucrose

(+ 66.5°)

 

H 2O

Invertase or

Mineral acid ˃

 

C6H12O6     +

 

C6Ht2O6

Fructose

(- 920)

 

Invert sugar(with specific rotation

Of – 200 at equilibrium)

Invert sugar [named due to optical inversion from dextro Sucrose (d) to levo invert sugar (l) due to more (l) nature of fructose is used as a substitute for honey.

 

Fermentation of sucrose yields

 

C12H22O11                 +

Sucrose

 

HP

Invertase

> C6H1O6 +

Glucose

Zymase

(yeast)

 

 

C6H12O6

Fructose

CO2

C 2 H5 OH             + CO2

(ii) Maltose: Its molecular formula is C12H22011 and is found in germinating seeds. commercially, it is obtained from starch:

Diastase (a-amylase)

Starch                                                  Maltose

It is a reducing sugar having a free -CHO group. Moreover, because of the presence of a free -CHO group and due to the relative positions of -OH group, two anomers, α- and β-maltose, are possible.

Its hydrolysis yields:

Maltase

Maltose                                   2 Glucose

It has α-1,4 glycosidic bond between two glucose units

(i)     Lactose: Its molecular formula is C12H22011 and is commonly known as milk sugar, as it is present in milk and synthesized in mammary glands. It may be present in urine during pregnancy. Souring of milk is due to the activity of certain bacteria that drop the pH of· milk because of conversion of lactose into lactic acid:

lactobacilli

Lactose                                    Lactic acid

It is a reducing sugar (having free -CHO ) and exhibits mutarotation to give α- and β-lactose in equimolar quantities. It has β1,4-glycosidic linkage between galactose and glucose

(i)                 Cellobiose: Commercially, it is obtained from cellulose and is a reducing  Sugar. The two glucose molecules are linked by β-1,4-glycosidic bond.

Glucose + Glucose                               Cellobiose

 

(i)                 Isomaltose: It is a reducing sugar where two glucose molecules are linked by α-1,6- glycosidic linkage.

Glucose + Glucose                              Isomaltose

2. Trisaccharides

           

They have three monomeric units. e.g ., raffinose, (C18H32  O16) consisting of D-Glucose, D Fructose and D -Galactose.