Carbohydrates Carbohydrates Poly hydroxy aldehydes or poly hydroxyketones

Carbohydrates

Carbohydrates • Poly hydroxy aldehydes or poly hydroxy-ketones of formula (CH 2 O)n And are usually classified according to their structure: w Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose, cannot be hydrolyzed to simpler carbohydrates; eg. Glucose or fructose. w Disaccharides - 2 monosaccharides covalently linked can be hydrolyzed into two monosaccharide units; eg. Sucrose, which is hydrolyzed into glucose and fructose. w Oligosaccharides - a few monosaccharides covalently linked. w Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units. eg Starch or cellulose.

Monosaccharides Aldoses (e. g. , glucose) have an Ketoses (e. g. , fructose) have aldehyde group at one end. a keto group, usually at C 2.

D vs L Designation D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde. The lower representations are Fischer Projections.

Sugar Nomenclature For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers.

Hemiacetal & hemiketal formation An aldehyde can react with an alcohol to form a hemiacetal. A ketone can react with an alcohol to form a hemiketal.

Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH. Glucose forms an intra-molecular hemiacetal, as the C 1 aldehyde & C 5 OH react, to form a 6 -member pyranose ring. These representations of the cyclic sugars are called Haworth projections.

Cyclization of glucose produces a new asymmetric center at C 1. The 2 stereoisomers are called anomers, a & b. Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C 1: w a (OH below the ring) w b (OH above the ring).

Glycosidic Bonds The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond: R-OH + HO-R' R-O-R' + H 2 O E. g. , methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl-glucopyranose).

Disaccharides: Maltose, a cleavage product of starch (e. g. , amylose), is a disaccharide with an a(1 4) glycosidic link between C 1 - C 4 OH of 2 glucoses. Cellobiose, a product of cellulose breakdown, is the otherwise equivalent b anomer (O on C 1 points up). The b(1 4) glycosidic linkage is represented as a zig- zag.

Other disaccharides include: w Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose. Because the configuration at the anomeric C of glucose is a (O points down from ring), the linkage is a(1 2). w Lactose, milk sugar, is composed of galactose & glucose, with b(1 4) linkage from the anomeric OH of galactose.

Polysaccharides: Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. Glucose storage in polymeric form minimizes osmotic effects. Amylose is a glucose polymer with a(1 4) linkages. The end of the polysaccharide with an anomeric C 1 not involved in a glycosidic bond is called the reducing end.

Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more a(1 6) branches. The highly branched structure permits rapid glucose release from glycogen stores, e. g. , in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants.

Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with b(1 4) linkages. Every other glucose is flipped over, due to b linkages. This promotes intra-chain and inter-chain H-bonds and van der Waals interactions, that cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles - microfibrils.

Classification upon reducing end Reducing sugars • can be oxidized by mild oxidizing agents because the oxidizing agent is reduced in the reaction • All monosaccharides Maltose, Lactose • Non-reducing sugars • is not oxidized by mild oxidizing agents. • Sucrose All polysaccharides •

Common Oxidizing Agents Benedict’s sol. Fehling’s sol. Tollen’s reagent

In-lab Experiments Picric Acid Test For reducing sugars Seliwanoff’s Test for Ketoses Bial’s (Orcinol) Test for pentoses Barfoed’s Test for monosaccharides Benedict’s Test For reducing sugars

1. Benedict's Test (positive for reducing sugars) • Principle: Benedict's reagent contains cupric ions, which in an alkaline environment, oxidize the aldehyde group to a carboxylic acid. Cupric ions are reduced to cuprous oxide, which forms a red precipitate RCHO + 2 Cu 2+ + 4 OH- ----> RCOOH + Cu 2 O + 2 H 2 O

Procedure 1. Place 1 m. L of the following 1% carbohydrate solutions in separate, labeled test tubes: glucose, fructose, sucrose, lactose, maltose, and starch. 2. Also place 1 ml of distilled water in another tube to serve as a control. 3. To each tube, add 1 ml of Benedict's reagent and heat the tubes in a boiling water bath for 5 minutes. 4. Remove the tubes from water bath. Note and record the results. v In the presence of a reducing sugar a precipitate which may be red, yellow or green will form.

2. Barfoed's Test (Used to distinguish between mono- & di-saccharides) • Principle Barfoed's reagent reacts with monosaccharides to produce cuprous oxide at a faster rate than disaccharides do: • RCHO + 2 Cu 2+ + 2 H 2 O -----> RCOOH + Cu 2 O + 4 H+

Procedure • Procedure: 1. Place 1 m. L of the following 1% carbohydrate solutions in separate, labeled test tubes: glucose, fructose, sucrose, lactose, and maltose. 2. To each tube, add 1 ml of Barfoed's reagent, and heat in a boiling water bath for 10 minutes. 3. Remove the tubes from water bath. Note and record your observations. v A red precipitate will form if the test is positive.

3. Bial's (Orcinol) Test for pentoses ( for the detection of pentoses) Principle • Pentoses are converted to furfural by this reagent, which forms a blue green color with orcinol.

Procedure 1. Add about 1 ml of 1% xylose, glucose, fructose, maltose, arabinose, and xylose solution to their respective labeled test tubes. 2. Add 1. 5 ml of Bial's reagent to each tube and mix well. 3. 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. v. A blue-green color indicates a positive result. Prolonged heating of some hexoses yields hydroxymethyl furfural which also reacts with orcinol to give colored complexes.

Bial's reagent (0. 1 % orcinol in concentrated HCl containing 0. 1 % Fe. Cl 3. 6 H 2 O).

4. Seliwanoff's (Resorcinol) Test (used for detection of Ketoses) • Principle Ketohexoses (such as fructose) and disaccharides containing a ketohexose (such as sucrose) form a cherry-red condensation product. Other sugars (e. g. aldose) may produce yellow to faint pink colors.

Procedure 1. Add about 3 ml of Seliwanoff's reagent to each labeled test tube. 2. Add 1 drop of the respective sugar solution to the appropriate test tubes, and mix well. 3. 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. v A positive result is indicated by the formation of a red color without the separation of a brown-red precipitate.

Seliwanoff's reagent (0. 5 % resorcinol in 3 N HCl).

5. Picric Acid Test (for reducing sugars) • Principle Picric acid (2, 4, 6 -trinitrophenol) or TNP reacts with reducing sugars to give a red colored picramic acid C 6 H 2. OH. NH 2(NO 2)2

Procedure 1. Into a test tube add 1 ml of maltose solution, into the second tube, 1 ml of sucrose solution. 2. Add into each tube 1 ml of a saturated solution of picric acid, and then add into each tube 0. 5 ml of sodium hydroxide solution. 3. Heat both samples in a boiling water bath. v In the presence of reducing sugars, the solution stains red; a sodium salt of picric acid is formed.