Download 24.1 Introduction to Carbohydrates

24.1 Introduction to Carbohydrates
• Carbohydrates (sugars) are abundant in nature:
– They are high energy biomolecules.
– They provide structural rigidity for organisms (plants,
crustaceans, etc.).
– The polymer backbone on which DNA and RNA are
assembled contains sugars.
• The term, carbohydrate, evolved to describe the
formula for such molecules: Cx(H2O)x.
• Carbohydrates are NOT true hydrates. WHY?
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• Carbohydrates (sugars) are polyhydroxy aldehydes or
ketones.
– Consider glucose, which is made by plants:
– Describe the potential energy change that occurs during
glucose photosynthesis.
– Is glucose a polyhydroxy aldehyde or ketone?
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24.2 Classification of
Monosaccharides
• Saccharides have multiple chiral centers, and they are
often drawn as Fischer projections.
– Designate each chirality center in
glucose as either R or S.
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• Saccharides have multiple chiral centers, and they are
often drawn as Fischer projections.
• What does the suffix, “ose” mean?
• Define the following terms:
– Aldose and ketose
– Pentose and hexose
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• Glyceraldehyde is a monosaccharide with one chirality
center.
– Natural glyceraldehyde is dextrorotatory (D): it rotates plane
polarized light in the clockwise direction.
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24.2 Classification of
Monosaccharides
• Naturally occurring larger sugars can be broken down
into glyceraldehyde by degradation.
• Such sugars are often called D-sugars.
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24.2 Classification of
Monosaccharides
• Recall that dextrorotatory versus levorotatory rotation
cannot be predicted by the R or S configuration.
• Here, D no longer refers to
dextrorotatory. Rather it
refers to the R configuration
at the chiral carbon farthest
from the carbonyl.
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24.3 Configuration of Aldoses
• There are four aldotetroses. Two are shown below.
• What are the other two structures?
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24.3 Configuration of Aldoses
• Aldopentoses have three chirality centers. The number
of isomers will be 23.
• Recall the 2n rule from Section 5.5.
• The D-sugars are naturally occurring.
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24.3 Configuration of Aldoses
• Ribose is a key building block of RNA.
– WHAT is RNA? More detail to come in Section 24.10.
• Arabinose is found in plants.
• Xylose is found in wood.
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24.3 Configuration of Aldoses
•
•
•
•
Based on the 2n rule, how many aldohexoses are there?
How many of the aldohexoses are D isomers.
Glucose is the most common aldohexose.
Mannose and galactose are also common.
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24.4 Configuration of Ketoses
• Relevant ketoses have between three and six carbons.
• For each naturally occurring D isomer, there is an L
enantiomer.
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24.4 Configuration of Ketoses
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24.5 Cyclic Structures of
Monosaccharides
• Recall from Section 20.5 that carbonyls can be attacked
by alcohols to form hemiacetals.
– The intramolecular reaction is generally favored for 5 and 6membered rings. WHY?
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24.5 Cyclic Structures of
Monosaccharides
• For the following compound, draw the mechanism and
resulting product that results from acid catalyzed ringclosing hemiacetal formation.
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24.5 Cyclic Structures of
Monosaccharides
• Monosaccharides, like glucose, can also undergo ringclosing hemiacetal formation.
• The equilibrium greatly favors the closed form called
pyranose.
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• Distinguish between the α and β anomers.
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Anomeric effect
• Which would you predict to be more stable?
– Beta 67% , alpha 33%, open 0.01%
19
24.5 Cyclic Structures of
Monosaccharides
• Ketoses form both furanose (5-membered) and
pyranose (6-membered) rings:
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24.5 Cyclic Structures of
Monosaccharides
70% β
2% α
0.7%
23%-β
5%-α
• The equilibrium concentrations in water are above.
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24.5 Cyclic Structures of
Monosaccharides
• The furanose form takes part in most biochemical
reactions.
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24.6 Reactions of Monosaccharides
• Monosaccharides are generally soluble in water. WHY?
• To improve their solubility in organic solvents, the
hydroxyl groups can be acetylated.
• WHY is pyridine added to the reaction?
• How might acetylation help in purification efforts.
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24.6 Reactions of Monosaccharides
• Monosaccharides can also be converted to ethers via
the Williamson ether synthesis.
• Ether linkages are more robust than ester linkages.
WHY?
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24.6 Reactions of Monosaccharides
• When treated with an excess of an alcohol, the
hemiacetal equilibrium can be shifted to give an acetal.
• When a sugar is used, alpha and beta
glycosides are formed.
