REACTIONS OF POLYSACCHARIDES

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REACTIONS OF POLYSACCHARIDES

 

                Polysaceharides are made up of many number of same or different monosaccharides linked by glycosidic linkage. The most commonly available polysaccharide is starch which is a mixture of amylose and amylopectin. The individual glucose units in amylose are linked by alpha 1, 4 glycosidic linkages. Amylopectin has branching points contributed by alpha 1, 6 glycosidic bonds. Starch is insoluble in cold water but forms a co1loidol solution in hot water which remains so even after cooling. Starch has no detectable reducing activity. Starch gives a blue colour with iodine solution. Glycogen is also called animal starch. It is present in liver and muscle. It is highly branched than starch.

 

REACTION OF STARCH

Perform the following tests:-

1) Molisch’s test: Purple coloured ring is observed.

2) Iodine test:

Principle: The amylose component  of starch has a helical structure when it is treated with iodine solution, iodine is trapped inside the coil and the complex has an intense blue colour, when amylose solution is heated, the helical conformation is disrupted  and it loses capacity to bind iodine on cooling, the original conformation is regained and the capacity to bind iodine is also recovered. Amylopectin and glycogen because of branched structure gives light purple or red colour with iodine.

                To 1 ml of starch add 2 or 3 drops dilute iodine solution. A blue colour is formed from starch – iodine complex. This is general test for identifying starch.

 

3) Effect of Alkaline and Acidic medium on Iodine test:

                Add 1ml of 5% of NaOH to 2ml starch to make it alkaline, now add 3 drops of iodine. There is no blue colour, acidify this solution with dropwise addition of glacial acetic acid the blue colour reappears.

Explanation:

                In the presence of NaOH free iodine is not available to form a starch iodine complex, hence no blue colour develops. When acid is added it neutralises the NaOH and releases the iodine for combining with starch.

 

4) Benedict’s test:

                Starch gives a negative test

 

5) Hydrolysis of Starch by Acid:

                To 3 ml of starch in a test tube add 3 drops of conc. HCl and boil for 2 minutes, cool, neutralise with 20% NaOH. Now perform Benedict test with one portion of the hydrolystate, the test will be positive because of hydrolysis of starch to glucose. Take the other portion of hydrolystate, add a drop of glacial acetic acid and two drops of iodine. No blue colour is noticed indicating that starch is completely hydrolysed by acid to reducing sugar namely glucose.

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