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57.4 DISACCHARIDES

These are the carbohydrates which give two units of monosaccharides on hydrolysis with dilute acids or enzymes. Some examples are: Sucrose, maltose and lactose.

H2O/H+

C12H22O11     →     C6H12O6 + C6H12O6

Sucrose                  Glucose       Fructose

 

H2O/H+

C12H22O11     →     C6H12O6 + C6H12O6

Lactose                  Galactose     Fructose

 

H2O/H+

C12H22O11     →     2C6H12O6

Maltose                  Glucose

This implies that a disaccharide is formed by condensation of two monosaccharide units. The two monosaccharide units in a disaccharide are joined together by an oxide (or ether) linkage formed by loss of a water molecule. Such a linkage between two monosaccharide units
through oxygen atom is called glycosidic linkage.

Now, let us study about some disaccharides in somewhat details.

SUCROSE (CANE-SUGAR), C12H22O11

Sucrose is the most widely occurring disaccharide. It is found in all photosynthetic plants. It is obtained commercially from sugarcane or sugarbeets. On hydrolysis with dilute acids or enzyme invertase, 1 mole of sucrose gives 1 mole of glucose and 1 mole of fructose.

H+

C12H22O11 + H2O     →     C6H12O6 + C6H12O6

                                                                                       Glucose             Fructose

Sucrose is a non-reducing sugar. Haworth (1927) suggested the following structure of sucrose.

Haworth’s Representation of Sucrose

 

MALTOSE,C12H22O11

When starch is hydrolysed by the enzyme diastase, maltose is formed as one of the products.

Diastase

2(C6H10O5)n + nH2O                     →                nC12H22O11

Starch                                                                      Maltose

On hydrolysis with dilute acids, one mole of maltose yields 2 moles of glucose.

H+

C12H22O11 + H2O     →     2C6H12O6

Maltose                               Glucose

Maltose is a reducing sugar. In maltose the two glucose units are linked through α-glycosidic linkage between C-1 of one glucose unit .and C-4 of the other. The free aldehyde group can be produced at C-1 of the second glucose in solution. Hence, maltose shows reducing properties.

The structure of maltose is given below:

LACTOSE, (MILK SUGAR),C12H22O11

Lactose is present in the milk and is also known as milk sugar. On hydrolysis with dilute acids one mole of lactose yields 1 mole of .glucose and one mole of galactose

H+

C12H22O11     →     C6H12O6 + C6H12O6

                                                                     +H2O     Glucose             Fructose

Lactose is a reducing sugar. In lactose, glucose and galactose units are linked through β-glycosidic linkage between C-1 of galactose and C-4 of glucose unit. The structure of lactose is given as follows: