Biomolecules & Polymers Carbohydrates

BIOMOLECULES & POLYMERS CARBOHYDRATES INTRODUCTION Carbohydrates received their name because of their general formula Cx(H2O)y, according to which they appear to be hydrates of carbon. xCO2 + yH2O Sunlight, chlorophyll Cx(H2O)y + xO2 Carbohydrate A polyhydroxy compound that has an aldehyde or a ketone functional group present, either free or as hemiacetal or acetal are called carbohydrate. Carbohydrates are substances with the general formula Cx(H2O)y, and were therefore called carbohydrates (hydrates of carbon) because they contained hydrogen and oxygen in the same proportion as in water. However, a number of compounds have been discovered which are carbohydrates by chemical behaviour, but do not conform to the formula Cx(H2O)y, e.g., 2-deoxyribose, C5H10O4. S KEEYYPPOOIINNTTS It is also important to note that all compounds conforming to the formula Cx(H2O)y are not necessarily carbohydrates, e.g., formaldehyde, CH2O ; acetic acid, C2H4O2 ; etc. Carbohydrates are often referred to as Saccharides (Latin, Saccharum = sugar) because of the sweet taste of the simpler members of the class, the sugars. CLASSIFICATION OF CARBOHYDRATE The carbohydrates are divided into three major classes depending on the number of simple sugar units present in their molecule. (i) Monosaccharide : A carbohydrate that cannot be hydrolyzed to simpler compounds is called monosaccharide. Monosaccharide which have six carbon are either aldohexoses or ketohexoses (ii) Oligosaccharides : Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaccharides. They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides, they provide on hydrolysis. Amongst these the most common are disaccharides. The two monosaccharides units obtained on hydrolysis on a disaccharide may be same or different. For example, sucrose on hydrolysis gives one molecule each of glucose and fructose whereas maltose gives two molecules of glucose only. (iii) Polysaccharide : A carbohydrate that can be hydrolyzed to many monosaccharide molecules is called a polysaccharide. Example : Starch, Cellulose, etc. Aldohexoses Their structure has been elucidated as follows : (i) Analysis and molecular-weight determinations show that the molecular formula of the aldohexoses is C6H12O6. (ii) When treated with acetic anhydride, aldohexoses form the penta-acetate. This indicates the presence of five hydroxyl groups. Ac2O (iii) Aldohexoses form an oxime when treated with hydroxylamine, or add molecule of HCN to form cyanohydrin and therefore contain a carbonyl group. (iv) When an aldohexose is oxidised with bromine-water or tollen’s reagent or fehling solution a pentahydroxy-acid of formula C6H12O7 is obtained. This indicates that the carbonyl group present is an aldehydic group. (v) When reduced with concentrated hydroiodic acid and red phosphorus at 100°C, aldohexoses give n-hexane. This indicates that the six carbon atoms in an aldohexose are in a straight chain. The above reactions show that structure of aldohexoses is CH2  C(H)  C(H)  C(H)  C(H)  CHO | | | | | OH OH OH OH OH Due to four asymmetric carbon atoms, there are sixteen optical isomers. or Eight pairs of enantiomers. (8D-variety & 8L- variety). D-variety of them are as follows H CHO OH HO H H OH H OH HO H OH H OH H OH H OH H OH CH2OH H OH CH2OH H OH CH2OH H OH CH2OH D(+)-allose (VII) CHO H D(+)-altrose (VIII) HO H H OH H H OH CH2OH D(–)-gulose (XI) CHO HO H CHO HO H HO H D(+)-mannose (X) CHO H H H D(+)-glucose (IX) CHO OH HO CHO OH H CHO OH HO H OH HO H HO H H HO H HO H OH CH2OH D(–)-idose (XII) H OH CH2OH D(+)-galactose (XIII) H OH CH2OH D(+)-talose (XIV) (XII) BIOMOLECULES & POLYMERS Note : 1. D-aldohexoses shown above have epimeric / diastereomeric relationship with each other 2. D-aldohexoses can be either dextro (+) or laevo (-) Aldoses : Monosaccharides containing aldehyde group are called aldoses. Ketoses : Monosaccharides containing ketonic group