Bioorganic chemistry Prepared by Dr. Khalid Shadid

Bioorganic chemistry Prepared by Dr. Khalid Shadid
6/17/2018 Carbohydrates Created By Prof. Gary F. Porter, Ph.D. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.
1. Introduction 1a. Classification of Carbohydrates Carbohydrates Are polyhydroxyl aldehydes and ketones in the monomeric form and Exist in hemiacetal and acetal forms. Sugars are Monosaccharides - 1 monomeric unit Disaccharides - 2 monomeric units) Oligosaccharides - 3 to 10 monomeric units Polysaccharides - more than 10 monomeric units Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

6/17/2018 1. Introduction 1B. Photosynthesis of Carbohydrate Metabolism Plants use photosynthesis (sunlight) to produce energy storing compounds (sugars). Plants use the energy storing compounds to drive metabolism. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

1B. Photosynthesis of Carbohydrate Metabolism
1. Introduction 1B. Photosynthesis of Carbohydrate Metabolism The energy produced during the breakdown of carbohydrates is captured in the form of ATP. ATP is a high energy compound whose hydrolysis exothermically drives reactions. ATP + H2O ADP + Pi + energy Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

2. Monosaccharides 2A. Classification of Monosaccharides Aldose - contains an aldehyde group Ketose - contains a ketone group Triose - contains 3 carbons Tetrose - contains 4 carbons Aldopentose - contains an aldehyde & 5 carbons Ketohexose - contains a ketone & 6 carbons Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

2A. Classification of Monosaccharides
Practice Problem 22.1 How many chirality centers are contained in (a) the aldotetrose and (b) the ketopentose above? (c) How many stereoisomers would you expect from each general structure? Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

2. Monosaccharides 2B. D- and L- Designations of Monosaccharides Two enantiomeric forms of glyceraldehyde exist. We designate D- and L- for these trioses. D-Form L-Form Note: These designations give no indication of their dexo- vs. levorotatory nature. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

2B. D- and L- Designations of Monosaccharides
Practice Problem 22.2 Identify a three-dimensional formula for the above and designate each as a D- or L- sugar. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

2C. Structural Formulas for Monosaccharides
Fischer projections are useful for monosaccharide analysis. Note the two orientations of the blue oxygens below. Haworth formulas are useful because monosaccharides exist in the cyclic form. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

The chair conformation of a cyclohexane ring has two distinct orientations for the bonds that project from the ring: axial and equatorial Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

These diastereomers Differ only at the hemiacetal carbon, and Are the α and β anomers of D-glucose. Ketoses form cyclic acetals with the same two anomeric forms. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

3. Mutarotation Negative test with Schiff’s reagent (for aldehyde) No observable of UV and IR absorption band for carbonyl group When a sugar is dissolved in water, the specific rotation of the solution gradually changes until it reaches a constant value due to equilibrium between a and b forms (b form is more positive value) e.g. freshly prepared solution of b -glucose has a specific rotation +112o. When this solution is allowed to stand the rotation falls till reach o. The equilibrium reached is: 36% a –D- glucose [a]D = 64% b –D- glucose [a]D = The mean is o The α and β anomers are in equilibrium. The value of that equilibrium is the mutarotation value. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

6/17/2018 3. Mutarotation This equilibrium favors the more stable β anomer. This shift in equilibrium is called the anomeric effect. Note: The open-chair form is negligible at equilibrium. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

6/17/2018 Monosaccharides In solution mainly in pyranoside form (76%) In nucleotides (or other combinations) mostly found in furanose form Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

4. Glycosidic Formation (carbohydrate acetals called Glycosides) (acetals of Glucose called Glucosides) Formation of glycosidic bonds are Acid-catalyzed and Include the addition of methanol. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Reactions of Monosaccharides Action of Acids: General Test for carbohydrates: Treatment with conc. mineral acid (HCl or H2SO4) undergoes dehydration to give furfural or furfural derivatives which on condensation with a- Naphtol yield a violet or purple colored complex . Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

