Download FOUNDATION MODULE 2012

FOUNDATION MODULE 2012
Isomerism and
Monosaccharide Derivatives
LEARNING OBJECTIVES
By the end of the lecture, the student will be able to :
 Explain isomerism in carbohydrates
 Describe the monosaccharide derivatives (sugar acids, sugar alcohols, aminosugars and de-oxysugars)
 Discuss the clinical importance of monosaccharide derivatives (GAGs,
detoxification, cataract, diabetes mellitus, hexosamines, DNA)
ISOMERS
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Compounds that have the same chemical but have different structural formula
For Example :
Fructose, Glucose, mannose, and galactose are all isomers of each other, having
the same chemical formula , C6 H12 O6.
Carbon satisfied with four different atoms/groups of atoms is called
chiral/asymmetric carbon
D, L Isomers
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Position of OH group around carbon adjacent to primary alcohol carbon
Epimers
Enantiomers
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Enantiomers are molecules that are mirror images of each other. The two
members of the pair are designated as a D – and an L – sugar.
Ring Structure
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In aqueous solutions sugar molecule acquires a ring structure
5 membered furanose (4C)
6 membered pyranose (5C)
- and - Anomers
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Functional group carbon becomes asymmetric in ring form known as Anomeric
carbon
-configuration/anomer: the OH group on anomeric carbon is below the
plane of the ring.
-configuration/anomer: the OH group on anomeric carbon is above the
plane of the ring.
Optical Isomers
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Plane polarized light is rotated upon passage through the sugar solution
Dextrorotatory (d, +)
To the right
Levorotatory (l,-)
Inverse sugar
Dextrorotatory nature of sucrose changes to levorotary upon hydrolysis because
of fructose is naturally occurring D(-) sugar
Oxidation Reactions of Monosaccharides
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Oxidized to acids
Aldoses and ketoses give positive tests when treated with Benedict’s reagent
Benedict’s reagent (an alkaline solution of cupric citrate complex) gives a brick
red precipitate of Cu2O
The Sugar Acids
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Selective oxidation of the aldehyde group of an aldose to the
corresponding carboxylic acid
An aldose becomes an Aldonic acid
Selective oxidation of the aldehyde group of carbon number
6 produces Uronic Acids
Dilute nitric acid oxidizes both the aldehyde and primary hydroxyl groups of an
aldose to an Aldaric acid
Significance
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Sugar acids are present in glycosaminoglycans.
Detoxification reactions
Drugs
Bilirubin
Glycosaminoglycans
Reduction Reactions of Monosaccharides
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Aldoses and ketoses can be reduced to alditols
Significance
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High blood sugar level
Sugar enters in the cell
Reduced to sorbitol
Osmotically active
Drags water in the medium
E.g.
Cataract
Diabetic neuropathy
Therapeutic use: Mannitol for cerebral edema
Deoxy Sugars
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Oxygen is removed from the hydroxyl group
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Dexoribo-Nucleotides
Deoxyribo-nucleic acids
Significance
Amino-Sugars
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Carbonyl group reacts with amino group to form amino sugars
Glycosaminoglycans
References
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Lippincott’s
Biochemistry .
chapter 7. Pg83-89
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Harper chapter “carbohydrates of physiologic significance”
Pages 113-120
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