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