5. Other Reactions of Monosaccharides Monosaccharides + aqueous base >>>>> enolizations and keto–enol tautomerizations =isomerizations. D-glucose + CaOH, D-fructose and D-mannose Lobry de Bruyn–Alberda van Ekenstein transformation stand for several days Enolization, Tautomerization, and Isomerization Base facilitates isomerization of monosaccharides. Enolization creates epimers. D-Glucose to D-Mannose Tautomerization creates structural isomers. D-Glucose to D-Fructose Note: Formation of a glysoside prevents enolization. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Enolization, Tautomerization, and Isomerization Tautomerization creates structural isomers D-Glucose to D- Fructose In base, the position of the C=O can shift. Hence use acidic or neutral solutions of sugars to preserve their identity. Note: Formation of a glycoside prevents enolization Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

5.Other Reactions of Monosaccharides Formation of Ethers Convert all -OH groups to -OR, using a modified Williamson synthesis (base+alkyl halide), after converting sugar to acetal, stable in base. Benzyl ethers are commonly used to protect hydroxyl groups in sugars. Benzyl halides are easily introduced because they are highly reactive in SN2 reactions. Sodium hydride (NaH) used as the base in an aprotic solvent such as DMF or DMSO. The benzyl groups can later be easily removed by hydrogenolysis using a palladium catalyst. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Other Reactions of Monosaccharides Conversions to Esters Treating a monosaccharide with excess acetic anhydride and a weak base (such as pyridine or sodium acetate) converts all of the hydroxyl groups, including the anomeric hydroxyl, to ester groups. If the reaction is carried out at a low temperature (e.g., 0°C), the reaction occurs stereospecifically; the a anomer gives the a-acetate and the b anomer gives the b-acetate. Acetate esters are common protecting groups for carbohydrate hydroxyls Base-catalyzed with pyridine or sodium acetate Treat with excess acetic anhydride Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

6. Oxidation Reactions of Monosaccharides Benedict’s Reagent: Reducing Sugars Aldoses & Ketose are reducing sugars. Methylated Aldoses & Ketoses are non-reducing sugars. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

22.6 Oxidation Reactions of Monosaccharides
Benedict’s Reagent: Reducing Sugars Gives a positive test for Aldoses & Ketoses, and Gives a negative test for Non-reducing sugars. 22.6 Oxidation Reactions of Monosaccharides Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

6. Oxidation Reactions of Monosaccharides Tollens’ Reagent: Reducing Sugars (Aldoses vs. Ketoses) Gives a positive test for Aldoses, and Gives a negative test for Ketoses. Aldehyde + Ag+(NH3)2HO- Carboxylate + Ag(↓) Clear Soln. Mirrored Silver Surface Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

6. Oxidation Reactions of Monosaccharides Practice Problem 22.6 How might you distinguish between α-D-glucopyranose (i.e., D-glucose), methyl α-D-glucopyranoside, and α-D-fructofuranose. Sugars Benedict's Test Tollens’ Test α-D-glucopyranose Pos. or Neg. α-D-fructofuranose Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

6. Oxidation Reactions of Monosaccharides Bromine water oxidizes aldehyde, but not ketone or alcohol. 6B. Bromine Water: The Synthesis of Aldonic Acid Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

6. Oxidation Reactions of Monosaccharides 6C. Nitric Acid Oxidation: Aldaric Acids Oxidizes –CHO and –CH2OH groups (terminal alcohol), and Forms –COOH groups. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

7. Reduction of Monosaccharides: Alditols The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2/M Reduction with NaBH4 - Practice Problem 22.11 (a) Would you expect D-glucitol to be optically active? (b) Write Fischer projection formulas for all of the D-aldohexoses that would yield optically inactive alditols. -Name the sugar alcohol by adding -itol to the root name of the sugar. -Reduction of D-glucose produces D-glucitol, commonly called D-sorbitol. -Reduction of D-fructose produces a mixture of D-glucitol and D-mannitol. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

