Carbohydrate Lecture 2 (University of Mosul, 2024-2025) PDF

University of Mosul College of Medicine Lecture: Lecture 2 Subject/year : Carbohydrate 2024-2025 Lecturer: Nashwan Sadeq Sulaiman Department: Biochemistry Date: 25/11/2024  Classification of stereoisomers The aim of this lecture is 1. to define the isomer 2. to define the Asymmetric...

University of Mosul College of Medicine Lecture: Lecture 2 Subject/year : Carbohydrate 2024-2025 Lecturer: Nashwan Sadeq Sulaiman Department: Biochemistry Date: 25/11/2024  Classification of stereoisomers The aim of this lecture is 1. to define the isomer 2. to define the Asymmetric (chiral)carbon atoms: 3. to classify the stereoisomer with examples of each type. 4. Enumerate the Chemical properties of monosaccharaides.  Intended learning outcomes: By the end of this lecture the student will be able to: 1. Predict , identify and distinguish between enantiomers and diastereoisomers 2. Fomulate a nomenclature of various stereoisomers 3. Select the optically active stereoisomers and describe its 4. Explain how the dehydration of sugars take place by acids 5. Indicate the enolization of monosaccharides by alkaline solution Isomers (Stereoisomers): Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H12O6. Asymmetric (chiral)carbon atoms: is a carbon that has four different groups or atoms attached to it. All the monosaccharaides except dihydroxyacetone contain one or more asymmetric or chiral carbon atoms. Dihydroxyacetone does not contain a chiral carbon ( it does not have a carbon atom that has four different groups attached to itself )  Question Which of the carbons in dihydroxyaciton is the chiral carbon? A. carbon number one B. carbon number two C. carbon number three D. dihydroxyacitone does not have any chiral carbon Types of stereoisomers are: 1. D and L isomer (enantiomer). 2. Functional Isomerism 3. Diastereomers 4. Epimers 5. Anomers 6. Optical Activity 1. D and L isomer (enantiomer). The orientation of- OH group around the carbon atom adjacent to the terminal primary alcohol carbon determines its D or L form.When the - OH group on this carbon is on the right, the sugar is a member of the D-series, when it is on the left, it is a member of the L-series. These D and L configuration are also called Enantiomers. Enantiomers: Stereoisomers that are not superimposable (mirror images) of each other, the configurations at all stereogenic centers are exactly opposite. All their chemical and physical properties are the same but the direction of optical rotation are opposite. L-Glucose is enantiomer of D-glucose because of having opposite configuration at all centers of chirality. 2. Functional isomers: have same molecular formulae but differ in their functional groups. For example, glucose and fructose have same molecular formulae C6H12O6, but glucose contains aldehyde as functional group and fructose contains keto group. Hence, glucose and fructose are functional isomers. This type of functional isomerism is also called as aldose-ketose isomerism because aldose is an isomer of ketose and vice versa. 3. Diastereoisomers: are stereoisomers whose molecules are not mirror images of each other. In another definition diastereoismers are pairs of isomers that have opposite configurations at more than one of the chiral centers but are not mirror images of each other. for example galactose and mannose are diasteroisomer because they differ in the position of –OH groups at two carbon atoms (carbon 2 and carbon 4) 4. Epimers : Are stereoisomers that differ in configuration of –OH group at only one center of chirality, they have the same configuration at all stereogenic centers except one. Glucose, galactose and mannoseare are examples for epimers. Galactose is an epimer of glucose because, configuration of hydroxyl group on 4th carbon atom of galactose is different from glucose. Similarly, mannose is an epimers of glucose because configuration of hydroxyl group on 2nd carbon atom of mannose is different from glucose 5. Anomers: they have identical configuration of OH group at every stereogenic Centre but they differ only in configuration at anomeric carbon atom. Because of cyclization of sugar , an additional asymmetric center created at anomeric carbon. This leads to formation of two isomers namely α and β. *The alpha anomer: Where- OH group is down *The beta anomer:Where- OH group is up  Question: Alpha D-glucopyranose and beta D-glucopyranose can be most accurately Described as what type of isomer? a. Functional isomer b. Enantiomers c. Epimers d. anomers 6. Optical activity 'When a beam of plane -polarized light is passed through a solution of an optical isomer, it will be rotated either to the right-or left in accordance to the type of compound and it is called optical activity' A compound which causes rotation of plane polarized light to the right is said to be dextrorotatory (+) isomer, and 'd' is used to designate the fact. Rotation of the plane polarized light to the left is called levorotatory isomer and designated by a (-) sign and 'l' is used to designate to the fact.  The rings can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl yielding two distinct configurations (α and β) Carbohydrates can change spontaneously between the configurations: a process known as mutarotation. Mutarotation :It is defined as the change in the specific optical rotation representing interconversion of α and β forms of D-Glucose to an equilibrium mixture. In an aqueous solution, an equilibrium mixture forms between the two anomers and the straight-chain structure of a monosaccharide in a process known as mutarotation. Chemical properties of monosaccharaides Some of the important chemical properties of monosaccharaides are: Furfural formation Enolization Oxidation Reduction Osazone formation Furfural formation by action of strong acids Sugars when treated with mineral acids like  Conc. Hydrochloric acid (HCL)  Conc. Sulfuric acid (H2SO4)  Conc. Nitric acid (HNO3) They undergoes dehydration loses three water molecules to form furfural derivatives This is the basis of the following reactions…… Molisch's test Seliwanoff's test Enolization / Tautomerization By action of alkalis When glucose is kept in alkaline solution for several hours it undergoes tautomerization to form D-Fructose and D-Mannose. This results in formation of a common intermediate –enediol The presses of shifting of hydrogen atom from one carbon atom to another to produce enediols is known as tautomerization. Enediols are good reducing agent and forms the basis of reducing property of sugars. Benedict҆s test Fehling҆s test Reference Chemical Basis of Life (David Fraser Harris) Principle of Biochemistry ( David L. Nelson Michael M. Cox)