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Carbohydrates

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Carbohydrate: a polyhydroxyaldehyde, a polyhydroxyketone, ... ribose. C. H. 2. O. H. C. H. O. O. H. O. H. H. H. C. H. 2. O. H. C. H. O. O. H. H. H. O. H. C. H. 2 ... – PowerPoint PPT presentation

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Title: Carbohydrates


1
Carbohydrates
Chapter 25
  • Chapter 25

2
Carbohydrates
  • Carbohydrate a polyhydroxyaldehyde, a
    polyhydroxyketone, or a compound that gives
    either of these compounds after hydrolysis.
  • Monosaccharide a carbohydrate that cannot be
    hydrolyzed to a simpler carbohydrate.
  • they 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
25.1 A. Monosaccharides
  • Monosaccharides are classified by their number of
    carbon atoms.

4
Monosaccharides
  • There are only two trioses.
  • Often the designations aldo- and keto- are
    omitted and these compounds are referred to
    simply as trioses, tetroses, and so forth.
  • although these designations do not tell the
    nature of the carbonyl group, they at least tell
    the number of carbons.

5
Monosaccharides
  • Glyceraldehyde contains a stereocenter and exists
    as a pair of enantiomers.

6
B. Fischer Projections
  • Fischer projection a two dimensional
    representation for showing the configuration of
    carbohydrates.
  • horizontal lines represent bonds projecting
    forward.
  • vertical lines represent bonds projecting to the
    rear.
  • the only atom in the plane of the paper is the
    stereocenter.

7
C. D,L Monosaccharides
  • In 1891, Emil Fischer made the arbitrary
    assignments of D- and L- to the enantiomers of
    glyceraldehyde.

8
D,L Monosaccharides
  • According to the conventions proposed by Fischer.
  • D-monosaccharide a monosaccharide that has the
    same configuration at its penultimate carbon as
    D-glyceraldehyde that is, its -OH is on the
    right when written as a Fischer projection.
  • L-monosaccharide a monosaccharide that has the
    same configuration at its penultimate carbon as
    L-glyceraldehyde that is, its -OH is on the left
    when written as a Fischer projection.

9
D,L Monosaccharides
  • Here are the two most abundant D-aldotetroses and
    the two most abundant D-aldopentoses in the
    biological world.

H
H
H
H
H
H
H
H
H
H
H
D-Erythrose
D-Threose
D-Ribose
10
D,L Monosaccharides
  • And the three most abundant hexoses.

H
H
C
O
H
H
H
H
H
H
H
H
H
11
D. Amino Sugars
  • Amino sugar a sugar that contains an -NH2 group
    in place of an -OH group.
  • only three amino sugars are common in nature.
  • N-acetyl-D-glucosamine is a derivative of
    D-glucosamine.

12
E. Physical Properties
  • Monosaccharides are colorless crystalline solids,
    very soluble in water, but only slightly soluble
    in ethanol.
  • sweetness relative to sucrose

13
25.2 Cyclic Structure
  • Monosaccharides have hydroxyl and carbonyl groups
    in the same molecule and exist almost entirely as
    five- and six-membered cyclic hemiacetals.
  • anomeric carbon the new stereocenter created as
    a result of cyclic hemiacetal formation.
  • anomers carbohydrates that differ in
    configuration at their anomeric carbons.

14
A. Haworth Projections
  • Haworth projections
  • five- and six-membered hemiacetals are
    represented as planar pentagons or hexagons, as
    the case may be, viewed through the edge.
  • they are most commonly written with the anomeric
    carbon on the right and the hemiacetal oxygen to
    the back right.
  • the designation ?- means that the -OH on the
    anomeric carbon is cis to the terminal -CH2OH
    ?- means that it is trans to the terminal -CH2OH.

15
Haworth Projections
  • six-membered hemiacetal rings are shown by the
    infix -pyran- so become pyranose.
  • five-membered hemiacetal rings are shown by the
    infix -furan- so become furanose.

O
O
Pyran
Furan
16
Haworth Projections
17
B. Conformational Formulas
  • five-membered rings are so close to being planar
    that Haworth projections are adequate to
    represent furanoses.

18
Conformational Formulas
  • other monosaccharides also form five-membered
    cyclic hemiacetals.
  • here are the five-membered cyclic hemiacetals of
    D-fructose.

19
Ascorbic Acid (Vitamin C)
  • L-Ascorbic acid (vitamin C) is synthesized both
    biochemically and industrially from D-glucose.

20
Ascorbic Acid (Vitamin C)
  • L-Ascorbic acid is very easily oxidized to
    L-dehydroascorbic acid. Both are physiologically
    active and are found in most body fluids.

H
H
O
O
oxidation
O
O
reduction
H
H
O
O
L-Dehydroascorbic acid
21
Conformational Formulas
  • for pyranoses, the six-membered ring is more
    accurately represented as a chair conformation.

22
Conformational Formulas
  • if you compare the orientations of groups on
    carbons 1-5 in the Haworth and chair projections
    of ?-D-glucopyranose, you will see that in each
    case they are up-down-up-down-up respectively.