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24.6 Reactions of Monosaccharides
• The mechanism of glycoside formation is analogous to
the acetal formation mechanism.
• Only the anomeric hydroxyl group is replaced.
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24.6 Reactions of Monosaccharides
• The mechanism of glycoside formation is analogous to
the acetal formation mechanism.
• What factors would you consider when trying to predict
whether the alpha or beta anomer will be the major
product?
• Practice with CONCEPTUAL CHECKPOINTs 24.28 and
24.29.
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24.6 Reactions of Monosaccharides
• Under strongly basic conditions, glucose and mannose
interconvert.
• Mannose and glucose are epimers because they only
differ in the configuration of one carbon center.
• Practice with CONCEPTUAL CHECKPOINT 24.30.
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24.6 Reactions of Monosaccharides
• Monosaccharides can be reduced to ALDITOLs shifting
the equilibrium to the right. HOW?
– D-sorbitol or D-glucitol are sugar substitutes.
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Reducing sugars
• If the sugar has an –OH attached to the anomeric
carbon, then the sugar is a reducing sugar
• If it has –OR, then it is not a reducing sugar
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24.6 Reactions of Monosaccharides
• Practice with SKILLBUILDER 24.4.
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24.7 Disaccharides
• Disaccharides form when two sugars connect through a
glycosidic linkage.
– The 1  4 glycosidic linkage is
most common.
– The bottom ring is capable of
mutarotation at its anomeric
position.
– Because the anomeric position of the bottom ring is a
HEMIACETAL rather than an acetal, it is in equilibrium with the
open form. Thus, maltose is a reducing sugar.
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24.7 Disaccharides
• Cellobiose is similar to maltose. WHAT are the
differences?
• Will cellobiose be a reducing sugar?
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24.7 Disaccharides
• Lactose is another disaccharide.
• Some people have trouble digesting lactose.
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24.7 Disaccharides
• Sucrose (table sugar)
is also a disaccharide.
– Honey bees can
convert sucrose into a
mixture of sucrose,
fructose, and glucose.
– Fructose is very sweet.
• Sucrose is not a
reducing sugar. WHY?
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24.8 Polysaccharides
• Cellulose is a polysaccharide containing 7000–12000
glucose units connected through glycosidic bonds.
• How is cellulose capable of giving plants like trees their
rigidity and strength?
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24.8 Polysaccharides
• Starch is a major components of grains and other foods,
like potatoes.
• What is the difference between
molecules of starch and molecules
of cellulose?
• Starch is made of
amylose and
amylopectin.
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24.8 Polysaccharides
• Amylopectin
has some 16α-glycoside
branches.
• We can eat corn
and potatoes,
but not grass or
trees. WHY?
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24.9 Amino Sugars
• Amino sugars like glucosamine are
important biomolecules.
• Acetylated glucosamine can form an
important polysaccharide called chitin.
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24.9 Amino Sugars
• The carbonyl groups in chitin allow for even stronger Hbonding between neighboring chains.
• Chitin is used in insect and arthropod exoskeletons.
WHY?
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24.10 N-Glycosides
• N-glycosides can be formed when sugars are treated
with an amine and an acid catalyst.
• RNA and DNA incorporate important N-glycosides called
nucleosides.
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24.10 N-Glycosides
• Ribose forms ribonucleosides in RNA.
• Deoxyribose forms deoxyribonucleosides in DNA.
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24.10 N-Glycosides
• There are four different heterocyclic amines that attach
to deoxyribose molecules to form DNA nucleosides.
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24.10 N-Glycosides
• In DNA, the nucleosides are attached to phosphate
groups forming nucleotides.
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24.10 N-Glycosides
• The phosphate
groups of the
nucleotides are
connected together
to make the DNA
strand or
POLYNUCLEOTIDE.
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24.10 N-Glycosides
• The nucleotides in DNA can attract one another through
H-bonding of the DNA base pairs.
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24.10 N-Glycosides
• WHY does DNA
form a double
helix?
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24.10 N-Glycosides
• RNA is structurally different from DNA :
– The sugar in RNA is ribose. WHAT is the sugar in DNA?
– RNA contains uracil instead of thymine.
• RNA translates the information stored in DNA
into working molecules (proteins and enzymes).
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24.10 N-Glycosides
• RNA strands generally do
not form double helices like
DNA.
• RNA strands can fold into
many different shapes, and
some even act as catalysts
called ribozymes.
• It is possible that RNA
evolved self-replication as
an early step in the
evolution of life from small
molecules.
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