Reactions of monosaccharides, osazone formation, reduction, oxidation, reaction with acids and alkalies, ester formation and formation of amino sugars, amino sugar acids and deoxy sugars.

1. Sugar derivatives and Reactions of
Monosaccharides-Lecture 3
,
Namrata Chhabra
M.D, MHPE
Principal-in-charge
Professor & Head,
Department of Biochemistry

2. Reactions of monosaccharides
1) Osazone formation
2) Reduction
3) Oxidation
4) Action of alkali
5) Action of acid
6) Glycoside formation
2

3. Reactions of monosaccharides
7) Ester formation
8) Amino sugar formation
9) Amino sugar acid formation
10) Deoxy sugar formation
3

4. 1. Osazone formation
● This test is used for the identification of sugars and involves the reaction
of monosaccharide with phenylhydrazine, a crystalline compound.
● All reducing sugars form osazones with excess of phenylhydrazine when
kept at boiling temperature.
● Each sugar has a characteristic crystal form of osazones.
4

5. Reactions involved in the formation of Osazone crystals
Three molecules of phenyl hydrazine are required,
the reaction takes place at first two carbon atoms.
In the general form of the osazone reaction, an alpha-carbon oxidation is involved with the formation
of a bis- phenylhydrazone, known as an osazone.
5

6. Osazone formation
● D-fructose and D-mannose give the same
osazone as D-glucose.
● The difference in these sugars present on the
first and second carbon atoms are masked
when osazone crystals are formed.
● Hence these three sugars form similar needle
shaped crystals arranged like sheaves of corn
or a broom.
● It is seldom used for identification these days .
● HPLC or mass spectrometry is used for the
identification of sugars present in the
biological fluids.
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7. Osazone formation
Galactose forms
rhombic plate like
crystals.
The difference in shape
of the crystals is due to
the difference in the
structure of galactose
from glucose at C4
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8. 2) Reduction of monosaccharides
Sugar alcohols are produced by the reduction of the carbonyl group
(Aldehyde or ketone) of monosaccharides.
The resultant product is a polyol or sugar alcohol
● Glucose forms sorbitol (glucitol)
● Mannose forms mannitol
● Fructose forms a mixture of mannitol and sorbitol
● Glyceraldehyde forms glycerol
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9. Sugar alcohols
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10. Sugar alcohols
● Reduction of aldoses takes place
at C-1 to form sugar alcohol
● Reduction of Ketose sugars takes
place at C-2 to form Sugar
Alcohol.
● An asymmetric Carbon atom is
produced after reduction of keto
sugars, which is why two types of
alcohols are produced.
● Fructose upon reduction
produces equimolar amounts of
Mannitol and Sorbitol
10

11. Significance of Sugar alcohols
1) Sorbitol
● In Diabetes Mellitus excess of
Glucose is converted to Sorbitol.
● The osmotic effect of Sorbitol is
responsible for many of the
complications of diabetes mellitus
e.g. Cataract formation in lens.
● Clinically sorbitol is dehydrated and
nitrated to form Isosorbide mono
and dinitrate, both of which are used
for the treatment in Angina.
11

12. Significance of Sugar alcohols
2) Mannitol- Mannitol is also
osmotically active and is used as an
infusion to lower the intracranial
tension by producing forced diuresis.
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13. Significance of Sugar alcohols
Dulcitol- excess of galactose
in galactosemia is converted
to Dulcitol. The osmotic
effect of Dulcitol is similar
to Sorbitol and is
responsible for premature
cataract formation in
affected patients of
galactosemia.
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14. Significance of Sugar alcohols
Xylitol- is produced in Uronic
acid pathway of Glucose
utilization; it is subsequently
oxidized to produce D-
Xylulose. Also used as a
sweetener
14

