Chapter 12 Carbohydrates

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Chapter 12 Carbohydrates
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Carbohydrates

• Carbohydrate A polyhydroxyaldehyde or
  polyhydroxyketone, or a substance that gives
  these compounds on hydrolysis.
• Monosaccharide A carbohydrate that cannot be
  hydrolyzed to a simpler carbohydrate.
• Monosaccharides have the general formula CnH2nOn,
  where n varies from 3 to 8.
• Aldose A monosaccharide containing an aldehyde
  group.
• Ketose A monosaccharide containing a ketone
  group.

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Monosaccharides

• The suffix -ose indicates that a molecule is a
  carbohydrate.
• The prefixes tri-, tetra, penta, and so forth
  indicate the number of carbon atoms in the chain.
• Those containing an aldehyde group are classified
  as aldoses.
• Those containing a ketone group are classified as
  ketoses.
• There are only two trioses

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Monosaccharides

• There are only two trioses
• Often aldo- and keto- are omitted and these
  compounds are referred to simply as trioses.
• Although triose does not tell the nature of the
  carbonyl group, it at least tells the number of
  carbons.

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Monosaccharide

• Monosaccharides with
• three carbons trioses
• Five carbons pentose
• Six carbons hexose
• And so on

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Monosacharides

• Figure 12.1 Glyceraldehyde, the simplest aldose,
  contains one stereocenter and exists as a pair of
  enantiomers.

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Enantiomers

• Enantiomers a molecule has a nonsuperimposable
  mirror image
• Chiral molecule has four different groups

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Monosaccharides

• Fischer projection A two-dimensional
  representation for showing the configuration of
  tetrahedral stereocenters.
• Horizontal lines represent bonds projecting
  forward from the stereocenter.
• Vertical lines represent bonds projecting to the
  rear.
• Only the stereocenter is in the plane.

(R)-Glyceraldehyde (3-D representation)
(R)-Glyceraldehyde (Fisher projection)
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Monosacharides

• In 1891, Emil Fischer made the arbitrary
  assignments of D- and L- to the enantiomers of
  glyceraldehyde.
• D-monosaccharide the OH is attached to the
  bottom-most assymetric center (the carbon that is
  second from the bottom) is on the right in a
  Fischer projection.

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Monosacharides

• L-monosaccharide the -OH is on the left in a
  Fischer projection.

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Table 12.1
Table 20-1 p532
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Table 12.2
Table 20-2 p532
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Examples

• Draw Fisher projections for all 2-ketopentoses.
  Which are D-2-ketopentoses, which are
  L-2-ketopentoses? Prefer to table 12.2 (your
  textbook) to write their names

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Amino Sugars

• Amino sugars contain an -NH2 group in place of
  an -OH group.
• Only three amino sugars are common in nature
  D-glucosamine, D-mannosamine, and
  D-galactosamine. N-acetyl-D-glucosamine is an
  acetylated derivative of D-glucosamine.

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Cyclic Structure

• Aldehydes and ketones react with alcohols to form
  hemiacetals
• Cyclic hemiacetals form readily when the hydroxyl
  and carbonyl groups are part of the same molecule
  and their interaction can form a five- or
  six-membered ring.

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Epimers

• Diastereomers that differ in configuration at
  only on asymmetric center

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Haworth Projections

• Figure 12.2 D-Glucose forms these two cyclic
  hemiacetals.

Same side
D-glucose
?-D-Glucopyranose ß-D-Glucose
?-D-Glucopyranose ?-D-glucose
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Haworth Projections

• A five- or six-membered cyclic hemiacetal is
  represented as a planar ring, lying roughly
  perpendicular to the plane of the paper.
• Groups bonded to the carbons of the ring then lie
  either above or below the plane of the ring.
• The new carbon stereocenter created in forming
  the cyclic structure is called the anomeric
  carbon.
• Stereoisomers that differ in configuration only
  at the anomeric carbon are called anomers.
• The anomeric carbon of an aldose is C-1 that of
  the most common ketose is C-2.

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Haworth Projections

• In the terminology of carbohydrate chemistry,
• A six-membered hemiacetal ring is called a
  pyranose, and a five-membered hemiacetal ring is
  called a furanose because these ring sizes
  correspond to the heterocyclic compounds furan
  and pyran.

