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Chapter 12 Carbohydrates

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Title: Chapter 12 Carbohydrates


1
Chapter 12 Carbohydrates
2
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.

3
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

4
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.

5
Monosaccharide
  • Monosaccharides with
  • three carbons trioses
  • Five carbons pentose
  • Six carbons hexose
  • And so on

6
Monosacharides
  • Figure 12.1 Glyceraldehyde, the simplest aldose,
    contains one stereocenter and exists as a pair of
    enantiomers.

7
Enantiomers
  • Enantiomers a molecule has a nonsuperimposable
    mirror image
  • Chiral molecule has four different groups

8
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)
9
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.

10
Monosacharides
  • L-monosaccharide the -OH is on the left in a
    Fischer projection.

11
Table 12.1
Table 20-1 p532
12
Table 12.2
Table 20-2 p532
13
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

14
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.

15
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.

16
Epimers
  • Diastereomers that differ in configuration at
    only on asymmetric center

17
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
18
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.

19
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.

20
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
21
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
22
Examples
  • Give structure of the cyclic hemiacetal formed by
  • 4-hydroxybutanal
  • 5-hydroxypentanal

23
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
24
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.

25
Chair Conformations
26
Examples
  • Which OH groups are in the axial position in
  • ß-D-mannopyranose
  • ß-D-idopyranose

27
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
28
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
29
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.

30
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.

31
Examples
  • Draw a Haworth projection and a chair
    conformation for methyl ?-D-mannopyranoside.
    Label the anomeric carbon and glycosidic bond

32
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

33
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.

34
Alditols
  • These three alditols are also common in the
    biological world. Note that only one of these is
    chiral.

Erythritol
D-Mannitol
Xylitol
35
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

36
Oxidation to Aldonic Acids
  • 2-Ketoses (e.g. D-fructose) are also reducing
    sugars.

37
Oxidation to Aldonic Acids
ß-D-Glucopyranose
D-Glucose
D-Gluconate an aldonic acid
38
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.

39
Formation of Phosphoric esters
40
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.

41
Sucrose
  • Table sugar, obtained from the juice of sugar
    cane and sugar beet.

?-1,2-Glycosidic bond
Sucrose
42
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
43
Maltose
  • From malt, the juice of sprouted barley and other
    cereal grains.

?-1,4-Glycosidic bond
Maltose
44
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.

45
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
46
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.

47
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.

48
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
49
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.

50
Example
  • Draw a chair conformation for a disaccharide in
    which two units of D-glucopyranose are joined by
    a ß -1,3-glycosidic bond

51
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.

52
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.

53
Heparin
  • Figure 12.5 The repeating pentasaccharide unit of
    heparin.

54
p546
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