Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 24 Carbohydrates Carbohydrates • Sugars and their derivatives are classified as carbohydrates – Examples: Glucose, Sucrose, Starch, Glycogen • Molecular formulas fit a hydrate of carbon pattern: Cn(H2O)m • Sucrose: C6H12O6 = C6(H2O)6 24.1 Properties and Classification of Carbohydrates 2 3 Monosaccharides • Simplest carbohydrates • Do not break down into other carbohydrates • Examples: glucose (dextrose), fructose, galactose, xylose, ribose • Usually colorless and water soluble • Cyclic and open chain versions 4 Classification of Monosaccharides • Classification by functional group – Either aldehydes or ketones • If ketone = ketose • If aldehyde = aldose 5 Classification of Monosaccharides • Classification by carbon chain length – Chains contain 3-8 carbons • Triose = 3 carbon sugar • Tetrose = 4 carbon sugar • Pentose = 5 carbon sugar • Hexose = 6 carbon sugar • Etc. 6 Classifying Monosaccharides • Functional group and chain length classifications can be combined • Examples: – Aldehyde + 5 carbons = aldopentose – Ketone + 6 carbons = ketohexose 7 Problems • Classify the following monosaccharides by both the number of carbons and functional group each contains. Glyceraldehyde Erythrulose Sedoheptulose 8 Fischer Projections • • • • • Convenient 2D representation of 3D carbohydrate molecules Carbon chain written vertically Most oxidized carbon toward top All bonds depicted horizontally and vertically Carbons are represented by crossing lines 9 • Vertical bonds go back • Horizontal bonds come forward 10 Manipulating Fischer Projections 1) A Fischer projection may be turned 180° in the plane of the paper 24.2 Fischer Projections 11 2) A Fischer projection may not be turned 90° in the plane of the page 3) A Fischer projetion may not be lifted from the plane of the paper and turned over. 12 4) A Fischer projection can be held steady while the groups at either end rotate in either a clockwise or a counterclockwise direction 13 5) An interchange of any two of the groups bound to an asymmetric carbon changes the configuration of that carbon 6) Meso compounds are a possibility • Will have a line of symmetry 24.2 Fischer Projections 14 Problems • Assign R or S stereochemistry to each chiral carbon 24.2 Fischer Projections 15 Fischer Projections – More Complex • Based on an eclipsed molecular conformation 16 Problem • Assign R or S stereochemistry to each chiral carbon in the following monosaccharide: 17 The D,L System • D-Glyceraldehyde rotates the plane of polarized light in a clockwise direction – Dextrarotatory (+ or D) • L-Glyceraldehyde rotates the plane of polarized light in a counterclockwise direction – Levorotatory (- or L) 18 • Almost all naturally occurring monosaccharides have the same R stereochemical configuration as D-glyceraldehyde at chiral center furthest from carbonyl group • When furthest chiral center has an OH drawn to the right, the sugar is D, when the chiral center has its OH drawn to the left, the sugar is L 19 • D and L notation have no relation to the direction in which a given sugar rotates plane-polarized light except for glyceraldehyde – D and L can be either dextrorotatory or levorotatory 20 Problems • Classify the following sugars as D or L 21 Cyclic Structures of the Monosaccharides • g- and d-hydroxy aldehydes exist predominantly as cyclic hemiacetals – 5 and 6 membered rings are very stable 24.3 Structures of the Monosaccharides 22 Fischer Projections Haworth Structures 23 Drawing Haworth Structures 1) Flip the sugar to the right 90° 2) Fold the chain into a hexagon (or pentagon) 24 3) Form the hemiacetals • 2 versions, α and β • Anomers 25 Problems • Draw the cyclic structures for the following sugars 26 27 Monosaccharide Anomers: Mutarotation • The two anomers of D-glucopyranose can be crystallized and purified • -D-glucopyranose melts at 146° and its specific rotation, []D = +112.2° • b-D-glucopyranose melts at 148–155°C with a specific rotation of []D = +18.7° • Rotation of solutions of either pure anomer slowly changes due to slow conversion of the pure anomers into a 37:63 equilibrium mixture of :b with a []D = +52.6° • called mutarotation 28 Conformational Representations of Pyranoses • Convert the Haworth form to a chair: 24.3 Structures of the Monosaccharides 29 Oxidation and Reduction of Carbohydrates • The aldehydes of aldoses may be reduced or selectively oxidized without impacting the other alcohols • Selective oxidation of the primary alcohol group may also be realized 24.8 Oxidation and Reduction Reactions of Carbohydrates 30 Common Oxidation and Reduction Products 24.8 Oxidation and Reduction Reactions of Carbohydrates 31 Disaccharides • Disaccharides consist of two monosaccharides 24.11 Disaccharides and Polysaccharides 32 Disaccharides • Note that the glycosidic linkage is an acetal and can be hydrolyzed with aqueous acid 24.11 Disaccharides and Polysaccharides 33 Disaccharides • C-1 of the glucose residue can be oxidized; however, C-1 of the galactose residue cannot • Reducing sugars: Carbohydrates that be oxidized (they reduce the oxidizing agent) 24.11 Disaccharides and Polysaccharides 34 Disaccharides • Another important disaccharide is (+)-sucrose • (+)-Sucrose is a nonreducing sugar as it cannot be oxidized with bromine water • It also cannot undergo mutarotation 24.11 Disaccharides and Polysaccharides 35 Polysaccharides • Sugars with many monosaccharide residues connected by glycosidic linkages are called polysaccharides • Cellulose is polymer of glucose 24.11 Disaccharides and Polysaccharides 36 Polysaccharides • Starch is a glucose polymer • It consists of two different types of glucose polymer 24.11 Disaccharides and Polysaccharides 37 Polysaccharides • Chitin is a polysaccharide that occurs widely in nature (e.g., shells of lobsters and crabs) 24.11 Disaccharides and Polysaccharides 38