Download 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
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.”
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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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4
D
and L Sugars
sugars can be degraded to the
dextrorotatory (+) form of glyceraldehyde.
L sugars can be degraded to the
levorotatory (-) form of glyceraldehyde.
• D
•
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The D Aldose Family
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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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Cyclic Structure for Glucose
Glucose cyclic hemiacetal formed by
reaction of -CHO with -OH on C5.
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D-glucopyranose
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Cyclic Structure for Fructose
Cyclic hemiacetal formed by reaction of
C=O at C2 with -OH at C5.
D-fructofuranose
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Anomers
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Mutarotation
Glucose also called
dextrose; dextrorotatory.
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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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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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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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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 - and -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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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.
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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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Kiliani-Fischer Synthesis
• This process lengthens the aldose chain.
• A mixture of C2 epimers is formed.
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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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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
HO
excess CH3I
Ag2O
CH2OHO
H H
HO
CH3O
OCH3
CH3O
H
H
+
CH3O
H H
CH3O
H
H3O
CH2OCH3
O
OH
OH
H
H
H
CH2OCH3
O
H H
CH3O
OH
CH3O
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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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.
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Maltose
Two glucose units linked 1-4’.
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Lactose
• Galactose + glucose linked 1-4’.
• “Milk sugar.”
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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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Cellulose
• Polymer of D-glucose, found in plants.
• Mammals lack the -glycosidase enzyme.
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Amylose
• Soluble starch, polymer of D-glucose.
• Starch-iodide complex, deep blue.
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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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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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Ribonucleosides
A -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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Structure of RNA
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43
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.
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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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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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End of Chapter 23
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