Isomerism, Functions, Classification, Properties and Reactions CARBOHYDRATES Isomerism, Functions, Classification, Properties and Reactions

Isomerism The existence of two or more compounds with the same composition but different constitution.

Types of Isomerism Structural Isomers 1. Skeletal or chain 2. Position isomers 3. Functional isomers Stereoisomers 1. Geometric or cis-trans isomerism 2. Optical isomers a. Enantiomers b. Diastereomers

Structural Isomers Skeletal or chain CH3 – C – CH3 CH3 H H H H H H – C – C – C – C – C –H H H H H H

Position Isomers CH3 – CH2 – CH2Cl CH3 – CH – CH3 Cl 1-chloropropane 2- chloropropane

Functional Isomers CH3 – CH2 – OH CH3 – O – CH3 ethyl alcohol dimethyl ether

Stereoisomers Geometric or cis-trans isomerism H – C – COOH H – C – COOH HOOC – CH H – C – COOH fumaric acid (trans) maleic acid (cis) MP = 302 oC MP = 130 oC Insoluble in water soluble in water

Optical isomers Enantiomers – are mirror images of each other and non superimposable. They have the same melting point, boiling point and solubility. Enantiomers are either dextro-rotatory or levorotatory. d – if the OH of the penultimate C is at the right l – of the OH of the penultimate C is at the left

Diastereomers are not mirror images of each other Diastereomers are not mirror images of each other. They have different MP, BP and solubilities.

Functions of Carbohydrates Skeletal or structural functions – for example cellulose, the major structural component of plant cell walls; and peptidoglycans of bacterial cell walls Storage, nutrient and reservoir of chemical energy, to be enlarged or depleted as the organism needs – e.g. glycogen and starch which are produced or consumed in line with the energy needs of the cell

Classification According to the number of sugar units Monosaccharides – simple sugars that could not be hydrolyzed further into smaller units under reasonably mild conditions. Ex. Glucose Oligosaccharides – are hydrolyzable polymers of monosaccharides containing 2-6 molecules of simple sugars. Ex. Sucrose Polysaccharides – very long chains or polymers of monosaccharides which may be linear or branched in structure; usually tasteless and water insoluble Homopolysaccharides – made up of only one kind of monosaccharide. Ex. Starch and glycogen Heteropolysaccharides – made up of 2 or more different kinds of monosaccharides. Ex. Hyaluronic acid

2. According to functional groups Aldose contains the aldehyde group Ketose contains the ketose group According to the number of carbon atoms Triose contains 3 carbon atoms Tetrose contains 4 carbon atoms Pentose contains 5 carbon atoms Hexose contains 6 carbon atoms

Aldotriose: glyceraldehyde Ketotriose: dihydroxyacetone Tetroses: List of monosaccharides Trioses: Aldotriose: glyceraldehyde Ketotriose: dihydroxyacetone Tetroses: Aldotetrose: erythrose and threose Ketotetrose: erythrulose Pentoses: Aldopentoses: arabinose, lyxose, ribose, deoxyribose, and xylose Ketopentoses: ribulose and xylulose Hexoses: Aldohexoses: allose, altrose, galactose, glucose, gulose, idose, mannose and talose Ketohexoses: fructose, psicose, sorbose and tagatose Heptoses: Keto-heptoses: mannoheptulose, sedoheptulose Octoses: octolose, 2-keto-3-deoxy-manno-octonate Nonoses: sialose

β-D-Glucopyranose (Haworth projection) α-D-Glucopyranose (Haworth projection)

Linkages in Disaccharides Maltose – contains 2 glucose units linked at α-1,4 Cellobiose – contains 2 glucose units linked at β-1,3 Isomaltose – contains 2 glucose units linked at α-1,6 Lactose – has one glucose and one galactose Sucrose – has one glucose and one fructose

Linkages in Polysaccharides Storage polysaccharides Starch – the storage polysaccharide of higher plants; consists of 2 fractions: amylose and amylopectin Glycogen – structure is similar to amylopectin but more highly branched Nutrient polysaccharides – ex. Inulin, found in bulbs of many plants, consists of fructofuranose unit joined together by glycosidic linkage

Structural Polysacchrides Cellulose – consists of d glucose units; it is found in cell walls of plants. Pectins – contain arabinose, galactose and galacturonic acid hemicelluloses – are not cellulose derivatives; a homopolymer of d-xylose chitin – a homopolymer of N-acetyl D glucosamine; found in the shell of crustaceans and scales of insects