____________________________________________________________________________________________________ Subject Chemistry Paper No and Title Paper 16, Bioorganic and biophysical chemistry Module No and Title 3, Sugars and polysaccharides Module Tag CHE_P16_M3 CHEMISTRY PAPER No. 16 Bioorganic and biophysical chemistry MODULE No.3: Sugars and polysaccharides ____________________________________________________________________________________________________ TABLE OF CONTENTS 1. Learning outcomes 2. Introduction 2.1 Definition of carbohydrates 3. Classification 3.1 Based on number of simple sugars (mono, di and polysaccharides) 3.2 Based on carbonyl function (aldose, ketose) 3.3 Based on number of carbon atom 4. Structure and stereochemistry of monosaccharide 4.1 Enantiomers, diastereomers and epimers (isomers) 5. Cyclic structure of sugars 5.1 Anomers, racemic mixture, mutorotation 6. Chemical properties of monosaccharaides 6.1 Action of acids and alkalis 6.2 Oxidation and reduction of sugars 6.3 Osazone formation 6.4 Glycoside formation 7. Derivatives of monosaccharaides 7.1 Amino sugar, deoxysugars, sugar acids, sugar alcohols, phosphoric acid esters of sugars 8. Disaccharides 8.1 Reducing 8.2 Non- reducing 9. Polysaccharides (glycan) 9.1 Homo polysaccharides 9.2 Hetero polysaccharides 10. Glycoproteins 11. Functions of carbohydrates 12. Summary CHEMISTRY PAPER No. 16 Bioorganic and biophysical chemistry MODULE No.3: Sugars and polysaccharides ____________________________________________________________________________________________________ 1. Learning Outcomes After studying this module, you shall be able to • Know about general definition and basis of classification of carbohydrates • Know about straight chain and cyclic structures of carbohydrates • Chemicalproperties of monosaccharaidesand their derivatives • Types of disaccharides and their properties • Polysaccharides and its types • Function of carbohydrates 2. Introduction The carbohydrates are an important class of naturally occurring organic compounds. They occur naturally in plants (where they are produced photosynthetically), when the word "carbohydrate" was coined, it originally referred to compounds of general formula Cn(H2O)n. However, only the simple sugars or monosaccharaides fit this formula exactly. The other types of carbohydrates, oligosaccharides, and polysaccharides, are based on monosaccharaides units and have slightly different general formula. Carbohydrates are also called "saccharides" which means sugar in Greek. Many commonly encountered carbohydrates are polysaccharides, including glycogen, which is found in animals, and starch and cellulose, which occur in plants. 2.1 What is carbohydrate? The carbohydrates are poly functional compounds. They contain following functional groups:- (1). Alcoholic hydroxy group, -OH (2). Aldehyde group, -CHO (3). Ketone, >C=O A precise definition of the term 'carbohydrate' can be given as:- Polyhydroxyaldehydes or polyhydroxy ketones, and large molecules that produce these compounds on hydrolysis. The above definition of carbohydrate is not entirely satisfactory. It envisages the presence of free carbonyl group in simple carbohydrate molecule which is not true. CHEMISTRY PAPER No. 16 Bioorganic and biophysical chemistry MODULE No.3: Sugars and polysaccharides ____________________________________________________________________________________________________ We understand that carbonyl compound (aldehyde or ketone) reacts with an alcohol to form hemiacetal. In carbohydrates, we have an aldehyde group which combines with an alcoholic OH of the same molecule to form an internal hemiacetal. We will also see that it is by elimination of H2O between the hemiacetal OH group of two sugars molecules that larger carbohydrate molecule are formed. In light of above, the definition of carbohydrate may be improved as: A polyhydoxy compound that has an aldehyde or a ketone function present, either free or as hemiacetal or acetal. 3.Classification of carbohydrates 3.1 Based on number of simple sugars (mono, di and polysaccharides) The carbohydrates are divided into three major classes depending on the number of simple sugar unit present in their molecule. In other words, the basis of classification of carbohydrate will be the number of simple sugar molecules on hydrolysis. The molecules so obtained may be of same or different sugars 3.1.1 Monosaccharaides (simple sugars) These are single unit carbohydrate that cannot be broken into simpler glucose and fructose. Examples: 3.1.2 Oligosaccharides They are made of 2 to 10 units of monosaccharaides or simple sugars. The oligosaccharides containing two monosaccharaides units are called disaccharide and those containing three units are trisaccharides. Thus sucrose, C12H22O11 is disaccharides because on hydrolysis it gives one molecule of glucose and one molecule of fructose. Oligosaccharides with more than three subunits are normally found in glycoproteins, such as blood group antigens. CHEMISTRY PAPER No. 16 Bioorganic and biophysical chemistry MODULE No.3: Sugars and polysaccharides ____________________________________________________________________________________________________ 3.1.3 Polysaccharides They contain more than ten monosaccharide units in molecule. Thus one molecule of starch or cellulose on hydrolysis gives very large number of glucose units. 