Carbohydrates Chemistry (Monosaccharides) Lecture Notes PDF

Lec. (1) Carbohydrates Chemistry (Monosaccharides) By Dr Dina Aboraya Definition of Carbohydrates: ▪ Carbohydrates are organic substances composed of carbon, hydrogen and oxygen (C, H, O). ▪ Hydrogen and Oxygen are present in the same ratio as in water (H2O = 2:1), so called...

Lec. (1) Carbohydrates Chemistry (Monosaccharides) By Dr Dina Aboraya Definition of Carbohydrates: ▪ Carbohydrates are organic substances composed of carbon, hydrogen and oxygen (C, H, O). ▪ Hydrogen and Oxygen are present in the same ratio as in water (H2O = 2:1), so called carbohydrates with the general formula (CH2O) n where n=3 or more.  Characteristics of Carbohydrates Carbohydrates are organic substances characterized by 3 features: 1- They are composed of three elements C, H & O 2- Presence of an aldehyde group or ketone group. If the sugar has an aldehyde group, it is classified as an aldose. If it has a ketone group, the sugar is classified as a ketose. 3- Presence of more than one hydroxyl group (- OH-) (polyhydroxy). The aldehyde group is always at the end of the carbon chain, and the ketone group is always on the second carbon of the chain  Classification of Carbohydrates On the basis of Number of structural units into: Monosaccharides Contain 1 sugar unit, as glucose, fructose. Disaccharides contain 2 sugar units as sucrose and lactose. Oligosaccharides Contains 3-10 sugar units as maltotriose and raffinose. Polysaccharides Contain more than 10 sugar units as starch and cellulose.  Monosaccharaides Definition: They are the simplest carbohydrate units which cannot by hydrolyzed into simpler units. The general formula of Monosaccharaides is CnH2nOn Monosaccharaides have the ending ose (glucose, galactose, etc…).  Classification of Monosaccharaides They are further classified according to : 1. Number of carbon atoms (n) in the sugar: triose, tetrose, pentose, and hexose. 2. Active group in the sugar: If its contain aldehyde group …. its called aldose, and if it contains an ketone group … its called ketose. Examples of monosaccharides 1. Trioses (3C) : Aldoses (Aldotriose) Ketoses (Ketotriose) e.g. glyceraldehyde e.g. dihydroxyacetone Examples of monosaccharides 2. Tetroses (4C) : Aldoses (Aldotetrose) Ketoses (Ketotetrose) e.g. Erythrose e.g. Erythrulose Examples of monosaccharides 3. Pentoses (5C) : Aldoses (Aldopentose) Ketoses (Ketopentose) e.g. Ribose , Arabinose and Xylose e.g. Ribulose and Xylulose  Examples of monosaccharides 4. Hexoses (6C) : Aldoses (Aldohexose) Ketoses (Ketohexose) e.g. Glucose, Mannose, Galactose e.g. Fructose ,Arabinose  Chiral carbon atom (Asymmetric carbon atom) Definition: A chiral carbon atom is a carbon atom in a molecule that attached to four different atoms or groups.  Chiral carbon atom (Asymmetric carbon atom) ▪ Glyceraldehyde contains one asymmetric carbon atom ▪ Glucose contains four asymmetric carbon atoms. ▪ All sugars contain asymmetric carbon atom except dihydroxy acetone. Carbohydrates properties Any compound having asymmetric carbon atom shows two properties: 1. Optical Activity. 2. Stereoisomerism.  Stereoisomers in carbohydrates Stereoisomers Definition : Compounds that have the same structural formula but differ in the spatial configuration i.e. they have the same number of C, H and O atoms but differ in the arrangement of groups and atoms in the space. The number of possible isomers of a compound depends on the number of asymmetric carbon atoms (n) and equal (2) n e.g. Trioses : 1 Asymmetric carbon = 2 1 = 2. Tetroses : 2 Asymmetric carbon atoms = 2 2 = 4. Pentoses : 3 Asymmetric carbon atoms = 2 3 = 8. Hexoses : 4 Asymmetric carbon atoms = 2 4 = 16. Heptoses : 5 Asymmetric carbon atoms = 2 5 = 32. N.B. The number of asymmetric carbon atoms : Aldoses= (Number of carbon atoms - 2) Ketoses= (Number of carbon atoms - 3) (ketoses is less than aldoses by one) e.g.: - Glucose has 4 asymmetric carbons (6 - 2=4) - Fructose has 3 asymmetric carbons (6 - 3=3) Number of isomers: e.g. Glucose has 4 asymmetric carbons …….. 