15. Significance of Sugar alcohols
Glycerol is produced from
Glyceraldehyde.
Glycerol is used for the formation of
Triglycerides and phospholipids.
Clinically glycerol is nitrated to form
Nitroglycerine which is used for the
treatment of angina.
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16. Significance of Sugar alcohols
Myo- Inositol- It is a hexahydroxy alcohol,
also considered a vitamin.
It is present in the plasma membrane and
acts as a second messenger for the action
of hormones.
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17. Significance of Sugar alcohols
7) Ribitol- is used in the formation of vitamin B2- (Riboflavin )
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18. 3) Oxidation of sugars
Sugar acids are produced by the oxidation of the monosaccharides.
Oxidation may involve :
1) Aldehyde group(C1) to form Aldonic acid, or
2) Primary Alcoholic group (C5) in an aldohexose to form uronic acid or
3) Both groups to form Saccharic acid.
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19. 3) Oxidation of sugars
1) Oxidation of Aldehyde group
● Under mild conditions, in the presence of Hypobromous acid, the
aldehyde group is oxidized to form Aldonic acid.
● Thus, Glucose is oxidized to Gluconic acid, Mannose to form Mannonic
acid and Galactose to form Galactonic acid.
● Formation of Gluconic acid by the activity of Glucose oxidase is the basis
for the Quantitative estimation of urinary and blood Glucose.
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20. Oxidation of Aldehyde group
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21. 3) Oxidation of sugars
2) Oxidation of Primary Alcoholic acid
● Under special conditions when the
aldehyde group is protected, and the
molecule is oxidized at the primary
alcoholic group the product is a
Uronic acid.
● Thus Glucose is oxidized to form
Glucuronic acid, Galactose to form
Galacturonic acid and Mannose is
oxidized to Mannuronic acid.
21

22. 3) Oxidation of sugars
2) Oxidation of Primary Alcoholic acid
Glucuronic acid is used in the body for conjugation
reactions to convert the toxic water insoluble
compounds in to nontoxic water-soluble form, which
can be easily excreted in urine.
Glucuronic acid and its epimer Iduronic acid are used
for the synthesis of heteropolysaccharides.
22

23. 3) Oxidation of sugars
3) Oxidation of both Aldehyde and Primary Alcoholic group
● Under strong acidic conditions (Nitric acid and heat) the first and the last
carbons are simultaneously oxidized to form dicarboxylic acids, known as
Saccharic acids.
● Glucose is thus oxidized to form Glucose Saccharic acid, Mannose to
Mannaric acid and
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24. Oxidation of galactose and mannose by strong nitric acid
● Galactose to Mucic Acid (aldaric acid)
● The mucic acid forms insoluble crystals and is the basis for a test for identification of Galactose.
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25. Oxidation of glucose
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26. 4) Action of alkali
● In mild alkaline conditions Enediols are formed.
● Enediols are highly reactive sugars and are powerful reducing agents. This process
allows the interconversion of D-mannose, D-fructose and D-glucose.
● This interconversion reaction is known as Lobry de Bruyn- Van Ekenstein
transformation.
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27. 4) Action of alkali
● Enediols obtained by the action of base are quite susceptible to oxidation
when heated in the presence of an oxidizing agent.
● Copper sulfate is frequently used as the oxidizing agent and a red
precipitate of Cu2O is obtained.
● Sugars which give this reaction are known as reducing sugars.
● This reaction forms the basis of detection of sugars in the biological fluids
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28. 4) Action of alkali
Some of the frequently used solutions for
detecting the presence of reducing sugars in
biological fluids are as follows-
1) Fehling’s solution: KOH or NaOH and
CuSO4
2) Benedict’s solution: Na2CO3 and CuSO4
3) Clinitest tablets are used to detect urinary
glucose in diabetics.
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29. 4) Action of alkali
Strong alkalis cause
CARAMELISATION
(decomposition)of sugars.
29

30. Action of acids on sugars
● Monosaccharides are normally stable to dilute acids, but are dehydrated
by strong acids.
● D-ribose when heated with concentrated HCl yields furfural
● D-glucose under the same conditions yields 5-hydroxymethyl furfural
● Forms the basis of Molisch test, Seliwanoff test and Bial’s Test
30

31. Molisch test, Seliwanoff test and Bial’s Test
31

32. 6) Glycoside formation
Acetal or ketal derivatives formed when a monosaccharide reacts with an
alcohol are called glycosides.
They are formed by the reaction of the hydroxyl group of anomeric carbon
(hemiacetal or hemiketal) of monosaccharide with hydroxyl group of second
molecule with the loss of an equivalent of water
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33. 6) Glycoside formation
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34. 6) Glycoside formation
The second molecule may be-
1) Another sugar (Glycon)- e.g. formation of disaccharides and
polysaccharides.
2) Non Carbohydrate (Aglycon)- such as Methanol, Glycerol, Sterol or
Steroids etc.
34