3.2 Further classification of monosaccharides 3.2.1 Based on carbonyl function Those containing the aldehyde function, -CHO are called aldoses and other containing keto group -Co are, called ketoses. Fig1. Structure of glucose and fructose 3.3 Based on number of carbon atom The monosaccharaides containing 3, 4, 5, 6 etc carbon atoms are designated as trioses, tetroses, pentoses, hexoses and so on as shown in table1. For example, glucose a six carbon sugar with aldehyde function is aldohexose, fructose, a six carbon having ketone function is a ketohexose. CHEMISTRY PAPER No. 16 Bioorganic and biophysical chemistry MODULE No.3: Sugars and polysaccharides ____________________________________________________________________________________________________ Table1. Classification of monosaccharaides 4. Structure and stereochemistry of monosaccharide 4.1 Enantiomers, diastereomers and epimers (isomers) The building blocks of all carbohydrates are simple sugars called monosaccharaides, which can be an aldose or ketose. The simplest aldose, glyceraldehyde, has only one asymmetric carbon atom or chiral centre (carbon-2) (Figure 2)and only two possible of OH and H of the CH2OH unit are possible. If the OH group is shown as projecting to left on penultimate carbon atom in the straight chain representation of the molecule, then by convention the molecule is called L isomer, while if OH group is represented as projecting to right it is known as D isomer. Fig2. Structure of L- Glyceraldehyde and D-Glyceraldehyde These two stereoisomers to same carbohydrates are enantiomers or mirror images of one another. CHEMISTRY PAPER No. 16 Bioorganic and biophysical chemistry MODULE No.3: Sugars and polysaccharides ____________________________________________________________________________________________________ 4.1.2 Diastereomers Glyceraldehyde has only one asymmetric centre (*) and the designation of D or L is determined by the orientation of H and OH groups about the carbon atoms. For carbohydrates with more than one asymmetric carbon atom, the prefix D or L, refers only to configuration about the highest numbered asymmetric carbon atom. For higher carbohydrate structures, each time number of carbon is increased by one (with the addition of CHOH group), a new carbohydrate is produced, which has a chemically distinct form and completely different name because this group may be added as either OH-C-H or H-C-OH, an extra asymmetric (chiral) centre is created within the molecule, so aldotetroses have two chiral carbon C-2 and C-3, and there are 22 or four possible stereoisomer. Two of isomers have the D-configuration, and the two have L-configuration the two D isomers have same configuration at C-3 but they differ in configuration (arrangement of the -OH group) at other chiral carbon, C-2. These two isomers are called D-erythrose and D-threose. They are not superimposable on each other, but neither is they mirror images of each other. Such non superimposable, non-mirror image stereoisomers are called diastereomers. Fig3. D-Erythrose and D-Threose 4.1.3 Epimers (isomers) The two L isomers or L-erythrose, and L-threose is the enantiomer of D-threose. L-threose is a diastereomer of both D and L-erythrose, and L-erythrose is a diastereomer of both D and L- threose. Diasteromers that differ from each other in configuration at only one chiral carbon are called epimers; erythrose and D-threose are epimers. Glucose, galactose are also examples of epimers. Aldopentoses have three chiral carbons and there are 23 or 8 possible stereoisomers - four D form and four L- form, similarly aldohexoses will have 16 stereoisomers. Some of the possible CHEMISTRY PAPER No. 16 Bioorganic and biophysical chemistry MODULE No.3: Sugars and polysaccharides ____________________________________________________________________________________________________ stereoisomers are much more common in nature than others. For example D sugars rather than L sugars, predominate in nature. Most sugars present in nature, especially in foods, contain either five or six carbon atoms. We shall discuss D - glucose (an aldohexose) because it is most abundant monosaccharide in nature. Fig4. Epimers