8. Reactions of Monosaccharides with Phenylhydrazine: Osazone Osazone The product is called a phenylasozone. Substitutions occur on C-1 and C-2. 1 2 Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

8. Reactions of Monosaccharides with Phenylhydrazine: Osazone Epimers Differ in chirality at a single carbon. i.e., D-Glucose & D-Mannose Osazone Formation Results in loss of chirality at C2. C6H5NHNH2 This experiment, first done by Emil Fischer, established that D-glucose and D-mannose have the same configurations about C3, C4, and C5. Diastereomeric aldoses that differ in configuration at only one carbon (such as D-glucose and D-mannose) are called epimers. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Synthesis and Degradation of Monosaccharides 9B. The Ruff Degradation Reduces the carbon chain by one carbon, and Is an oxidative decarboxylation. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Disaccharides Bioorganic chemistry Prepared by Dr. Khalid Shadid 6/17/2018 12A. Sucrose Is ordinary table sugar, and Is found in photosynthetic plants Glucose (as an α-pyranose) a glycosidic linkage C1 of glucose and C2 of fructose, Sucrose is a nonreducing sugar; it gives negative tests with Benedict’s and Tollens’ solutions. Sucrose does not form an osazone and does not undergo mutarotation. (no hemiacetal group) both carbonyl groups present as full acetals (i.e., as glycosides). Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Disaccharides 12B. Maltose Is a hydrolysis product from starch -Maltose is a reducing sugar; it gives positive tests with Fehling’s, Benedict’s, and Tollens’ solutions. -Maltose also reacts with phenylhydrazine to form a monophenylosazone (i.e., it incorporates two molecules of phenylhydrazine). -Maltose exists in two anomeric forms: α-(+)-maltose, [α]D25 = + 168, and b -(+)- maltose, [α]D25 = The maltose anomers undergo Mutarotation to yield an equilibrium mixture, [α]D25 = + 136 Glucose (as an α-pyranose) Glucose (as an α-pyranose) Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Polysaccharides Polysaccharides are sugar units that are linked by glycosidic (acetal) bonds. Homopolysaccharides Are polymers with a single type of monosaccharide, With glucose monomeric units are glucans, and With galactose monomeric units are galactans. Heteropolysaccharides Are polymers containing more than one type of monosaccharide building block. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Polysaccharides Amylose typically consists of more than 1000 D-glucopyranoside units connected in a linkages between C1 of one unit and C4 of the next 13A. Starch Is found in plants, and Is an energy storage sugar form helical structures when α- linked as in starches, Partial structure of amylose Partial structure of amylopectin Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Polysaccharides 13B. Glycogen Is found in animals Is an energy storage sugar, and Has more α (1→6) branching than starch Partial structure of amylopectin Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Polysaccharides 13C. Cellulose Is found in plants, and Is a structural sugar rigid linear structures when b- linked as in cellulose. The digestive enzymes of humans cannot attack its linkages. Hence, cellulose cannot serve as a food source for humans, as can starch. Cows and termites, however, can use cellulose (of grass and wood) as a food source because symbiotic bacteria in their digestive systems furnish b- glucosidase enzymes. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

Other Biologically Important Sugars Ribose and 2-Deoxyribose are Found in RNA and DNA respectively. Uronic Acids found in sulfonated polysaccharide called heparin. Heparin is important in controlling blood clotting. specific oxidation of C6 of glucose to a carboxyl group converts glucose to glucuronic acid. In the same way, specific oxidation of C6 of galactose would yield galacturonic acid: Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

15. Sugars that Contain Nitrogen 15A. Glycosamine Anomeric hydrogens are replaced by nitrogen. Nucleoside has Ribose and 2-Deoxyribose bonded to a nitrogenous ring system. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

15. Sugars that Contain Nitrogen 15B. Amino sugars have non-anomeric hydrogens replaced by nitrogen. β-D-glucosamine is found in chitin. NAG & NAM is found in bacterial cell walls. Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

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