Qualitative Analysis of Carbohydrates (Polysaccharides) PDF

Qualitative Analysis of Carbohydrates (polysaccharides) Homo‐polysaccharides (Starch and Dextrin) Starch Starch is used for energy storage in plants. It is found in all plant seeds. Composed of two subunits 1. Amylose 20 % ( inner layer ) 2. Amylopectin 80 % ( outer layer) Complet...

Qualitative Analysis of Carbohydrates (polysaccharides) Homo‐polysaccharides (Starch and Dextrin) Starch Starch is used for energy storage in plants. It is found in all plant seeds. Composed of two subunits 1. Amylose 20 % ( inner layer ) 2. Amylopectin 80 % ( outer layer) Complete hydrolysis by amylase enzyme or acid of both amylose and amylopectin yields only D‐glucose.  Dextrins are a group of low molecular weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α-(1→4) or α-(1→6) glycosidic bond Dextrins Most can be detected with iodine solution.  Aim : to differentiate between polysaccharide and others Differentiate also between starch, glycogen, dextrin and cellulose This test is used for the detection of starch in the solution. Iodine Test The blue -black color is due to the formation of starch iodine complex. The violet color is due to the formation of dextrin iodine complex Iodine ‐ KI Reagent: Iodine is not very soluble in water; So The iodine reagent is made by dissolving iodine in water in the presence of potassium iodide. This makes a linear triiodide ion complex with is soluble that slips into the coil of the starch causing an intense blue‐black color. Principle Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. The iodine molecule slips inside of the amylose coil. Procedures 1. In a test tube take 5ml of solution 2. add few drops of iodine A blueblack color is observed which is indicative of presence of Starch Violet color for dextrin Red color if the tube contain glycogen result Interpretion +ve (blue to black It’s starch (polysaccharide) color) +ve (violet) it’s dextrin +ve (red color) it’s glycogen -ve ( iodine ) faint It’s cellulose brown Basic Lead acetate test: This test is used for the detection of starch in the solution. Reagent :basic lead acetate. ( NaoH +(CH3COO)2Pb ) Principle: Addition of lead acetate to starch paste leads to formation of white precipitate which is discriminative for starch due to the high molecular weight. Procedures: 1. To 2 ml starch or dextrin paste, add 3 ml basic lead acetate. A white precipitate is observed only with starch. Acid hydrolysis of starch This experiment illustrates the conversion of starch (non‐reducing sugar) to a reducing sugar by the action of hydrochloric acid at boiling point. The longer the starch is exposed to the acid the further hydrolysis proceeds Principle Heating starch solution in acid medium hydrolyses the glycosidic bonds giving different product along the hydrolysis period. Amylose Amylo dextrin Erythro dextrin Achro dextrin maltose glucose tests Before hydrolysis After hydrolysis Molisch test + ve purple to violet ring +ve purple to violet ring Iodine test +ve blue black color -ve no color change Benedict -ve no color change +ve yellow or red ppt Barfoed -ve no color change +ve red ppt Seliwanoffs -ve no color change ??????????????