Carbohydrate Chemistry PDF

CARBOHYDRATE CHEMISTRY 1  INTRODUCTION Carbohydrates consist of the elements carbon (C), hydrogen (H) and oxygen (O). with a ratio of hydrogen twice that of carbon and oxygen. Carbohydrates include sugars, starches, cellulose and many other compounds found...

CARBOHYDRATE CHEMISTRY 1  INTRODUCTION Carbohydrates consist of the elements carbon (C), hydrogen (H) and oxygen (O). with a ratio of hydrogen twice that of carbon and oxygen. Carbohydrates include sugars, starches, cellulose and many other compounds found in living organisms.  In their basic form, carbohydrates are simple sugars or monosaccharides. 2 These simple sugars can combine with each other to form more complex carbohydrates. The combination of two simple sugars is a disaccharide. Carbohydrates consisting of two to ten simple sugars are called oligosaccharides. Those with a larger number are called 3 Biochemically carbohydrates may be defined as polyhydroxyl aldehyde or polyhydroxyl ketone or compounds that yield them upon hydrolysis. 4 Structure of Carbohydrates There are three types of structural representations of carbohydrates: Open chain structure:- It is the long straight chain form of carbohydrates. Hemi- acetal structure:- Here the 1st carbon of the glucose condenses with the -OH group of the 5th carbon to form a ring structure. Haworth structure:- It is the presence of pyranose or furanose ring structure. 5 D-Glucose Open (Fischer) Hemi acetal Haworth α- D a Glucose 6  Classification of carbohydrates They are broadly classified into three major groups: This categorization is based on the number of sugar units. 7 1) Mono saccharides: Simple sugars they cannot be hydrolyzed into smaller units. Depending upon no. of carbon in a unit, mono saccharides are subdivided. Number of Category Name Examples Carbons 3 Triose Glyceraldehyde, Dihydroxy 4 Tetrose acetone Erythrose, Threose 5 Pentose Arabinose, Ribose, Ribulose, Xylose, Xylulose, Lyxose 6 Hexose Allose, Altrose, Fructose, Galactose, Glucose, Gulose, Idose, Mannose, Sorbose, Talose, Tagatose 7 Heptose Sedoheptulose, Mannoheptulose 8 Monosaccharide may also be classified into two based on the functional group: Aldose Ketose Tetroses D-Erythrose D-Threose 9  EXAMPLES OF PENTOSES D-Ribose D-Xylose D-Lyxose D-Arabinose 10 EPIMERS EPIMERS ribonucleic acid (RNA). Deoxyribose, which is missing oxygen at position 2, is a component of deoxyribonucleic acid (DNA). In nucleic acids, the hydroxyl group attached to carbon number 1 is replaced with nucleotide bases. Ribose Deoxyribose 11  Aldo Hexoses D-Allose D-Altrose 12  D-Mannose D-Glucose EIPEMER D-Galactose D-Idose D-Talose 13  Keto Hexoses D-Tagatose D-Fructose 14  EPIMERS Structures that have opposite configurations of a hydroxyl group at only one position,  such as glucose and mannose, are called epimers. 15 Sedoheptulose has the same structure as fructose, but it has one extra carbon. Sedoheptulose is found in carrots. Mannoheptulose is a monosaccharide found in avocados. D-Sedoheptulose D-Mannoheptulose 16  Chain and Ring forms The glucose ring form is created when the oxygen on carbon number 5 links with the carbon comprising the carbonyl group (carbon number 1) and transfers its hydrogen to the carbonyl oxygen to create a hydroxyl group. The rearrangement produces alpha glucose when the hydroxyl group is on the opposite side of the -CH2OH group, or beta glucose when the hydroxyl group is on the same side as the - CH2OH group. 17 Chain and Ring forms CONT. Isomers, such as these, which differ only in their configuration about their carbonyl carbon atom are called anomers. The little D in the name derives from the fact that natural glucose is dextrorotary, i.e., it rotates polarized light to the right. Monosaccharides forming a five-sided ring, like ribose, are called furanoses. Those forming six-sided rings, like glucose, are called pyranoses. 18 D-Glucose Cyclation of Glucose α-D-Glucose β-D-Glucose 19  2. Oligosaccharide Oligosaccharides are polymers of mono saccharides containing two to ten residues accumulate in vacuole. The monomers are joined together by glycosidic dond a) Disaccharides: Yield two mono saccharides on hydrolysis. i Reducing Disaccharides: With free aldehyde or ketone group e.g. Maltose (Glucose + Glucose), Lactose (Galactose + Glucose). α-D-glucopyranosyl-(1 4)- α-D-glucopyranoside 20 ii) Non Reducing Disaccharides: With no free aldehyde or ketone group e.g. Sucrose (Glucose + Fructose) α-D-glucopyranosyl-(1 2)- β-D-fructofuranoside 21 b) Tri saccharides: e.g. Raffinose (Glucose +Fructose + Galactose) found in cotton seed and sugar beet. c) Tetra saccharides: Yield 4 mono saccharides on hydrolysis. e.g. stachyose (Glucose + Fructose + Galactose + Galactose). 