2 4 =16 Fructose has 3 asymmetric carbons …….. 2 3 = 8  4- Epimers 1- D and L 2- Aldose and isomers ketose (enantiomers) 3- α and isomers (mirror image) β isomers (anomers)  1. Enantiomers (D and L isomers) (mirror image) Definition: They are isomers which are mirror images of each other e.g. D and L sugar. D and L refer to the configuration of the pre-last carbon (carbon atom before the last one, i.e. preterminal). If the OH attached to the pre- last C is directed to: ❑Right → D-sugar e.g. D-Glucose ❑Left → L-sugar e.g. L-Glucose.  1. Enantiomers (D and L isomers) (mirror image) N.B Most sugars in human are in D form except L-fucose, L-arabinose, L-xylulose and L-iduronic acid.  2- Aldose and ketose isomers Definition: They are isomers that differ in the position of the free active group carbon atom whether it is : an aldehyde group at C1 Or ketone group at C2 Eg. 1. Glyceraldehyde and Dihydroxyacetone 2. Erythrose and Erythrulose 3. Ribose and Ribulose 4. Glucose and Fructose  3- α and β isomers (anomers) They are isomers that differ in the position of OH group around the anomeric carbon. If OH group is attached below the ring plane (right) , it's α anomer. If OH group is attached above the ring plane (left) , it's β anomer e.g. α-glucopyranose , β-glucopyranose  4- Epimers They are isomers that differ in the position of one OH group at one asymmetric carbon atom only (epimeric carbon atom). e.g. D-glucose and D-mannose at C2 (C2 epimer) D-glucose and D-galactose at C4 (C4 epimer) SUGAR DERIVATIVES 1. Sugar Acids : Oxidations of monosaccharide 2. Sugar Alcohols : Reductions of monosaccharide 3. Deoxy Sugars : Replacement of OH group with H  1. Sugar Acids A- Aldonic acid Oxidation of first (C1) aldehyde group to carboxylic group. e.g.: ( C1 ) of Glucose → Gluconic acid N.B.: - Occur by mild oxidation - If the first aldehyde group is free  1. Sugar Acids B- Uronic acid Oxidation of the last hydroxyl group to carboxylic group e.g.: ( C6 ) of Glucose → Glucuronic acid N.B.: If the first aldehyde group is not free Importance of Glucuronic Acid: 1- Detoxification of toxic compounds by conjugation. 2- Formation of Mucopolysaccharides 3- Metabolism of bilirubin (conjugation of bilirubin in the liver to increase its solubility). 4- Excretion of steroids  1. Sugar Acids C- Aldaric acid Oxidation of both carbonyl carbon (C1) and Last hydroxyl carbon (C6) produces dicarboxylic acid or aldaric acid (saccharic acid; dicarboxylic acid) e.g. Glucose → Glucaric acid N.B.: Occur by strong oxidation  Sugar Acids Oxidation of the Oxidation of the Oxidation of both first first aldehyde last hydroxyl aldehyde group and last group. group. hydroxyl group. ALDONIC ACID URONIC ACID ALDARIC ACID D-glucose (by weak D-glucose (by D-glucose (by oxidizing agents) enzymes) strong oxidizing agents) D-gluconic acid D-glucuronic acid D-glucaric acid  2. Sugar Alcohol (Alditol) They are the products of Reduction of monosaccharide.  2. Sugar Alcohol (Alditol) Inositol (cyclitol) Its cyclic alcohol derived from glucose. A member of vitamin B complex. Function: 1) It is the main muscle sugar (myoinositol). 2) It enters in the formation of phosphatidyl inositol (phospholipid) which is a platelet activating factor and also needed as a second messenger in the mechanism of hormone action.  3. Deoxy Sugars Definition: They are monosaccharides with one hydroxyl group replaced by hydrogen, i.e. there is one oxygen missed. Examples: If the sugar is pentose (at carbon 2) e.g. ribose which gives deoxyribose (component of DNA). If the sugar is hexose (at carbons 6) e.g. L-galactose which gives L-fucose (component of glycoprotein). Biological Importance of Monosaccharaides Biological Importance of Monosaccharaides Galactose Biological Importance of Monosaccharaides Questions: Carbohydrates are Classified into …………. Enumerate different types of isomers of carbohydrates with mentioning examples. Mention the Importance of Glucuronic acid. Mention the Importance of ribose. Define Chiral Carbon atom.