CHAPTER 22 CARBOHYDRATES

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CHAPTER 22 CARBOHYDRATES
22.1 INTRODUCTION
21.1A CLASSIFICATION OF CARBOHYDRATES
Carbodydrares polyhydroxy aldehydes and ketones
or substances that
hydrolyze to yield polyhydroxy aldehydes and
ketones.
Monosaccharides simple carbohydrates cannot be
hydrolyzed into
smaller simpler carbohydrates.
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Disaccharides on a molecular basis,
carbohydrates that undergo
hydrolysis to produce only two molecules of
monosaccharide.
Trisaccharides those carbohydrates that yield
three molecules of
monosaccharide.
Polysaccharide carbohydrates that yield a large
number of molecules
of monosaccharide (?10).
Disaccharides Trisaccharides and Polysaccharide
are easily Hydrolysis to monosaccharide .
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Carbohydrares are the most abundant organic
constitutes of plants. We encounter carbohydrates
at almost every turn of our daily life.
21.1B PHOTOSYNTHESIS AND CARBOHYDRATE
METABOLESM
Carbohydrates are synthesized in green plants by
photosynthesis
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Carbohydrates can be released energy when animals
or plants metabolize them to carbon dioxide and
water.
Much of the energy is conserved in ATP. Plants
and animals can use the energy of ATP to carry
out all of their energy-requiring process. When
the energy in ATP is used, a coupled reaction
takes place in which ATP is hydrolyzed
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22.2 MONOSACCHARIDES
22.2A CLASSIFICATION OF MONOSACCHARIDES

• Monosaccharides are classified according to
• The number of carbon atoms present in the
  molecular.
• whether they contain an aldehyde or keto group.

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These two classification are frequently combined.
For example
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22.2B D AND L DESIGNATIONS OF MONOSACCHARIDES
Glyceraldehyde exists two enantiomeric forms
which have the absolute configurations
()-Glyceraldehyde should be designated (R)-()-
Glyceraldehyde and (-)-Glyceraldehyde should be
designated (S)-(-)- Glyceraldehyde (section 5.5)
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Other system designated ()-Glyceraldehyde as
D-()- Glyceraldehyde and (-)-Glyceraldehyde as
L-(-)-Glyceraldehyde.
D and L designations are not necessarily related
to the optical rotations of the sugars to which
they are applied.
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22.2C ATRUCTURAL FORMULAS FOR MONOSACCHARIDES
Fisher projection formula horizontal lines
project out towards the reader and vertical
lines project behind the plane of the page.
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Open chair structure (1, 2, or 3) exists
equilibrium with two cyclic forms 4 and 5 or 6
and 7.
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The cyclic forms of D-()-Glucose are hemiacetals
formed by an intramolecular reaction of the OH
group at C-5 with the aldehyde group.
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Notes
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(1) These two cyclic forms are diastereomers that
differ only in the configuration of C-1.
(2) In carbohydrate chemistry diastereomers of
this type are called anomers, and the hemiacetal
carbon atom is called the anomeric Carbon atom
( 3) In the orientation shown the aanomer has the
OH down and the ßanomer has the OH up.
(4) The actual conformations of the rings are the
chair forms. In the ß anomer of D-glucose, all
of the large substituents, -OH, or CH2OH , are
equatorial. In the a anomer, the only bulky
axial substituent is the -OH at C-1
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22.3 MUTAROTATION
The optical rotations of aand ßforms are found to
be significantly different,but when an aqueous
solution of either form is allowed to stand, its
rotation changed.
Mutarotation the change in rotation towards an
equilibrium value.
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Ordinary D-()-glucose has the a configuration at
the anomeric carbon atom and that higher melting
form has the ßconfiguration.
The percentage of the a andßanomers present at
equilibrium.
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22.4 GLYCOSIDE FORMATION
When a small amount of gaseous hydrogen chloride
is passed into a solution of D-()-glucose in
methanol, the reaction as follows
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The mechanism for the formation of the methyl
glucosides
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In acidic solutions, however, glycosides undergo
hydrolysis to produce a sugar and alcohol
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22.5 REACTIONS OF MONOSACCHARIDES
Dissolving monosaccharides in aqueous base causes
them to undergo a series of keto-enol
tauomerizations that lead to isomerizastions.
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22.5A FORMATION OF ETHERS
A methyl glucoside can be converted to the
derivative by treating it with excess dimethyl
sulfate in aqueous sodium hydroxide.
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The methoxy groups at C-2,C-3,C-4 and C-6 atoms
are stable in dilute aqueous acid, but C-1is
different from the others because it is Part of
an acetal linkage.
Under dilute aqueous acid the methoxy group at
C-1 will hydrolyze
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The oxygen at C-5 dose not bear a methyl group
brcause it was originally a part of the cyclic
hemiacetal linkage of D-glucose
25.5B CONVERSION TO ESTERS
Under excess acetic anhydride and a weak base
monosaccharide converts all of the hydroxyl
groups to ester groups
If the reaction is carried out at a low
temperature, the reaction occurs
stereospecificallythe aanomer gives the
a-acetate and the ßanomer gives the ß-acetate.
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22.5C CONVERSION TO CYCLIC ACETALS AND KETALS
Aldehydes and ketones react with open-chain
1,2-diols to produce cyclic acetals and ketals.
If the 1,2-diol is attached to a ring, as in a
monosaccharide, formation of the cyclic acetal
or ketal occurs only when the vicinal hydroxyl
froups are cis to each other.
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This reaction can be used to protect certain
hydroxyl groups of a sugar while reactions are
carried out on other parts of the molecule.
22.6 OXIDATION REACTIONS OF MONOSACCHARIDES