are called ketoses. Photosynthesis 6CO2 + 6H2O + 18 ATP Sunlight, chlorophyll C6H12O6 + 6O2 Cellular Respiration C6H12O6 + 6O2 6CO2 + 6H2O + 38 ATP (36 ATP net gain) enzymes CLASSIFICATION AND STRUCTURE OF CARBOHYDRATES Carbohydrates are polyhydroxy aldehydes and ketones and substances which hydrolyse to polyhydroxy aldehydes and ketones. The simplest carbohydrates are called sugars or saccharides, (Latin: Saccharum, sugar). Carbohydrates can be classified as monosaccharides, oligosaccharides and polysaccharides. GENERAL CHARACTERISTIC OF MONOSACCHARIDES The important characteristics of monosaccharides as follows: (i) All monosaccharides are water soluble due to the presence of hydrogen bonding between the different OH groups and surrounding water molecules. (ii) Monosaccharides have sweet taste and upon heating they get charred and give the smell of burning sugar. (iii) Monosaccharides are optically active in nature due to the presence of chiral carbon atoms. (iv) The chemical characteristics of monosaccharides are due to OH groups and carbonyl group which may be either aldehydic or ketonic group. Glyceraldehyde contains one asymmetric carbon atom (marked by an asterisk) and can thus exist in two optically active forms, called the D-form and the L-form. Clearly, the two forms are mirror images that cannot be superimposed, that is they are enantiomers. CHO H CHO C OH H2C OH D-Glyceraldehyde HC1 H O C 2 OH H2C OH 3 D-Glyceraldehyde OH C H OH H2C L-Glyceraldehyde D Means on the right OH HC1 O C 2 H H2C 3 L Means on the left OH L-Glyceraldehyde 110 All four isomers have been prepared synthetically. The D-and L-erythrose are mirror images, that is, they are enantiomers. They have exactly the same degree of rotation but in opposite directions. Equal amounts of the two would constitute a racemic mixture, that is a mixture that would allow a plane-polarised light to pass through the solution unchanged. C — C — C — C — C – CHO OH OH OH OH OH Supplying hydrogen atoms to the five carbon atoms to satisfy their tetravalency, following structure (open chain) may be assigned to glucose: (* indicates asymmetric carbon atom). H H H H H H C — C — C — C — C – CHO OH OH OH OH OH GLUCOSE Glucose is the most common monosaccharide. It is known as Dextrose because it occurs in nature principally as the optically active dextrorotatory isomers. It is act as a reducing agent (reduces both Fehling’s solution and ammonical silver nitrate solution). When heated with sodium hydroxide, an aqueous solution of glucose turns brown. It is known as dextrose and found as grapes, honey, cane sugar, starch and cellulose. Configuration of Glucose: Since the above structure possesses four asymmetric carbon atoms (shown by asterisks), it an exist in 2 4 = 16 optically active forms, i.e., eight pairs of enantiomers. All these are known and correspond to the D- and L-forms of glucose, mannose, galactose, allose, glucose, idose and talose. The naturally occurring dextrorotatory glucose (+)-glucose is only one of the 16-stereoisomers. CHO | H— C — OH 2 | HO—3C — H | H— 4 C — OH | H— 5 C — OH 6| H2C — OH D(+)Glucose 1 CHO | H— C — OH | H2C — OH (+)-Glyceraldehyde Notations D- and L- for denoting configuration were given by Rosanoff; according to this convention any compound whose bottom asymmetric carbon atoms has the configuration similar to the configuration of dextrorotatory glyceraldehyde (drawn above, i.e. the bottom carbon atom has –OH to the left and H to the right is given Lconfiguration. Remember that the symbols D-and L- have no relation with the specific rotation value, i.e., with (+) or (–) value. For example, the natural (–) fructose belongs to D-series, i.e., it is D(–)-fructose) BIOMOLECULES & POLYMERS Objections to open-chain structure of glucose: Even through open chain structure of (+) glucose explains most of its reactions, it fails to explain the following facts about it. (i) Glucose does not restore