35. Examples of glycosides
● Glycosides are present in many drugs, spices and in the constituents of
animal tissues.
● Glycosides comprise several important classes of compounds such as
hormones, sweeteners, alkaloids, flavonoids, antibiotics, etc.
● The glycosidic residue can be crucial for their activity or can only improve
pharmacokinetic parameters.
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36. Examples of glycosides
1) Cardiac Glycosides
Cardiac glycosides all contain steroids or genin
component as aglycone in combination with sugar
molecules.
These include derivatives of digitalis (digoxin) and
strophanthus such as ouabain.
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37. Examples of glycosides
2) Streptomycins are used as antibiotics.
3) Phloridzin is another glycoside which is
obtained from the root and bark of apple
tree.
It displaces Na+ from the binding site of
“carrier protein”, prevents the binding of
sugar molecule and produces Glycosuria.
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38. Examples of glycosides
3) Glycosides of vitamins, both hydrophilic and lipophilic often occur in
nature.
Glycosylated vitamins have an advantage over the respective aglycone in their
better solubility in water (especially the lipophilic ones), stability against
UV-light, heat and oxidation, reduction of the bitter taste and odor(e.g.,
thiamine), and resistance to an enzymatic action.
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39. 7) Ester formation
The –OH groups of monosaccharides can act as alcohols and react with
acids (especially phosphoric acid) to form esters.
Hydroxyl groups of sugars can be esterified to form Acetates,
phosphates, benzoates etc.
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40. 7) Ester formation
Sugars are phosphorylated at terminal C1 hydroxyl group or at other
places .
● At terminal hydroxyl: Glucose-6-P or ribose-5-P.
● At C1 hydroxyl group: Glucose-1-P .
● At both places: Fructose 2,6 bisphosphate
● Metabolism of sugars inside the cells starts with phosphorylation.
● Sugar phosphates are also components of nucleosides and nucleotides.
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41. Sugar esters
41

42. 8) Amino sugars
● Amino groups may be substituted for
hydroxyl group of sugars to give rise
to amino sugars.
● Generally, the amino group is added
to the second carbon of the hexoses.
● The most common amino sugars are
Glucosamine and Galactosamine.
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43. Acetylated Amino sugars
● The amino group in the sugar maybe
further acetylated to produce N-Acetylated
sugars such as N-AcetylGlucosamine
(GluNac ) and
N-Acetyl-Galactosamine(GalNAc), etc.
● These are important constituents of
glycoproteins, mucopolysaccharides and
cell membrane antigens.
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44. Acetylated Amino sugars
Glucosamine is the chief constituent of cell wall
of fungi and a constituent of shells of crustaceans
(Crabs, Lobsters etc), where it is found as a
polymer of N-Acetyl Glucosamine called Chitin.
Hence this amino sugar is also called
Chitosamine.
44

45. Acetylated Amino sugars
Galactosamine occurs as N-Acetyl
Galactosamine in Chondroitin
sulphates which are present in
cartilages, bones, tendons and heart
valves.
Hence Galactosamine is also called
Chondrosamine.
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46. Amino sugars
● Certain antibiotics, such as Erythromycin,
Carbomycin contain amino sugars.
● In some amino sugar the anomeric OH group
is replaced by amino group. e.g. Ribosyl
Amine, which is used for the de novo
synthesis of Purine nucleotides.
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47. 9) Amino sugar acids
Amino sugar acids are produced by condensation of amino sugar with Pyruvic
or lactic acid.
● Muramic acid is produced by the condensation of lactic acid with D-
Glucosamine. Certain bacterial cell walls contain Muramic acid.
● N-Acetylneuraminic acid is formed from the condensation of Pyruvic acid
with N-Acetyl Mannosamine.
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48. Amino sugar acids
● N-Acetylneuraminic acid (NANA),
also called Sialic acid, is a nine
carbon derivative and is an important
component of glycoproteins and
gangliosides (lipids).
● Neuraminidase is the enzyme which
removes NANA from its binding with
other compounds.
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49. Muramic acids
49

50. 10) Deoxy Sugars
Deoxy Sugars are monosaccharides which lack one or more hydroxyl groups
on the molecule.
They are formed by the removal of oxygen, generally from OH group present
at C-2 or other locations of monosaccharides .
Examples of deoxy Sugars
1) One quite ubiquitous deoxy sugar is 2’-deoxy ribose which is the sugar
found in DNA.
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51. Deoxy Sugars
51

52. Deoxy Sugars
2) 6-deoxy-L-mannose
(L-rhamnose) is used as a
fermentative reagent in
bacteriology.
3) L-Fucose
(6-deoxy.L-galactose) is a
component of glycoproteins and
gangliosides of cell membranes.
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