Carbohydrates

Introduction Carbohydrates are sugars and starches, which provide energy for humans and animals, and cellulose which make up many plant structures. There are three types of carbohydrates: Simple, or monosaccharides: Found in fruits and milk products are more easily digested by the body. Also often found in processed refined foods such as white sugar and white bread. A monosaccharide is a triose, tetrose, pentose or hexose, cannot be hydrolyzed to simpler carbohydrates

Sucrose: common table sugar = glucose + fructose Glucose: "blood sugar" Galactose: a sugar in milk Fructose: a sugar found in honey. Disaccharides : monosaccharides covalently linked can be hydrolyzed into two monosaccharide units. Sucrose: common table sugar = glucose + fructose Lactose: major sugar in milk = glucose + galactose Maltose: product of starch digestion = glucose + glucose . Note: Oligosaccharides: a few monosaccharides covalently linked

Starches are polymers of glucose: Two types are found: Complex, or polysaccharides: which take longer for the body to digest, are most commonly found in vegetables (cellulose), whole grain, breads, rice and legumes. Starches are polymers of glucose: Two types are found: amylose consists of linear, unbranched chains of several hundred glucose residues (units). The glucose residues are linked by a glycosidic bond between their #1 and #4 carbon atoms.

amylopectin differs from amylose in being highly branched amylopectin differs from amylose in being highly branched. a short side chain is attached by a glycosidic bond to the #6 carbon atom (the carbon above the ring). The total number of glucose residues in a molecule of amylopectin is several thousand. Glycogen Animals store excess glucose by polymerizing it to form glycogen. The structure of glycogen is similar to the amylopectin, although the branches in glycogen tend to be shorter and more frequent. Glycogen is broken back down into glucose when energy is needed (a process called glycogenolysis). Cellulose

Structure of Monosaccharides Aldoses ketoses

D vs L Designation

Reducing and Non-reducing sugars Sugars exist in solution as an equilibrium mixture of open-chain and closed-ring (or cyclic) structures. Sugars that can be oxidized by mild oxidizing agents are called reducing sugars because the oxidizing agent is reduced in the reaction. A non-reducing sugar is not oxidized by mild oxidizing agents. All common monosaccharides are reducing sugars. The disaccharides maltose and lactose are reducing sugars. The disaccharide sucrose is a non-reducing sugar. Common oxidizing agents used to test for the presence of a reducing sugar are: Benedict's solution, Fehling's solution.

Ex(1). Benedict's Test Benedict's test determines whether a monosaccharide or disaccharide is a reducing sugar. To give a positive test, the carbohydrate must contain a hemiacetal which will hydrolyse in aqueous solution to the aldehyde form. Benedict's reagent is an alkaline solution containing cupric ions, which oxidize the aldehyde to a carboxylic acid. In turn, the cupric ions are reduced to cuprous oxide, which forms a red precipitate. This solution has been used in clinical laboratories for testing urine. An aldehyde or ketone plus an alcohol may exist in equilibrium with a hemiacetal. General Equation for Hemiacetal Formation: Aldehyde + Alcohol <===> Hemiacetal The hemiacetal group can be recognized by finding a carbon with both an alcohol and an ether functional group attached to it. RCHO  +  2Cu2+  +  4OH- ----->   RCOOH  +  Cu2O  +  2H2O

Hemiacetal & hemiketal formation

Procedure Place 1 ml of carbohydrates solutions in test tube. To each tube, add 1 ml of Benedict's reagent. Heat the tubes in a boiling water bath for 3-5 minutes. Remove the tubes from water bath and record the results. Result: In the presence of a reducing sugar a precipitate which may be red , yellow or green will form.

Ex(2). Barfoed's Test Barfoed's test is similar to Benedict's test, but determines if a carbohydrate is a monosaccharide or a disaccharide. Barfoed's reagent reacts with monosaccharides to produce cuprous oxide at a faster rate than disaccharides do: RCHO  +  2Cu2+  +  2H2O ----->   RCOOH  +  Cu2O  +  4H+

Procedure Place 1ml of the following 1% carbohydrate solutions in test tubes: glucose, fructose, sucrose, lactose, and maltose. To each tube, add 1 ml of Barfoed's reagent. Heat in a boiling water bath for 10 minutes. Note and record your observations. Result: A red precipitate will form if the test is positive.

Ex(3). Bial's (Orcinol) Test for Pentoses This test is used to distinguish pentoses from hexoses. Bial's reagent contains {orcinol (5-methylresorcinol) in concentrated HCl with a small amount of FeCl3 catalyst}. 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 product. Not: Be extremely careful when heating the test tubes. Bial's reagent contains concentrated hydrochloric acid. Do not point the tube toward yourself or any of your fellow lab workers. -Arabinose , -Lyxose , -Xylose Bial's reagent (0.1 % orcinol in concentrated HCl containing 0.1 % FeCl3.6H2O).

Procedure Add about 2 ml of 1% carbohydrates solutions. Add 3 ml of Bial's reagent to each tube and mix well. Carefully heat each tube (with some agitation) directly over the burner flame. Hold the tube at a diagonal and heat along the sides of the tube rather than at the bottom to prevent eruption of the liquid from the tube. Move the tube diagonally in and out of the flame, until the mixture just begins to boil. Stop heating when the mixture begins to boil.

Two negative tests (left, middle) and a positive test (right) Result A bluish color indicates a positive result. Prolonged heating of some hexoses yields hydroxymethyl furfural which also reacts with orcinol to give colored complexes. Two negative tests (left, middle) and a positive test (right)

Ex(4). Seliwanoff's (Resorcinol) Test For Ketoses This test can differentiate between ketoses and aldoses. Seliwanoff's reagent (resorcinol dissolved in 6 M HCl), The test reagent dehydrates ketohexoses to form 5-hydroxymethylfurfural. 5-hydroxymethylfurfural further reacts with resorcinol present in the test reagent to produce a red product within two minutes. Aldohexoses react to form the same product, but do so more slowly. Ketohexoses (such as fructose) and disaccharides containing a ketohexose (such as sucrose) form a cherry-red condensation product. Other sugars may produce yellow to faint pink colors. Seliwanoff's reagent (0.5 % resorcinol in 3N HCl).

a negative test (left) and a positive test (right) Procedure Add about 3 ml of Seliwanoff's reagent to each labeled test tube. Add 1 drop of the respective sugar solution to the appropriate test tubes and mix well. Place all the test tubes in the boiling water bath at the same time and heat for 3 min after the water begins to boil again. Record your observations. A positive result is indicated by the formation of A red color with or without the separation of a brown-red precipitate. a negative test (left) and a positive test (right)