Schiff’s reagent colour. (ii) Glucose does not form a bisulphite and aldehyde-ammonia compound. (iii) Glucose forms two isomeric penta-acetates neither of which reacts with carbonyl reagents. (iv) The existence of the two isomeric glucoses and the change in specific rotation (mutarotation) is not explained by an open-chain formula. (v) Glucose reacts with methanol in presence of dry HCl gas to form two isomeric glucosides. Since glucose is less soluble in ethanol, it separates out on cooling the reaction mixture. Commercially, it is obtained by the hydrolysis of starch which is available from relatively inexpensive source such as maize, potatoes and rice. Constitution of Glucose: Molecular Formula: By the usual analytical methods, the molecular formula glucose is found to be C6H12O6. Straight Chain of six carbon atoms: (i) Reduction of glucose with conc. HI and phosphorus gives 2-iodohexane and n-hexane. This indicates that six carbon atoms in glucose are present in a straight chain C6H12O6 Glucose HI/P Prolonged heating CH3 – (CH2)4 – CH3 n -h exane (ii) Glucose when oxidized with bromine water gives gluconic acid which when reduced with excess of HI gives n-hexanoic acid, CH3.(CH2)4.COOH confirming the presence of a straight chain of six carbon atoms in glucose. Presence of five hydroxyl groups: When treated with acetic anhydride, glucose forms penta-acetate indicating the presence of 5 – OH groups and since glucose is a stable compound, the five –OH groups must be attached to 5 different carbon atoms. C6H12 O6 Glucose C6H5NHNH2 C6H5N CHO | (CHOCOCH3)4 | CH2OCOCH3 Glucose pentaacetate Presence of an aldehydic groups: (i) Glucose forms a cyanohydrin with hydrogen cyanide and a mono-oxime with hydroxylamine suggesting the presence of a carbonyl group. C6H12O6 Glucose HCN CN | (CHOH)5 | CH2OH Glucose cyanohydrin (ii) Glucose reduces Fehling solution and Tollen’s reagent indicating that the carbonyl group is aldehydic in nature. (iii) The presence of aldehydic group in glucose is confirmed by its oxidation to gluconic acid having the same number of carbon atoms. C 5H 11 . CHO + (O) Br2 Glucose C 5 H11O 5 . COOH Gluconic acid Now since aldehydic group is monovalent, it must be present on the end of the chain. Open chain structure: On the basis of the above points, glucose may be assigned following part structure orientation shown in the  anomer has the –OH trans to the –CH2OH group and the  anomer has the –OH cis to the –CH2OH group. FRUCTOSE Also said to be fruit sugar (1) It occur both in combined as well as free state. (2) Fructose is named as fruit sugar because it is present in honey and most sweet fruit in free state. (3) It is sweetest monosaccharide and present in cane sugar and insulin in combined state. (4) It is also known as -Laevulose i.e. natural occurring compound is laevorotatory PREPARATION (1) By acid hydrolysis of cane sugar : C12H22O11 + H2O H C6H12O6 + C6H12O6 Glucose Fructose (2) By enzymatic action of sucrose Sucrose (C12H22O11) Invertase (Disaccharide) Note: Glucose + Fructose Invert Sugar Glucose and fructose obtained by acid hydrolysis of sucrose can be separated by treating with Ca(OH)2 which forms calcium glucosate & calcium fructosate. Calcium fructosate being water insoluble. It is separated out easily PROPERTIES (1) It is colourless crystalline solid. (2) It is soluble in water but insoluble in ether ketone and benzene. (3) It is pentahydroxy ketone and shows mutarotation like glucose. BIOMOLECULES & POLYMERS CHEMICAL REACTION Due to OH group at 2nd carbon : (1) It forms fructose pentaacetate with acetyl chloride : (CH3CO)2O CH2OH | C=O | (CHOH)3 | CH2OH or CH3COCl O || CH2–O–C–CH3 | C=O O fructose penta acetate | || (CH–O–C–CH3)3 | CH2–O–C–CH3 || O CH3OH/H + & -methyl fructoside (2) Reaction due to keto group : It also form osazone with excess of phenyl hydrazine thus we can say that osazone formation is characteristic of -hydroxy carbonyl