22  3. Polysaccharides Polysaccharides are polymeric anhydrides of mono saccharides joined together by glycosidic bonds. The long chain polymers are either straight chain or branched. Classification of Polysaccharides: 1) On the Basis of Function: a) Storage e.g. Starch, glycogen b) Structural e.g. Cellulose, Pectin 23 a) Homo polysaccharides b) Hetero polysaccharides. a) Homo Polysaccharides: On hydrolysis gives single monosaccharide units i) Pentosan: Contains pentoses (C5 H8 04). ii) Hexosans: Contains hexoses (C6 H10 05) Homo polysaccharides subdivided in to: A) Glucosans: Polymer of glucose e.g. Starch, Glycogen B) Fructosans: Polymer of fructose e.g. Inulin C) Galactans: Polymer of galactose e.g. Galactan D) Mannans: Polymer of mannose e.g. 24 b) Hetero Polysaccharide: e.g. Hyaluronic acid, Chondroitin sulphates. A) Gum: Consist of arabinose, rhamnose, galactose and glucoronic acid. B) Agar: The sulphuric acid esters of galactans consists of galactose, galactouronic acid. C) Pectins: Fundamental unit is pectic acid, consist of arabinose, galactose, galactouronic acid. 25 26  Functions of Polysaccharides 1) They serve as structural components of the cells 2) They serve as stored form of energy 3) They serve as nutrient. 27 Structure and Properties of Starch Consist of two components, Amylose and Amylopectin. Amylose is a long chain polysaccharides containing α - D glucose molecules linked by 1- 4 glycosidic linkages, produce blue colour with iodine. Amylopectin is a branched chain polysaccharides consisting α –D glucose molecules linked by 1- 4 glycosidic linkage and branches by 1-6, linkage produce purplish colour with iodine and forms a gel with hot water. Cellulose: It is structural polysaccharide found in cell walls of plants, made up of long chains of β-D-Glucose molecules linked by β 1-4 glycosidic bond, no branching, yield on hydrolysis crystalline D- glucose.28  Functions of Carbohydrates 1. Carbohydrates are chief energy source, in many animals , they are instant source of energy. Glucose is broken down by glycolysis / kreb' s cycle to yield ATP. 2. Glucose is the source of storage of energy. It is stored as glycogen in animals and starch in plants. Stored carbohydrates acts as energy source instead of proteins. 3. Carbohydrates are intermediates in biosynthesis of fats and proteins. 4. Carbohydrates aid in regulation of nerve tissue and is the energy source for brain. 5. Carbohydrates get associated with lipids and proteins to form surface antigens, receptor molecules, vitamins and antibiotics. 29 Functions of Carbohydrates 6. They form structural and protective components, like in cell wall of plants and microorganisms. 7. In animals they are important constituent of connective tissues. 8. They participate in biological transport, cell- cell communication and activation of growth factors. 9. Carbohydrates that are rich in fibre content help to prevent constipation. 10. Also they help in modulation of immune system. 30  Physical Properties of Carbohydrates Stereoisomers are compounds that have the same structural formulae but differ in their spatial configuration. A carbon is said to be asymmetric when it is attached to four different atoms or groups. The number of asymmetric carbon atoms (n) determines the possible isomers of a given compound which is equal to 2n. e.g glucose contains 4 asymmetric carbons, and thus has 16 isomers. 31  Physical Properties of Carbohydrates 1. Enantiomers:- are a special type of steroisomers that are mirror images of each other.  Example: Glucose has two isomers with respect to penultimate carbon atom.  They are D -glucose and L- glucose. 2. Optical Activity - It is the characteristic feature of compounds with asymmetric carbon atoms. Rotation of plane polarized light forming (+) glucose and (-) glucose. The term dextrorotatory(+) and levorotatory (-) are used to compounds that respectively rotate the plane of polarized light to the right or to the left. An optical isomer may be designated as D(+), D(-), L(+) and L(-).  Racemic mixture: if D and L- isomers are present in equal concentration, it is known as racemic mixture or DL mixture.  Racemic mixture does not exhibit any optical activity, since the + and – activities cancel each others. 32  Physical Properties of Carbohydrates Diastereo isomers – They are stereoisomers that are not mirror images of one another. It the configurational changes with regard to C 2, C 3, or C 4 in glucose. Example: Mannose, galactose. Annomerism - It is the spatial configuration with respect to the first carbon atom in aldoses and second carbon atom in ketoses. Mutarotation is defined as the change in the specific optical rotation.  Representing the interconversion of α and β forms of D- glucose to an equilibrium mixture.33 Chemical Properties of Carbohydrates Osazone formation with phenylhydrazine. Benedicts test. Oxidation Reduction to alcohols 34