23
C. Mutarotation
  • Mutarotation the change in specific rotation
    that occurs when an ? or ? form of a carbohydrate
    is converted to an equilibrium mixture of the two.

24
(No Transcript)
25
25.3 A. Glycosides
  • Glycoside a carbohydrate in which the -OH of
    the anomeric carbon is replaced by OR.
  • methyl ?-D-glucopyranoside (methyl ?-D-glucoside).

26
Glycosides
  • Glycosidic bond the bond from the anomeric
    carbon of the glycoside to an -OR group.
  • Glycosides are named by listing the name of the
    alkyl or aryl group bonded to oxygen followed by
    the name of the carbohydrate with the ending -e
    replaced by ide.
  • methyl ?-D-glucopyranoside.
  • methyl ?-D-ribofuranoside.

27
N-Glycosides
  • The anomeric carbon of a cyclic hemiacetal also
    undergoes reaction with the N-H group of an amine
    to form an N-glycoside.
  • N-glycosides of the following purine and
    pyrimidine bases are structural units of nucleic
    acids.

28
N-Glycosides
  • the b-N-glycoside formed between D-ribofuranose
    and cytosine.

29
B. 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/M.

30
Reduction to Alditols
  • other alditols common in the biological world are

31
C. Oxidation to Aldonic Acids
  • The -CHO group can be oxidized to COOH.

32
Oxidation to Aldonic Acids
  • 2-Ketoses are also oxidized to aldonic acids.
  • under the conditions of the oxidation, 2-ketoses
    equilibrate with isomeric aldoses. This
    equilibration occurs in base (NaOH).

An aldose
An enediol
A 2-ketose
33
D. Oxidation to Uronic Acids
  • Enzyme-catalyzed oxidation of the terminal -OH
    group gives a -COOH group and is labelled as a
    uronic acid.

H
H
O
H
H
H
H
H
H
34
Oxidation to Uronic Acids
  • in humans, D-glucuronic acid is an important
    component of the acidic polysaccharides of
    connective tissue.
  • it is also used by the body to detoxify foreign
    hydroxyl-containing compounds, such as phenols
    and alcohols one example is the intravenous
    anesthetic propofol.

35
E. Oxidation by HIO4
  • Periodic acid cleaves the C-C bond of a glycol.

C
O

I
C
A 1,2-diol
C
O
C
O
O

I
C
O
C
O
36
Oxidation by HIO4
  • it also cleaves ?-hydroxyketones.
  • and ?-hydroxyaldehydes.

37
Oxidation by HIO4
  • Oxidation of methyl ?-D-glucoside consumes 2
    moles of HIO4 and produces 1 mole of formic acid,
    which indicates 3 adjacent C-OH groups.

38
25.4 Disaccharides A. Sucrose
  • Table sugar, obtained from the juice of sugar
    cane and sugar beet.

39
Disaccharides B. Lactose
  • The principle sugar present in milk.
  • about 5 - 8 in human milk, 4 - 5 in cows milk.

40
Disaccharides C. Maltose
  • Disaccharide from starch or glycogen. From malt,
    the juice of sprouted barley and other cereal
    grains.

41
Disaccharides D. Cellibiose
  • Disaccharide from cellulose. Cellulose is the
    structural carbohydrate in plants.

4
O
O
O
1
(b )
42
25.5 Homopolysaccharides A. Starch
  • Starch is used for energy storage in plants.
  • it can be separated into two fractions amylose
    and amylopectin each on complete hydrolysis
    gives only D-glucose.
  • Amylose is composed of continuous, unbranched
    chains of up to 4000 D-glucose units joined by
    ?-1,4-glycosidic bonds.
  • amylopectin is a highly branched polymer of
    D-glucose chains consist of 24-30 units of
    D-glucose joined by ?-1,4-glycosidic bonds and
    branches created by ?-1,6-glycosidic bonds.

43
Homopolysaccharaides B. Glycogen
  • Glycogen is the reserve carbohydrate for animals.
  • like amylopectin, glycogen is a nonlinear polymer
    of D-glucose units joined by ?-1,4- and
    ?-1,6-glycosidic bonds bonds.
  • the total amount of glycogen in the body of a
    well-nourished adult is about 350 g (about 3/4 of
    a pound) divided almost equally between liver and
    muscle.

44
Homopolysaccharaides C. Cellulose
  • Cellulose is a linear polymer of D-glucose units
    joined by ?-1,4-glycosidic bonds.
  • it has an average molecular weight of 400,000
    g/mol, corresponding to approximately 2800
    D-glucose units per molecule.
  • both rayon and acetate rayon are made from
    chemically modified cellulose.

45
25.6 Heteropolysaccharides A. Acidic
  • Hyaluronic acid an acidic polysaccharide
    present in connective tissue, such as synovial
    fluid and vitreous humor.

46
B. Acidic Heteropolysaccharides
  • Heparin
  • its best understood function is as an
    anticoagulant.
  • following is a pentasaccharide unit of heparin.

47
Carbohydrates
End Chapter 25
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