compounds. CH2OH | C=O H2N–NH–C6H5 H2N–NH–C6H5 CH2–OH –NH3 & –C6H5–NH2 | C=N–NH–C6H5 (CHOH)3 | CH2OH (CHOH)3 | CH2OH H2 N–NH–C6H5 CH=O | C=N–NH–C6H5 (CHOH)3 | CH2OH (A) Fructose cyanohydrin Br2 /H2O (CHOH)3 | CH2OH (CHOH)3 | CH2OH Fructose osazone HCN CH2OH | C=O CH=N–NH– C6H5 | CH=N–NH–C6H5 No reaction Na-Hg/H2O or NaBH4 (A) HI/P + Sorbitol n-hexane + 2-iodohexane Strong oxidising agent CH2–OH | COOH glycollic acid Mannitol + COOH COOH | | (CHOH)3 (CHOH)2 | + | COOH COOH Tartaric acid trihydroxy glutaric acid A mixture Fermentation C H OH STRUCTURE CH2 – OH | C=O | HO – C – H | H – C – OH | H – C – OH | CH2 – OH + Open chain structure -D-fructose -D-fructose of fructose Specific Rotation (– 92º) Specific Rotation (– 21º) Specific Rotation (– 133º) HAWORTH PROJECTION : FRUCTOFURANOSE -fructose -fructose Epimer : The change of configuration of one asymmetric carbon atom in a compound containing two or more asymmetric carbon atoms is known as epimerisation and compounds are known epimer. D-Glucose and D-mannose are empimer (Change in configuration at IInd carbon atom only). CH2OH CH2OH H 5 4 H OH HO 3 H O H 1 2 OH D-glucose H OH H 5 O 4 H HO OH 1 HO 2 3 H H H OH D-Mannose Such sugars are known as epimers e.g. epimerisation of glucose into mannose. BIOMOLECULES & POLYMERS CHO | H – C – OH Br2 | H (CHOH)3 2O | CH2OH D(+)-glucose CO2H | H – C – OH | (CHOH)3 | CH2OH + +H gluconic acid CHO | HO – C – H | (CHOH)3 | CH2OH D(+)-mannose Note : + –H C5H5N O C | O¯ (i) HCl HO – C – H (ii) lactone reduced | (CHOH)3 | CH2OH mannonic acid anion (1) Unlike other ketones, fructose can reduced Fehling sol. and Tollen’s reagents it is probably due to formation of an equilibrium between glucose, mannose and fructose. (2) Aldoses which produce the same osazones must have identical configuration on all their asymmetric carbon atoms except the alpha (since only the aldehyde group and -carbon atoms are involved in osazone formation). Conversion of Ketose into Aldose : H2 / Ni [O] HNO3 heat Na-Hg e ; H+ Conversion of Aldose into Ketose : (i) Ba(OH)2 HCN (ii) H2SO4 Chain lengthening : Chain shortening : Br2 /H2O COOH | CHOH | (CHOH)3 | CH2OH Ca salt 2+ H2O2/Fe heat Na/Hg H2SO 4 BIOMOLECULES & POLYMERS SUCROSE (Sucrose, cane-sugar C12H22O11) (i) Sucrose is a white crystalline solid, soluble in water. (ii) When heated above its melting point, it forms a brown substance known as caramel. (iii) Concentrated sulphuric acid chars sucrose, the product being almost pure carbon. (iv) Sucrose is dextrorotatory, its specific rotation being + 66.5°. (v) On hydrolysis with dilute acids sucrose yields an equimolecular mixture of D(+)-glucose and D(–)-fructose : HCl C12H22O11 + H2O C6H12O6 + C6H12O6 Glucose Fructose Since D(–)-fructose has a greater specific rotation than D(+)-glucose, the resulting mixture is laevorotatory. Because of this, hydrolysis of cane-sugar is known as the inversion of cane-sugar this is not to be confused with the Walden inversion, and the mixture is known as invert sugar. The inversion (i.e., hydrolysis) of canesugar may also be effected by the enzyme invertase which is found in yeast. (vi) Sucrose is not a reducing sugar, e.g., it will not reduce Fehling’s solution ; it does not form an oxime or an osazone, and does not undergo mutarotation. This indicates that neither the aldehyde group of glucose nor the ketonic group of fructose is free in sucrose. CH2OH H HO O H H OH H H OH HOCH2 O O H H 1 OH HO 2 CH2OH H 1 Sucrose Structure formulas for monosaccharides: Although many of the properties of D(+)-glucose can be explained in terms of an open-chain structure (1, 2, or 3), a considerable body of evidence indicates that the open-chain structure exists, primarily, in equilibrium with two cyclic forms. These can be represented by structures 4 and 5 or 6 and 7. The cyclic forms of D(+)-glucose are hemiacetals formed by an intramolecular reaction of the –OH group at C5 with the aldehyde group. Cyclisation