Carbohydrates: Types, Properties, and Identification Tests, Study notes of Biochemistry

Download Carbohydrates: Types, Properties, and Identification Tests and more Study notes Biochemistry in PDF only on Docsity! Qualitative tests of Carbohydrates-l- BCH302 [Practical] 2 • Carbohydrates are defined as the polyhydroxy aldehydes or polyhydroxy ketones. • Most , but not all carbohydrate have a formula (CH2O)n (hence the name hydrate of carbon). • Sugars ends with –ose. • In human body, the D-glucose is used. • Biological role: 1. Are the key source of energy used by living things. 2. Also serve as extracellular structural elements as in cell wall of bacteria and plant. 5 • Reducing and non Reducing sugar :  If the oxygen on the anomeric carbon of a sugar is not attached to any other structure, that sugar can act as a reducing agent and is termed a reducing sugar. FREE anomeric carbon Reducing sugar attached anomeric carbon Non-Reducing sugar FREE anomeric carbon Reducing sugar 6 • All monosaccharides are reducing sugars; they all have a free reactive carbonyl group. • Some disaccharides have exposed carbonyl groups and are also reducing sugars like lactose. While other disaccharides such as sucrose are non-reducing sugars and will not react with Benedict's solution. • Large polymers of glucose, such as starch, are not reducing sugars, since the concentration of hemiacetal groups is very low. 7 • Monosaccharide and disaccharide can be dissolved freely in water because water is a polar substance. • Polysaccharide cannot be dissolved easily in water, because, it has high molecular weight , which give colloidal solutions in water. 10 Objective: • To identify the carbohydrate from other macromolecules lipids and proteins (this test is specific for all carbohydrates). Principle: • Two solutions are added : H2SO4, α-naphthol 1- The test reagent (H2SO4) dehydrates pentose to form furfural and dehydrates hexoses to form 5- hydroxymethyl furfural. 2- The furfural and 5- hydroxymethyl furfural further react with α-naphthol present in the test reagent to produce a purple product. 11 Furfural 5- hydroxymethyl furfural α-naphthol [Present in the reagent ] α-naphthol Pentose sugar Hexose sugar (2)(1) 12 Method: 1. Two ml of a sample solution is placed in a test tube. 2. 0.5 ml drops of the Molisch reagent (which α-napthol in 95% ethanol) is added. 3. The solution is then poured slowly into a tube containing two ml of concentrated sulfuric acid so that two layers form, producing violet ring appear as liaison between the surface separations. Results: ObservationTube Glucose Lactose Starch Concentrated sulfuric acid is extremely corrosive and can cause serious burns when not handled properly. 15 Objective: • This test is performed to distinguish between reducing monosaccharides, reducing disaccharides and non reducing di- and polysaccharides. Principle: • Barfoed’s test used copper (II) ions in a slightly acidic medium. • Reducing saccharides are oxidized by the copper ion in solution to form a carboxylic acid and a reddish precipitate of copper (I) oxide. • Different types of reducing sugars react at different rates. Reducing monosaccharides react quickly with Barfoed’s reagent (acidic condition), but reducing disaccharides react very slowly or not at all. • The non-reducing sugars give negative result. 16 Method: 1. Place one ml of a sample solution in a test tube. 2. Add 3 ml of Barfoed's reagent (a solution of cupric acetate and acetic acid. 3. Heat the solution in a boiling water bath for 6 minutes (after the 3 min check the tubes). Results: ObservationTube Glucose Lactose Sucrose Glucose (+) 17 Objective: • To distinguish between pentose monosaccharide and hexose monosaccharide (to detect pentoses). Principle: • Bial's reagent (a solution of orcinol, HCl and ferric chloride). • Bial’s test uses concentrated HCl as a dehydrating acid and orcinol + traces of ferric chloride [FeCl3] as condensation reagent. • The test reagent dehydrates pentoses to form furfural. Furfural further reacts with orcinol and the iron ion present in the test reagent to produce a bluish or green product, while hexoses yield muddy-brown to grey condensation product.