• The most important oxidizing agents are
• Benedicts or Tollens reagent
• bromine water
• nitric acid
• periodic acid.

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Each of these reagents produces a different and
usually specific effect.
22.6A BENEDICTS OR TOLLENSREAGENTS
REDUCING SUGARS
Benedicts and Tollens reagent give positive
tests with aldoses and ketoses.
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Sugars that give positive tests with Tollensor
Benedicts solutions are known as reducing
sugars, and all carbohydrates that contain a
hemiacetal group or a hemoketal group give
positive tests.
Carbohydrates that contain only acetal or ketal
group do not give positive tests with Tollensor
Benedicts solution.
But neither of these reagents is useful as a
preparative reagent in carbohydrate oxidations.
Oxidations with both reagents take place in
alkaline solution, and in alkaline solutions
sugars undergo a complex series of reactions
that lead to isomerization.
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22.6B BROMINE WATER THE SYNTHESIS
OF ALDONIC ACIDS
Bromine water is a general reagent that
selectively oxidizes -CHO group to a COOH group.
Bromine water specifically oxidizes the ßanomer,
and the initial product that forms is a
daldonolactone.
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This compound may then hydrolyze to an aldonic
acid, and the aldonic acid may undergo a
subsequent ring closure to form a ?
aldonolactone.
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22.6C NITRIC ACID OXIDATIONALDARIC ACIDS
Dilute nitric acid oxidizes both the CHO group
and the terminal -CH2OH group of an aldose to
COOH groups.
It is not known whether a lactone is an
intermediate in the oxidation of an aldose to an
aldaric acid however, aldaric acids from
?andd-lactones readily
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The aldaric acid obtained from D-glucose is
called D-glucaric acid
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22.6D PERIODATE OXIDATIONS OXIDATIVE CLEAVAGE
OF POLYHYDROXY COMPOUNDS
Compounds that have hydroxyl groups on adjacent
atoms undergo oxidative cleavage when they are
treated with aqueous periodic acid.
Carbon-carbon bonds breaks and carbonyl
compounds produced.
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This reaction usually takes place in quantitative
yield. By measuring the number of molar
equivalents valuable that are consumed in the
reaction, information can often be gained.
1. Three CHOH groups gives one molar
equivalent of formiv acid and two equivalents
of formaldehyde.
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2. Oxidative cleavage also takes place when an
OH group is adjacent to the carbonyl group
of an aldehyde or ketone(but no that of an
acid or an ester).
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3. Periodic acid dose not cleave compounds in
which the hydroxyl groups are separated by an
intervening CH2 group, nor those in which a
hydroxyl group is adiacent to an ether or acetal
function.
22.7 REDUCTION OF MONOSACCHARIDESALDITOLS
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Aldoses( and ketoses) can be reduced with sodium
borohydride to compounds called alditols.
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22.8 REACTIONS OF MONOSACCHARIDES WITH
PHENYLHYDRAZINE OSAZONES
The aldehyde group of an aldose react with such
carbonyl reagents as hydroxylamine and
phenylhydrazine.
Osazone formation results in a loss of the
stereocenter at C-2 but dose not affect other
stereocenters.
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22.9 SYNTHESIS AND DEGRADATION OF
MONOSACCHARIDES
22.9A KILIANI-FISCHER SYNTHESIS
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Kiliani-fischer synthesis the method of
lengthening the carbon chain
of the an aldose.
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We can be sure that the aldotetroses that we
obtain from kiliani-fischer synthesis are both D
sugar because the starting compound is
D-glyceraldehyde and its stereocenter is
unaffected.
22.9B THE RUFF DEGRADATION
The Ruff degradation can be used to shorten the
chain by a similar unit.