creates a new stereogenic centre at C1, and this stereogenic centre explains how two cyclic forms are possible. These two cyclic forms are diastereomers that different only in the configuration of C1. In carbohydrate chemistry diastereomers of this type of called anomers, and the hemiacetal carbon atom is called the anomeric carbon atom. Structures 4 and 5 for the glucose anomers are called Haworth formulas and, although they do not give an accurate picture of the shape of the six-membered ring, they have many practical uses. Demonstrates how the representation of each stereogenic centre of the open-chain form can be correlated with its representation in the Haworth formula. Each glucose anomer is designated as an  anomer or a  anomer depending on the location of the –OH group of Cl. When we draw the cyclic forms of a D sugar in the BIOMOLECULES & POLYMERS Mutarotation : Ordinary D(+)-glucose has a melting point of 146°C. However, when D(+)-glucose is crystallized by evaporating an aqueous solution kept above 98°C, a second form of D(+)-glucose with a melting point of 150°C can be obtained. When the optical rotations of these two forms are measured, they are found to be significantly different, but when an aqueous solution of either form is allowed to stand, its rotation changes. The specific rotation of one form decreases and the rotation of the other increases, until both solutions. Show the same value. A solution of original D-(+)glucose (mp 146°C) has an initial specific rotation of +112°, but, ultimately, the specific rotation of this solution falls to +52.7°. A solution of second form of D(+) glucose (mp 150°C) has an initial specific rotation of +18.7°, but slowly, the specific rotation of this solution rises to +52.7°. This change in rotation towards an equilibrium value is called mutarotation. The explanation for this mutarotation lies in the existence of an equilibrium between the open-chain form of D(+) glucose an the  and  forms of the cyclic hemiacetals. Conversion to Esters: Treating a monosaccharide with excess acetic anhydride and a weak base (such as pyridine or sodium acetate) converts all of the hydroxyl groups, including the anomeric hydroxyl, to ester groups. If the reaction is carried out at a low temperature (e.g., 0°C), the reaction occurs stereospecifically; the  anomer gives the -acetate and the  anomer gives the -acetate. Acetate esters are common protecting groups for carbohydrate hydroxyls. BENEDICT’S OR TOLLENS’ REAGENTS: REDUCING SUGARS Benedict’s reagent (A alkaline solution containing a cupric citrate complex ion) and Tollen’s solution [Ag+(NH3)2OH]– oxidize and thus give positive tests with aldoses and ketoses. The tests are positive even though aldoses and ketoses exist primarily as cyclic hemiacetals. BIOMOLECULES & POLYMERS Sugars that give positive tests with Tollen’s or Benedict’s solutions are known as reducing sugars, and all carbohydrates that contain a hemiacetal group given positive tests. In aqueous solution these hemiacetals exist in equilibrium with relatively small, but not insignificant, concentration of noncyclic aldehydes or -hydroxy ketones. It is the latter two that undergoes the oxidation until one reactant is exhausted. Carbohydrates that contain only acetal groups do not given positive tests with Benedict’s or Tollen’s solutions, and they are called non-reducing sugars. Acetals do not exist in equilibrium with aldehydes or -hydroxy ketones in the basic aqueous media of the test reagents. Bromine Water : The synthesis of Aldonic Acid Monosaccharides do not undergo isomerization and fragmentation reactions in mildly acids solution. Thus, a useful oxidizing reagent for preparative purposes is bromine in water (pH 6.0). Bromine water is a general reagent that selectively oxidizes the –CHO group to a –CO2H group. It converts an aldose to an aldonic acid. Experiments with aldopyranoses have shown that the actual course