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Haworth Projections

• Aldopentoses also form cyclic hemiacetals.
• The most prevalent forms of D-ribose and other
  pentoses in the biological world are furanoses.
• The prefix deoxy means without oxygen. at C2

?-D-Ribofuranose ?-D-Ribose
ß-2-Deoxy-D-ribofuranose ?-2-Deoxy-D-ribose
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Haworth Projections

• D-Fructose (a 2-ketohexose) also forms a
  five-membered cyclic hemiacetal.

?-D-Fructofuranose ?-D-Fructose
D-Fructose
ß-D-Fructofuranose ß-D-Fructose
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Examples

• Give structure of the cyclic hemiacetal formed by
• 4-hydroxybutanal
• 5-hydroxypentanal

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Chair Conformations

• For pyranoses, the six-membered ring is more
  accurately represented as a strain-free chair
  conformation.

ß-D-Glucopyranose
?-D-Glucopyranose
D-Glucose
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Chair Conformations

• In both Haworth projections and chair
  conformations, the orientations of groups on
  carbons 1- 5 of b-D-glucopyranose are up, down,
  up, down, and up.

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Chair Conformations
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Examples

• Which OH groups are in the axial position in
• ß-D-mannopyranose
• ß-D-idopyranose

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Mutarotation

• Mutarotation The change in specific rotation
  that accompanies the equilibration of a- and
  b-anomers in aqueous solution.
• Example When either a-D-glucose or b-D-glucose
  is dissolved in water, the specific rotation of
  the solution gradually changes to an equilibrium
  value of 52.7, which corresponds to 64 beta
  and 36 alpha forms.

ß-D-Glucopyranose
D-Glucose
ß-D-Glucopyranose
?-D-Glucopyranose
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Formation of Glycosides

• Treatment of a monosaccharide, all of which exist
  almost exclusively in cyclic hemiacetal forms,
  with an alcohol gives an acetal.

Glycosidic bond
Glycosidic bond
ß-D-Glucopyranose ß-D-Glucose
Methyl ß-D-glucopyranoside Methyl ß-D-glucoside
Methyl ?-D-glucopyranoside Methyl ?-D-glucoside
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Formation of Glycosides

• A cyclic acetal derived from a monosaccharide is
  called a glycoside.
• The bond from the anomeric carbon to the -OR
  group is called a glycosidic bond.
• Mutarotation is not possible for a glycoside
  because an acetal, unlike a hemiacetal, is not in
  equilibrium with the open-chain
  carbonyl-containing compound.

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Formation of Glycosides

• Glycosides are stable in water and aqueous base,
  but like other acetals, are hydrolyzed in aqueous
  acid to an alcohol and a monosaccharide.
• Glycosides are named by listing the alkyl or aryl
  group bonded to oxygen followed by the name of
  the carbohydrate in which the ending -e is
  replaced by -ide.

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Examples

• Draw a Haworth projection and a chair
  conformation for methyl ?-D-mannopyranoside.
  Label the anomeric carbon and glycosidic bond

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Reduction to Alditols

• The carbonyl group of a monosaccharide can be
  reduced to an hydroxyl group by a variety of
  reducing agents, including NaBH4 and H2 in the
  presence of a transition metal catalyst.
• The reduction product is called an alditol.
• Alditols are named by changing the suffix -ose to
  -itol

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Alditols

• The product formed when the CHO group of
  monosaccharide is reduced to CH2OH group

• Sorbitol is found in the plant world in many
  berries and in cherries, plums, pears, apples,
  seaweed, and algae.
• It is about 60 percent as sweet as sucrose (table
  sugar) and is used in the manufacture of candies
  and as a sugar substitute for diabetics.

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Alditols

• These three alditols are also common in the
  biological world. Note that only one of these is
  chiral.

Erythritol
D-Mannitol
Xylitol
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Oxidation to Aldonic Acids

• The aldehyde group of an aldose is oxidized under
  basic conditions to a carboxylate anion.
• The oxidation product is called an aldonic acid.
• A carbohydrate that reacts with an oxidizing
  agent to form an aldonic acid is classified as a
  reducing sugar (it reduces the oxidizing agent).
• Itself is being oxidized

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Oxidation to Aldonic Acids

• 2-Ketoses (e.g. D-fructose) are also reducing
  sugars.

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Oxidation to Aldonic Acids
ß-D-Glucopyranose
D-Glucose
D-Gluconate an aldonic acid
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Oxidation to Aldonic Acids

• The body uses glucuronic acid to detoxify foreign
  alcohols and phenols.
• These compounds are converted in the liver to
  glycosides of glucuronic acid and then excreted
  in the urine.
• The intravenous anesthetic propofol is converted
  to the following water-soluble glucuronide and
  excreted.

