Carbohydrates
Contents
Reactions of monosaccharides
Oxidation
Lactone
Reduction
Isomerization
Schiff base formation
Glycation
Esterification
Glycosidic bonds
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Reactions of Monosaccharides
Oxidation
Aldoses and ketoses were historically
referred to as reducing sugars because
they responded positively in a chemical
test that effected oxidation of their
aldehyde and hydroxyketone
functionalities, respectively.
 In the presence of oxidizing agents, metal
ions such as Cu2+ and certain enzymes,
monosaccharides readily undergo
oxidation reaction.

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Oxidation

Depending on the oxidizing agent used,
the terminal aldehyde (or keto) or
terminal alcohol or both the groups may
be oxidized.
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Aldonic Acid

The carboxylic acid formed by oxidation
of the aldehyde in an aldose is referred to
as a glyconic acid (e.g., gluconic acid is the
oxidation product of glucose).
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Uronic Acid
It is also possible to oxidize the hydroxy
groups of monosaccharides, most notably
the terminal OH group (i.e., C-6 of
glucose).
 In this reaction, a glycuronic acid is
produced

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Oxidation of both aldehyde and CH2OH
gives an aldaric acid.
 In case of glucose it is glycaric acid.

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Oxidation
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Lactone
The carbonyl groups in both aldonic and
uronic acids can react with an OH group
in the same molcule to form a cyclic ester
known as lactone.
 Lactones are commonly found in nature.
For example ascorbic aid (vit C) is a
lactone derivative of D-glucuronic acid.

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Oxidized forms of D-glucose
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Glucose reacts with oxidizing agents such
as cupric ion (Cu2+) (Benedict’s
reagent)because the open-chain form has
a free aldehyde group that is readily
oxidized.
 This reaction occurs only with sugars that
an revert to the open chain form, all
monosaccharides are reducing sugars.

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Dr. Nikhat Siddiqi
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Reactions of Momosaccharides
Reduction
Reduction

When treated with reducing agents such
as sodium amalgam, the aldehyde or keto
group of monosaccharide is reduced to
corresponding alcohol.
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Reduction
Reduction of
aldehyde and ketone
groups yields sugar
alcohol called
alditols.
 Reduction of Dglucose yields Dsorbitol

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Sugar alcohols, also known as polyols,
polyhydric alcohols, or polyalcohols.
 For example, glucitol, also known as sorbitol,
has the same linear structure as the chain
form of glucose, but the aldehyde (-CHO)
group is replaced with a -CH2OH group.
 Other common sugar alcohols include the
monosaccharides erythritol and xylitol and
the disaccharides lactitol and maltitol.
 Sugar alcohols have about half the calories of
sugars and are frequently used in low-calorie
or "sugar-free" products.

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Sugar Alcohols
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Isomerization
Isomerization
After several hours an alkaline solution of
D-glucose also contains D-mannose and
D-fructose.
 It involves an intramolecular shift of a
hydrogn atom and relocation of a double
bond.
 The intermediate is called as enediol.

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Isomerization


The reversible
transformtion of
glucose to fructose is
an aldose-ketose
interconversion.
Because the
configuration at a
single asymetric
carbon changes, the
conversion of glucose
to mannose is
epimerization.
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Tautomerization or enolization


The process of shifting
a hydrogen atom from
one carbon to another
to produce enediols is
known as
tautomerization.
Sugars possessing
anomeric carbon
undergo
tautomerization in
alkaline solution.
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When glucose is kept in alkaline solution
for several hours, it undergoes
isomerization to form D-fructose and Dmannose.
 This results in the formation of a
common inetrmidiate namely enediol for
all the three sugars.

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Schiff Base Formation
Schiff Base Formation

The aldehyde and
ketone groups of
monosaccharides can
also undergo Schiff
base formation with
amines or hydrazides,
forming imines and
hydrazones,
respectively .
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Conjugation of a monosaccharide to an amino
group by formation of an imine.
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This reaction is often used to conjugate
the monosaccharide to proteins (via their
lysine residues).
 The imines formed with amino groups are
not stable to water and are typically
reduced with sodium cyanoborohydride
(NaCNBH3) in a process termed reductive
amination.

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Glycation
Glycation
As aldehydes, reducing sugars can also
form Schiff bases with amino groups of
the lysine residues in proteins.
 This nonenzymatic process that links
glycans to proteins is termed “glycation”
and is distinct from “glycosylation,” which
involves the formation of a glyco-sidic
bond between the sugar and protein.

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Glycation


Glycation products can undergo further
reactions that lead to the formation of
protein cross-links, and these can have
pathogenic consequences (i.e., they are
immunogenic and change the properties of
the protein).
Glycation products of glucose accumulate at
higher levels in diabetics than in healthy
individuals because of elevated blood glucose
levels. These modified proteins are thought
to underlie some of the pathologies
associated with diabetes.
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Esterification
Esterification
Alcohols readily form esters when
reacted with acids, anhydrides, or acyl
halides.
 The most important types of sugar esters
that occur in nature are
 (1) phosphate esters (including
diphosphate esters),
 (2) acyl esters (with acetic acid or fatty
acids), and
 (3) sulfate esters.

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Esterification
Sulphate esters of carbohydrate moleules
are found predominantly in proteoglycan
components of connective tissue.
 Sulphate esters are charged so they bind
large amounts of water and small ions.
 They also participate in forming salt
bridges between carbohydrate chains.

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Glycosidic Bonds
O-Glycosidic Bonds

Monosaccharides can be
modified by reaction
with alcohols and amines
to form adducts. For
example, d-glucose will
react with methanol in
an acid-catalyzed
process: the anomeric
carbon atom reacts with
the hydroxyl group of
methanol to form two
products, methyl α-dglucopyranoside and
methyl β-dglucopyranoside.
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O- Glycosidic Bond
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O-Glycosidic Bonds


These two
glucopyranosides differ
in the configuration at
the anomeric carbon
atom.
The new bond formed
between the anomeric
carbon atom of
glucose and the
hydroxyl oxygen atom
of methanol is called a
glycosidic bond—
specifically, an Oglycosidic bond.
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Glycosidic Bonds

Glycosidic bonds formed between
monosaccharide units are the basis for
the formation of oligosaccharides and
polysaccharides.
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N-glycosidic bond.

The anomeric carbon
atom of a sugar can
be linked to the
nitrogen atom of an
amine to form an Nglycosidic bond.
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The End