of the reaction is somewhat more complex than we have indicated above. Bromine water specifically oxidizes the  anomer, and the initial product that forms is a aldonolactone. This compound may then hydrolyze to an aldonic acid, and the aldonic acid may undergo a subsequent ring closure to form a -aldonolactone: NITRIC ACID OXIDATION: ALDARIC ACID Dilute nitric acid – a stronger oxidizing agent than bromine water – oxidizes the both –CHO group and the terminal –CH2OH group of an aldose to –CO2H groups. These dicarboxylic acids are known as aldaric acids: O H H OH HO O H H OH H OH H2O O C HC OH HC OH CH O HC or CH HC O OH O CH2 HC OH OH C C CH2 OH CH O OH This aldaric acid obtained from D-glucose is called D-glucaric acid. PERIODATE OXIDATION: OXIDATIVE CLEAVAGE OF POLYHYDROXY COMPOUNDS Compounds that have hydroxyl groups on adjacent atoms undergo oxidative cleavage when they are treated with aqueous periodic acid (HIO4). The reaction breaks carbon-carbon bonds and produces carbonyl compounds (aldehydes, ketones, or acids). H C OH O + HIO4 2 C C + HIO3 + H2O OH Since the reaction usually takes place in quantitative yield, valuable information can often be gained by measuring the number of molar equivalents of periodic acid that is consumed in the reaction as well as by identifying the carbonyl products. Periodate oxidations are though to take place through a cyclic intermediate: BIOMOLECULES & POLYMERS Before we discuss the use of periodic acid in carbohydrate chemistry, we should illustrate the course of the reaction with several simple examples. Notice in these periodate oxidations that for every C – C bond broken, a C – O bond 1. 2. is formed at each carbon. When three or more –CHOH groups are continuous, the internal ones are obtained as formic acid. Periodate oxidation of glycerol, for example, gives two molar equivalents of formaldehyde and one molar equivalent of formic acid; Oxidative cleavage also take place when an – OH group is adjacent to the carbonyl group of an aldehyde or ketone (but not that of an acid or an ester). Glyceraldehyde yields two molar equivalents of formic acid and one molar equivalent of formaldehyde, while dihydroxyacetone gives two molar equivalents of formaldehyde and one molar equivalent of carbon dioxide. Periodic acid does not cleave compound in which the hydroxyl groups are separated by an intervening –CH2 group, nor those in which a hydroxyl group is adjacent to an ether or acetal. REDUCTION OF MONOSACCHARIDES: ALDITOLS Aldoses (and ketoses) can be reduced with sodium borohydride to compounds called alditols: Reduction of D-glucose, for example, yields D-glucitol. REACTIONS OF MONOSACCHARIDES WITH PHENYLHYDRAZINE: OSAZONES The aldehyde group of an aldose reacts with such carbonyl reagents as hydroxylamine and phenylhydrazine. With hydroxylamine, the product is the expected oxime. With enough phenylhydrazine, however, three molar equivalents of phenylhydrazine are consumed and a second phenylhydrazone group is introduced at C2. The product is called a phenylosazone. Phenylosazones crystallize readily (unlike sugars) and are useful derivatives for identifying sugars. The mechanism for osazone formation probably depends on a series of reaction in which very much like C O in giving a nitrogen version of an enol. C N behaves BIOMOLECULES & POLYMERS A MECHANISM FOR THE REACTION Phenylosazone Formation H HC A: H N C A N HC 6 H 5 tautomerization OH H H HC N N C O C 6 H5 H : A Formed from the aldose HC (-C6 H5 NH 2 ) NH O C (+ 2C6H5 NHNH2) C NHHC6H5 C NHHC6 H5 + NH3 + H2O Osazone formation result in a loss of the stereogenic centre at C2 but does not affect other stereogenic carbons; Dglucose and D-mannose, for example, yield the same phenylosazone: CHO H CH OH HO H H OH H OH CH2 OH D-Glucose C C 6H5NHNH2 HO NHHC6H5 NHHC6H5 H C6H5NHNH2 CHO HO H HO H H OH H OH H OH H OH CH2 OH Same Phenylosazone CH2 OH D-Mannose This experiment, first done by Emil