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CARBOHYDRATE CHEMISTRY
Dr. Mohammed Shakil Akhtar
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Lecture Outlinesy Introduction and General Characteristics.
y
Function of Carbohydrates.y Classification of Carbohydrates.
y Properties of Monosaccharides.
y Sterioisomerism
y Optical Activity
y Cyclic Structures
y Anomers and anomeric carbon
y Reducing Sugars
y Glycosides
y Disaccharides
y Polysaccharides
y Glycoproteins
y Sugar Derivatives
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Introduction and General Characteristics
y Carbohydrates are the single most abundant class
of or ganic molecules found in nature, constitutingone of the four major classes of biomolecules.
y They are µhydrates of carbon¶ having the general
molecular formula name (CH2O)n where n= number of
carbon atoms ( 3 or more). They are composed of C, Hand O.
y Most carbohydrates are found naturally in bound
form rather than as simple sugars e.g starch,
glycoproteins and glycolipids.
y Carbohydrates are µaldehyde¶ or µketone¶ compounds
with multiple hydroxyl (- OH ) groups.
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y When the functional group is
an µaldehyde group¶ they areknown as µaldoses¶ e.g
glyceraldehyde, glucose .
yWhen the functional group
is a µketo group¶ they are
referred to as µketoses¶ e.g.
dihydroxyacetone, fructose
The aldehyde or ketone group of a carbohydrate isalso known as a µcarbonyl group¶ and the the carbon
atom of the aldehyde or ketone group is known as the
µcarbonyl carbon¶.
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ALDOSES
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KETOSES
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Functions of Carbohydratesy Carbohydrates serve as ener gy stores and fuels.
Breakdown of carbohydrates provides the ener gy thatmaintains life. Glucose is main source of ener gy for
the brain.
y Carbohydrates participate in the structure of cell
membranes (glycoproteins and glycolipids).y Pentose sugars (ribose and deoxyribose) form the
structure of nucleic acids.
y Carbohydrates play important roles in recognition
between cell types. Recognition events are importantin normal cell growth, fertilization and transformation
of cells.
y Carbohydrates act as intermediates in the biosynthesisfats and proteins.
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CL ASSIFICATION OF CARBOHYDRATES
They are classified according to the ³number of
structural units´ into:
y Monosaccharides
y Disaccharides
y Oligosaccharides and
y Polysaccharides
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Monosaccharides
y
The smallest and the simplest ones, for which n = 3,are glyceraldehyde and dihydroxyacetone. They are
trioses.
y Glyceraldehyde is also an aldose because it contains
an aldehyde group; whereas dihydroxyacetone is aketose because it contains a keto group, (the suffix-
ose means sugar)
y Monosaccharides are the carbohydrates that cannotbe hydrolysed into simpler units (have one structural
unit). Also known as simple sugars.
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Classification of Monosaccharides
They are classified according to :1. Number of carbon atoms in the molecule: They may
be subdivided into, trioses (3C), tetroses(4C),
pentoses(5C), hexoses(6C), heptoses(7C) or
octoses(8C).
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2. Type of functional group as aldosugars and
ketosugars.
y Examples of aldoses are glyceraldehyde, erythrose,
ribose, glucose.
y Examples of ketoses are dihydroxyacetone,
Erythrulose, Ribulose, Fructose.
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POL YSACCHARIDES
y They yield more than ten molecules of
monosaccharides on hydrolysis. They may be either
linear or branched polymers and may contain hundredsor even thousands of monosaccharide units . Examples
are starch, glycogen, dextran.
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Properties of Monosaccharide
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1. Monosaccharides are crystals and dissolve in water.
2. Monosaccharides have a Sweet taste.
3. Monosaccharides contain asymmetric carbon atom
(which is a carbon atom attached to four different
chemical groups) except dihydroxyacetone.
y Due to presence of asymmetric carbon atom,
monosaccharides have two main physical properties
which are:
y
Optical Isomerism or Stereoisomerismy Optical Activity
4. Cyclic structure of Monosaccharides
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Asymmetric Carbon
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No Asymmetric
Carbon
4 Asymmetric
Carbon
3 Asymmetric
Carbon
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y Optical isomerism or stereoisomerism is the ability of substances to be present in more than one form. It is due
to the presence of asymmetric carbon atom. These forms
are named optical isomers.
Optical Isomerism or Stereoisomerism
y So optical isomers or stereoisomers or isomers are
compounds that have same structural formula but differ
in their configuration around the asymmetric carbon
atom.
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y A monosaccharide containing one or more than one
asymmetric carbon atom can exist in a number of
isomers equal to 2n, where n equal to the number of
asymmetric carbon atoms.
e.g Glucose has 4 asymmetric carbon atoms, so
the number of optical isomers equal to 24
=2x2x2x2 = 16.
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ENANTIOMERS
y Enantiomers are a type of isomers that are mirror
images of each other by differing in the configuration
around the asymmetric carbons. They are designated
as ³D´ and ³L´ isomers based on the position of ±OH
group before the last ( sub-terminal) carbon.
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y In D isomer (D-glucose) the -OH group attached to
the asymmetric carbon ( sub-terminal C5 )is towards
the right while in L isomer ( L-glucose) -OH group ison the left.
³The majority of the sugars in humans are D-sugars´.