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Formation of Phosphoric esters
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What are Disaccharides and Oligosaccharides?

• Disaccharide A carbohydrate containing two
  monosaccharide units joined by a glycosidic bond
• Oligosaccharide A carbohydrate containing from
  six to ten monosaccharide units, each joined to
  the next by glycosidic bond
• Polysaccharide A carbohydrate consisting of
  large numbers of monosaccharide units joined by
  glycosidic bonds.

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Sucrose

• Table sugar, obtained from the juice of sugar
  cane and sugar beet.

?-1,2-Glycosidic bond
Sucrose
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Lactose

• The principle sugar present in milk.
• About 5 - 8 in human milk, 4 - 5 in cows milk.
• Has no sweetness

ß-1,4-Glycosidic bond
ß-1,4-Glycosidic bond
Lactose
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Maltose

• From malt, the juice of sprouted barley and other
  cereal grains.

?-1,4-Glycosidic bond
Maltose
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Polysaccharides

• Starch A polymer of D-glucose.
• Starch can be separated into amylose and
  amylopectin.
• Amylose is composed of unbranched chains of up to
  4000 D-glucose units joined by a-1,4-glycosidic
  bonds.
• Amylopectin contains chains up to 10,000
  D-glucose units also joined by a-1,4-glycosidic
  bonds at branch points, new chains of 24 to 30
  units are started by a-1,6-glycosidic bonds.

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Polysaccharides

• Figure 12.3 Amylopectin is a branched polymer of
  D-glucose units joined by a-1,4-glycosidic bonds.
  Branches consist of D-glucose units that start
  with an a-1,6-glycosidic bond.

?-1,6-Glycosidic bond
?-1,4-Glycosidic bonds
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Polysaccharides

• Glycogen is the energy-reserve carbohydrate for
  animals.
• Glycogen is a branched polysaccharide of
  approximately 106 glucose units joined by a-1,4-
  and a-1,6-glycosidic bonds.
• The total amount of glycogen in the body of a
  well-nourished adult human is about 350 g,
  divided almost equally between liver and muscle.

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Polysaccharides

• Cellulose is a linear polysaccharide of
  D-glucose units joined by b-1,4-glycosidic bonds.
• It has an average molecular weight of 400,000
  g/mol, corresponding to approximately 2200
  glucose units per molecule.
• Cellulose molecules act like stiff rods and align
  themselves side by side into well-organized
  water-insoluble fibers in which the OH groups
  form numerous intermolecular hydrogen bonds.
• This arrangement of parallel chains in bundles
  gives cellulose fibers their high mechanical
  strength.
• It is also the reason why cellulose is insoluble
  in water.

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Polysaccharides

• Figure 12.4 Cellulose is a linear polysaccharide
  of D-glucose units joined by b-1,4-glycosidic
  bonds.

ß-1,4-Glycosidic bonds
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Polysaccharides

• Cellulose (contd)
• Humans and other animals can not digest cellulose
  because their digestive systems do not contain
  b-glycosidases, enzymes that catalyze the
  hydrolysis of b-glycosidic bonds.
• Termites have such bacteria in their intestines
  and can use wood as their principal food.
• Ruminants (cud-chewing animals) and horses can
  also digest grasses and hay.
• Humans have only a-glucosidases hence, the
  polysaccharides we use as sources of glucose are
  starch and glycogen.
• Many bacteria and microorganisms have
  b-glucosidases.

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Example

• Draw a chair conformation for a disaccharide in
  which two units of D-glucopyranose are joined by
  a ß -1,3-glycosidic bond

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Acidic Polysaccharides

• Acidic polysaccharides a group of
  polysaccharides that contain carboxyl groups
  and/or sulfuric ester groups, and play important
  roles in the structure and function of connective
  tissues.
• There is no single general type of connective
  tissue.
• Rather, there are a large number of highly
  specialized forms, such as cartilage, bone,
  synovial fluid, skin, tendons, blood vessels,
  intervertebral disks, and cornea.
• Most connective tissues are made up of collagen,
  a structural protein, in combination with a
  variety of acidic polysaccharides.

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Acidic Polysaccharides

• Heparin (contd)
• Heparin is synthesized and stored in mast cells
  of various tissues, particularly the liver,
  lungs, and gut.
• The best known and understood of its biological
  functions is its anticoagulant activity.
• It binds strongly to antithrombin III, a plasma
  protein involved in terminating the clotting
  process.

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Heparin

• Figure 12.5 The repeating pentasaccharide unit of
  heparin.

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