Fischer, established that D-glucose and D-mannose have the same configuration about C3, C4 and C5. Diastereomeric aldoses that differ in configuration at only one carbon (such as D-glucose and D-mannose) are called epimers. In general, any pair of diastereomers that differ in configuration at only a single tetrahedral stereogenic carbon can be called epimers. Epimers: Many common sugars are closely related, differing only by the stereochemistry at a single carbon atom. For example, glucose and mannose differ only at C2, the first asymmetric carbon atom. Sugars that differs only by the stereochemistry at a single carbon are called epimers. and the carbon atom where they differ is generally stated. If the number of a carbon atom is not specified, it is assumed to be C2. Therefore, glucose and mannose are “C2 epimers” or simply “epimers”. The C4 epimer of glucose is galactose and the C2 epimer of erythrose is threose. Cyclic structure of Fructose: Like glucose, fructose also has a cyclic structure. Since fructose contains a keto group, it forms an intramolecular hemiketal. In the hemiketal formation, C5–OH of the fructose combines with C2-keto group. As a result, C2 becomes chiral and thus has two possible arrangements of CH2OH and OH group around it. Thus, D-fructose exists in two stereoisomeric forms, i.e., -fructopyranose and -D fructopyranose. However in the combined state (such as sucrose), fructose exists in furanose form as shown below: Hydrolysis of Sucrose: 6 CH2OH 5 H 4 OH O H OH H 1 H -link 2 3 H OH (Glucose unit) CH2OH O H linkage -link H OH OH H (Fructose unit) O Glycosidic CH2OH Sucrose BIOMOLECULES & POLYMERS (Invert Sugar or Invertose). Hydrolysis of sucrose with hot dilute acid yields D-glucose and D-fructose. C12H22O11 + H2O H + C6H12O6 + C6H12O6 Sucrose D(+)-glucose D(–)-Fructose []D = +66.5° []D = +53° []D = –92° Invert Sugar []D = (+53°) – (–92°) = 39° Sucrose is dextrorotatory, its specific rotation being +66.5%, D-glucose is also dextrorotatory, []D = +53°, but Dfructose has a large negative rotation, []D = –92°. Since D-fructose has a greater specific rotation than D-glucose, the resulting mixture is laevorotatory. Because of this the hydrolysis of sucrose is known as the inversion of sucrose, and the equimolecular mixture of glucose and fructose is known as invert sugar or invertose. POLYSACCHARIDES Polysaccharides are the polymers of monosaccharides. The natural polysaccharide generally contain about 1003000 monosaccharide units. The three most abundant natural polysaccharides-cellulose, starch and glycogen are derived from the same monomer, i.e., glucose. Starch: It is a polymer of glucose. Its molecular formula is (C6H10O5)n where the value of n(200 – 1000) varies from source to source. It is the chief food reserve material or storage polysaccharide of plants and is found mainly in seeds, roots, tubers, etc. Wheat, rice, potatoes, corn, bananas etc., are rich source of starch. Starch is not a single compound but is a mixture of two components – a water soluble component called amylose (20%) and a water insoluble component called amylopectin (80%). Both amylose and amylopectin are polymers of -D glucose. Amylose is a linear polymer of -D glucose. It contains about 200 glucose units which are linked to one another through -linkage involving C1 of one glucose unit with C4 of the other. Amylopectin, on the other hand, is a highly branched polymer. It consists of a large number (several branches) of short chains each containing 20-25 glucose units which are joined together through -linkages involving C1 of one glucose unit with C4 of the other. The C1 of terminal glucose unit in each chain is further linked to C6 of the other glucose. Unit in the next chain through C1-C6 -linkage. This gives amylopectin a highly branched structure as shown below: C1 – C4 -linkage Glucose – Glucose – Glucose Glucose – Glucose – Glucose Glucose – Glucose – Glucose Hydrolysis: