Chapter 23 Carbohydrates and Nucleic Acids Organic Chemistry, 5th Edition L. G. Wade, Jr. Chapter 23 Carbohydrates and Nucleic Acids Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003, Prentice Hall

Carbohydrates Synthesized by plants using sunlight to convert CO2 and H2O to glucose and O2. Polymers include starch and cellulose. Starch is storage unit for solar energy. Most sugars have formula Cn(H2O)n, “hydrate of carbon.” => Chapter 23

Classification of Carbohydrates Monosaccharides or simple sugars polyhydroxyaldehydes or aldoses polyhydroxyketones or ketoses Disaccharides can be hydrolyzed to two monosaccharides. Polysaccharides hydrolyze to many monosaccharide units. E.g., starch and cellulose have > 1000 glucose units. => Chapter 23

Monosaccharides Classified by: aldose or ketose number of carbons in chain configuration of chiral carbon farthest from the carbonyl group fructose, a D-ketohexose => glucose, a D-aldohexose Chapter 23

D and L Sugars D sugars can be degraded to the dextrorotatory (+) form of glyceraldehyde. L sugars can be degraded to the levorotatory (-) form of glyceraldehyde. => Chapter 23

The D Aldose Family => Chapter 23

Erythro and Threo Terms used for diastereomers with two adjacent chiral C’s, without symmetric ends. For symmetric molecules, use meso or d,l. => Chapter 23

Epimers Sugars that differ only in their stereochemistry at a single carbon. => Chapter 23

Cyclic Structure for Glucose Glucose cyclic hemiacetal formed by reaction of -CHO with -OH on C5. => D-glucopyranose Chapter 23

Cyclic Structure for Fructose Cyclic hemiacetal formed by reaction of C=O at C2 with -OH at C5. => D-fructofuranose Chapter 23

Anomers => Chapter 23

Mutarotation Glucose also called dextrose; dextrorotatory. => Chapter 23

Epimerization In base, H on C2 may be removed to form enolate ion. Reprotonation may change the stereochemistry of C2. => Chapter 23

Enediol Rearrangement In base, the position of the C=O can shift. Chemists use acidic or neutral solutions of sugars to preserve their identity. => Chapter 23

Reduction of Simple Sugars C=O of aldoses or ketoses can be reduced to C-OH by NaBH4 or H2/Ni. 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. => Chapter 23

Oxidation by Bromine Bromine water oxidizes aldehyde, but not ketone or alcohol; forms aldonic acid. => Chapter 23

Oxidation by Nitric Acid Nitric acid oxidizes the aldehyde and the terminal alcohol; forms aldaric acid. => Chapter 23

Oxidation by Tollens Reagent Tollens reagent reacts with aldehyde, but the base promotes enediol rearrangements, so ketoses react too. Sugars that give a silver mirror with Tollens are called reducing sugars. => Chapter 23

Nonreducing Sugars Glycosides are acetals, stable in base, so they do not react with Tollens reagent. Disaccharides and polysaccharides are also acetals, nonreducing sugars. => Chapter 23

Formation of Glycosides React the sugar with alcohol in acid. Since the open chain sugar is in equilibrium with its - and -hemiacetal, both anomers of the acetal are formed. Aglycone is the term used for the group bonded to the anomeric carbon. => Chapter 23

Ether Formation Sugars are difficult to recrystallize from water because of their high solubility. Convert all -OH groups to -OR, using a modified Williamson synthesis, after converting sugar to acetal, stable in base. => Chapter 23

Ester Formation Acetic anhydride with pyridine catalyst converts all the oxygens to acetate esters. => Chapter 23

Osazone Formation Both C1 and C2 react with phenylhydrazine. => Chapter 23

Ruff Degradation Aldose chain is shortened by oxidizing the aldehyde to -COOH, then decarboxylation. => Chapter 23

Kiliani-Fischer Synthesis This process lengthens the aldose chain. A mixture of C2 epimers is formed. => Chapter 23

Fischer’s Proof Emil Fischer determined the configuration around each chiral carbon in D-glucose in 1891, using Ruff degradation and oxidation reactions. He assumed that the -OH is on the right in the Fischer projection for D-glyceraldehyde. This guess turned out to be correct! => Chapter 23

Determination of Ring Size Haworth determined the pyranose structure of glucose in 1926. The anomeric carbon can be found by methylation of the -OH’s, then hydrolysis. => Chapter 23

Periodic Acid Cleavage Periodic acid cleaves vicinal diols to give two carbonyl compounds. Separation and identification of the products determine the size of the ring. => Chapter 23

Disaccharides Three naturally occurring glycosidic linkages: 1-4’ link: The anomeric carbon is bonded to oxygen on C4 of second sugar. 1-6’ link: The anomeric carbon is bonded to oxygen on C6 of second sugar. 1-1’ link: The anomeric carbons of the two sugars are bonded through an oxygen. => Chapter 23

Cellobiose Two glucose units linked 1-4’. Disaccharide of cellulose. A mutarotating, reducing sugar. => Chapter 23

Maltose Two glucose units linked 1-4’. => Chapter 23

Lactose Galactose + glucose linked 1-4’. “Milk sugar.” => Chapter 23

Gentiobiose Two glucose units linked 1-6’. Rare for disaccharides, but commonly seen as branch point in carbohydrates. => Chapter 23

Sucrose Glucose + fructose, linked 1-1’ Nonreducing sugar => Chapter 23

Cellulose Polymer of D-glucose, found in plants. Mammals lack the -glycosidase enzyme. => Chapter 23

Amylose Soluble starch, polymer of D-glucose. Starch-iodide complex, deep blue. => Chapter 23

Amylopectin Branched, insoluble fraction of starch. => Chapter 23

Glycogen Glucose polymer, similar to amylopectin, but even more highly branched. Energy storage in muscle tissue and liver. The many branched ends provide a quick means of putting glucose into the blood. => Chapter 23

Chitin Polymer of N-acetylglucosamine. Exoskeleton of insects. => Chapter 23

=> Nucleic Acids Polymer of ribofuranoside rings linked by phosphate ester groups. Each ribose is bonded to a base. Ribonucleic acid (RNA) Deoxyribonucleic acid (DNA) Chapter 23

Ribonucleosides A -D-ribofuranoside bonded to a heterocyclic base at the anomeric carbon. => Chapter 23

Ribonucleotides Add phosphate at 5’ carbon. Chapter 23

Structure of RNA => Chapter 23

Structure of DNA -D-2-deoxyribofuranose is the sugar. Heterocyclic bases are cytosine, thymine (instead of uracil), adenine, and guanine. Linked by phosphate ester groups to form the primary structure. => Chapter 23

Base Pairings => Chapter 23

Double Helix of DNA => Two complementary polynucleotide chains are coiled into a helix. Described by Watson and Crick, 1953. Chapter 23

DNA Replication => Chapter 23

Additional Nucleotides Adenosine monophosphate (AMP), a regulatory hormone. Nicotinamide adenine dinucleotide (NAD), a coenzyme. Adenosine triphosphate (ATP), an energy source. => Chapter 23

End of Chapter 23 Chapter 23