Chapter 23 Carbohydrates and Nucleic Acids

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.”

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1 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. =>

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Monosaccharides

• Classiﬁed by:  aldose or ketose  number of carbons in chain  conﬁguration of chiral carbon farthest from the carbonyl group

glucose, a D-aldohexose

fructose, a D-ketohexose =>

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2 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.

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The D-Aldose Family

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3 Erythro and Threo

• Terms used for diastereomers with two adjacent chiral C’s, without symmetric ends. • For symmetric molecules, use meso or d,l.

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Epimers

Sugars that differ only in their stereochemistry at a single carbon.

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4 Cyclic Structure for Glucose

Glucose cyclic hemiacetal formed by reaction of -CHO with - OH on C5.

=> D-glucopyranose Chap 23 Carbohydrates and Nucleic Acids Slide 23-9

Cyclic Structure for Fructose

Cyclic hemiacetal formed by reaction of C=O at C2 with -OH at C5.

D-fructofuranose

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5 Anomers

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Mutarotation

Glucose also called dextrose; dextrorotatory. =>

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6 Epimerization

In base, H on C2 may be removed to form enolate ion. Reprotonation may change the stereochemistry of C2.

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Enediol Rearrangement

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

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7 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. =>

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Oxidation by Bromine

Bromine water oxidizes aldehyde, but not ketone or alcohol; forms aldonic acid.

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8 Oxidation by Nitric Acid

Nitric acid oxidizes the aldehyde and the terminal alcohol; forms aldaric acid.

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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.

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9 Nonreducing Sugars

• Glycosides are acetals, stable in base, so they do not react with Tollens reagent. • Disaccharides and polysaccharides are also acetals, nonreducing sugars.

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Formation of Glycosides

• React the sugar with alcohol in acid. • Since the open chain sugar is in equilibrium with its α- and β-hemiacetal, both anomers of the acetal are formed. • Aglycone is the term used for the group bonded to the anomeric carbon.

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10 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.

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Ester Formation

Acetic anhydride with pyridine catalyst converts all the oxygens to acetate esters.

=> Chap 23 Carbohydrates and Nucleic Acids Slide 23-22

11 Osazone Formation

Both C1 and C2 react with phenylhydrazine.

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Ruff Degradation

Aldose chain is shortened by oxidizing the aldehyde to -COOH, then decarboxylation.

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12 Kiliani-Fischer Synthesis

• This process lengthens the aldose chain. • A mixture of C2 epimers is formed.

=> Chap 23 Carbohydrates and Nucleic Acids Slide 23-25

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! =>

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13 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.

H excess CH3I H + H CH OH H O CH OCH 2 O CH2OCH3 3 2 3 HO Ag2O O O CH3O CH3O H H H OH H H H HO OCH3 CH O OH OH CH3O 3 CH O CH3O H H 3 H H H H

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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.

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14 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. =>

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Cellobiose

• Two glucose units linked 1-4’. • Disaccharide of cellulose. • A mutarotating, reducing sugar.

=> Chap 23 Carbohydrates and Nucleic Acids Slide 23-30

15 Maltose

Two glucose units linked 1-4’.

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Lactose

• Galactose + glucose linked 1-4’. • “Milk sugar.”

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16 Gentiobiose

• Two glucose units linked 1-6’. • Rare for disaccharides, but commonly seen as branch point in carbohydrates.

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Sucrose

• Glucose + fructose, linked 1-1’ • Nonreducing sugar

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17 Cellulose

• Polymer of D-glucose, found in plants. • Mammals lack the β-glycosidase enzyme.

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Amylose

• Soluble starch, polymer of D-glucose. • Starch-iodide complex, deep blue.

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18 Amylopectin

Branched, insoluble fraction of starch.

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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.

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19 Chitin

• Polymer of N-acetylglucosamine. • Exoskeleton of insects.

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Nucleic Acids

• Polymer of ribofuranoside rings linked by phosphate ester groups. • Each ribose is bonded to a base. • Ribonucleic acid (RNA) • Deoxyribonucleic acid (DNA)

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20 Ribonucleosides

A β-D-ribofuranoside bonded to a heterocyclic base at the anomeric carbon.

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Ribonucleotides

Add phosphate at 5’ carbon.

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21 Structure of RNA

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Structure of DNA

• β-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.

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22 Base Pairings

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Double Helix of DNA

• Two complementary polynucleotide chains are coiled into a helix. • Described by Watson and Crick, 1953.

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23 DNA Replication

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Additional Nucleotides

• Adenosine monophosphate (AMP), a regulatory hormone. • Nicotinamide adenine dinucleotide (NAD), a coenzyme. • Adenosine triphosphate (ATP), an energy source.

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24 End of Chapter 23

Homework: 54, 56-58, 63, 65, 68, 69, 73, 76, 77

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