• The Ruff degradation involves
• Oxidation of the aldose to an aldonic acid.
• Oxidative decarboxylation of the aldonic acid to
  the next lower
• aldose.

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22.10 THE D FAMILY OF ALDOSES
We can place all of the aldose into families or
family trees based on their relation to D- or
L-glyceraldehyde
Most, but not all, of the naturally occurring
aldose belong to the D family with D-(-)-glucose
being by far the most common.
22.11 FISCHERS PROOF OF THE CONFIGURATION OF
D-()-GLUCOSE
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Fischers assignment was based on the following
reasoning.

• Nitric acid oxidation of ()-glucose gives an
  optically active
• aldaric acid.
• (2) Degradation of ()-glucose gives
  (-)-arabinose, and nitric acid
• oxidation of (-)-arabinose gives an
  optically active aldaric acid.
• (3) A Kiliani-Fischer synthesis beginning with
  (-)-arabinose gives
• ()-glucose and ()-mannose nitric acid
  oxidation of
• ()-mannose gives an optically active
  aldaric acid.

(4) Fischer had already developed a method for
effectively interchanging the two end
groups(CHO and CH2OH) of an aldose chain.
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22.12 DISACCHARIDES
22.12A SUCROSE
Sucrose the most widely occurring disaccharide
of ordinary table sugar.
Structure
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The structure of sucrose is based on the
following evidence

• Sucrose has the molecular formula C12H22O11
• Acid-catalyzed hydrolysis of 1 mol of sucrose
  yields 1 mol
• of D-glucose and 1 mol of D-frutose.
• Sucrose is a nonreducing sugar. Neither the
  glucose not the
• fructose portion of sucrose has a
  hemiacetal or hemiketal
• group, thus the two hexoses must have a
  glycoside linkage
• that involves C-1of glucose and C-2 of
  fructose.
• The hydrolysis of sucrose indicates an a
  configuration at the
• glucoside portion and an enzyme known to
  hydrolyze a
• ß-fructofuranosides.
• Methylation of sucrose gives an octamethyl
  derivative that,
• on hydrolysis, gives 2,3,4,6-tetra-O-methyl
  -D-glucose and
• 1,3,4,6-tetra-O-methyl-D-fructose.

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22.12B MATOSE
Structure
or
Notes
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• When 1 mol of maltose is subjected to
  acid-catalyzed hydrolysis,
• it yield 2 mol of D-()-glucose.
• Maltose is a reducing sugar.
• Maltose exists in two anomeric forms
  a-()-maltose,
• , and
  ß-()-maltose,

• Maltose reacts with bromine water to form a
  monocarboxylic
• acid, maltose acid.
• Methylation of maltose acid followed by
  hydrolysis gives
• 2,3,4,6-tetra-O-methyl-D-glucose and
  2,3,5,6-tetra-O-methyl-D-
• gluconic acid.
• Methylation of maltose itself, followed by
  hydrolysis, gives
• 2,3,4,6-tetra-O-methyl-D-glucose and
  2,3,4,6-tri-O-methyl-
• D-glucose.

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22.12C CELLOBIOSE
Structure
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or
Notes

• Cellobiose is a reducing sugar.
• Cellobiose also undergoes mutarotation and forms
  a
• phenylosazone.
• Cellobiose is hydrolyzed by ß-glucosidases. This
  is indicate
• that the glycosidic linkage in cellobiose
  is ß.