Hydrolysis of starch with hot dilute acids or by enzymes give dextrins of varying complexity, maltose and finally D-glucose. Starch does not reduce Tollen’s reagent and Fehling’s solution. Uses: It is used as a food. It is encountered daily in the form of potatoes, bread, cakes, rice etc. It is used in coating and sizing paper to improve the writing qualities. Starch is used to treat textile fibres before they are woven into cloth so that they can be woven without breaking. It is used in manufacture of dextrins, glucose and ethyl alcohol. Starch is also used in manufacture of starch nitrate, which is used as an explosive. Cellulose: Cellulose is the chief component of wood and plane fibres; cotton, for instance, is nearly pure cellulose. It is insoluble in water and tasteless; it is a non-reducing carbohydrate. These properties, in part at least, are due to extremely high molecular weight. H OH O O HO H OHH H H HO HO O OH O HO O HO O H OH O H Cellulose Cellulose has the formula (C6H10O5)n. Complete hydrolysis by acid yields D(+)-glucose as the monosaccharide. Hydrolysis of completely methylated cellulose gives a high yield of 2, 3, 6-tri-O-methyl-D-glucose. Like starch, therefore, cellulose is made up of chains of D-glucose units, each unit joined by a glycoside linkage of C-4 of the next. Cellulose differs from starch, however, in the configuration of the glycoside linkage. Upon treatment with acetic anhydride and sulfuric acid, cellulose yields octa-O- acetyl cellobiose, there is evidence that all glycoside linkages in cellulose, like the one in (+) cellobiose, are beta linkages. Physical methods give molecular weights for cellulose ranging from 250000 to 1000000 or more; it seems likely that there are at least 1500 glucose units per molecule. End group analysis by both methylation and periodic acid oxidation gives a chain length of 1000 glucose units or more. X-ray analysis and electron microscopy indicate that these long chains lie side by side in bundles, undoubtedly held together by hydrogen bonds between the numerous neighbouring –OH groups. These bundles are twisted together to form. Rope like structure which themselves are grouped to from the fibers we can see. In wood these cellulose “ropes” are embedded in lignin to give a structure that has been likened to reinforced concrete. Properties of Cellulose: We have seen that the glycoside linkages of cellulose are broken by the action of acid, each cellulose molecule yielding many molecules of D(+)-glucose. Now let us look briefly at reactions of cellulose in which the chain remains essentially intact. Each glucose unit in cellulose contains three free – OH groups; these are the positions at which reactions occurs. These reactions of cellulose, carried out to modify the properties of a cheap, available ready-made polymer, are of tremendous industrial importance. BIOMOLECULES & POLYMERS Like any alcohol, cellulose forms esters. Treatment with a mixture of nitric and sulfuric acid converts cellulose into cellulose nitrate. The properties and uses of the product depend upon the extent of nitration. Guncotton, which is used in making smokeless powder, is very nearly completely nitrated cellulose, and is often called cellulose trinitrate (three nitrate groups per glucose unit). Pyroxylin is less highly nitrated material containing between two and three nitrate groups per glucose unit. It is used in the manufacture of plastics like celluloid and collodion, in photographic film, and in lacquers. It has the disadvantage of being flammable, and forms highly toxic nitrogen oxides upon burning. Industrially, cellulose is alkylated to ethers by the action of alkyl chlorides (cheaper than sulfates) in the presence of alkali. Considerable degradation of the long chain is unavoidable in these reactions. Methyl, ethyl, and benzyl others of cellulose are important in the production of textiles, films, and various plastic objects.