22.12D LACTOSE
Lactose is a reducing sugar that hydrolyzes to
yield D-glucose and D-galactose the glycosidic
linkage is ß.
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Structure
or
22.13 POLYSACCHARIDES
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Homopolysaccharides polysaccharides that are
polymers of a single
monosaccharide.
Heteropolysaccharides those made up of more than
one type of
monosaccharide.
Glucan a homopolysaccharide consisting of
glucose monomeric units.
Galactan a homopolysaccharide consisting of
galactose units
Three important polysaccharides, all of which are
glucans, glycogen, starch and cellulose.
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22.13A STARCH
Heating starch with water produce amylose
(10-20)and amylopectin(80-90).
Structure of amylose
In amylopectin the chains are branched. Branching
takes place between C-6 and C-1at intervals of
20-25 glucose units.
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Partical structure of amylopectin
The molecular weight is about 1-6 milion, include
hundreds of interconnecting chains of 20-25
glucose units.
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22.13B GLYCOGEN
In glycogen the chain are much more highly
branched and the molecular weights as high as
100 million.

• The size and structure of glycogen suits its
  function
• Its size makes it too large to across cell
  membranes.
• The structure of glycogen solves the enormous of
  osmotic
• pressure within the cell.
• (3) The high branch structure of glycogen
  simplify the cells
• logistical problems.

Glucose (from glycogen) is highly water soluble
and as an ideal Source of ready energy.
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22.13C CELLULOSE
A portion of cellulose structure
Special property
The outside OH groups are ideally situated to
zip the chains make together by forming
hydrogen bonds.
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Zipping many cellulose chains together in this
way gives a highly insoluble.
22.13D CELLULOSE DERIVATIVES
Most of the cellulose derivatives include two or
three free hydroxyl groups of each glucose unit
which have been converted to an eater or an
ether.
Rayon is made by treating cellulose with carbon
disulfide in base solution.
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The solution of cellulose xanthate is then passed
through a small Orifice or slit into an acidic
solution.
22.14 OTHER BIOLOGICALLY IMPORTANT SUGARS
Uronic acids monosaccharide derivatives in which
the CH2OH group at C-6 has been specifically
oxidized to a carboxyl group.
For example
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Deoxy sugars monosaccharides in which an OH
group has been replaced by H.
22.15 SUGARS THAT CONTAIN NITROGEN
22.15A GLYCOSYLAMINES
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Glycosylamine sugars in which an amino group
replaces the anomeric
OH. For example
Nucleoside glycosylamines in which the amino
component is a pyrimidine
or a purine and in which the sugar
component is either D-ribose or
2-deoxy-D-ribose.
22.15B AMINO SUGARS
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Amino sugar a sugar in which an amino group
replaces a nonanomeric
OH group.
D-glucosamine can be obtained by hydrolysis of
chitin. The repeating units in chitin is
N-acetylglucosamine and the glycosidic linkages
are ß, 14. The structure of chitin is smaller
than that of cellulose.
D-glucosamine can also be isolated from heparin.
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22.16 GLYCOLIIPIDS AND GLYCOPROTEINS OF THE
CELL SURFACE
Glycolipids the carbohydrates joined through
gltcosidic linkages to
lipids.
Glycoproteins the carbohydrates joined through
gltcosidic linkages to
proteins.
Glycolipids and glycoproteins on the cells are
known to be the agents by which cells interact
with other cells and with invading bacteria and
viruses.
The A,B and O blood types are determined,
respectively, by the A, B and H determinants on
the blood cell surface.
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The A,B and H antigens differ only in the
monodacchride units at their nonreducing ends.
Type A antigens carry anti-B antibodies in their
serum type B antigens carry anti-A antibodies
in their serum type AB cells have both A and B
antigens but have neither anti-A nor
anti-B antigens type O cells have neither A nor
B antigens but have both anti-A and anti-B
antigens.
22.17 CARBOHYDRATE ANTIBIOTICS
Streptomycin isolation of the carbohydrate
antibiotic.
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Streptomycin is made up of the following three
subunits
Other members of this family are antibiotics
called kanamycins, neomycins, and gentamicins.
All are based on an amino cyclitol linked to one
or more amino augars. The glycosidic linkage is
nearly always a.