Epimer
Carbohydrates act as the most important source of energy for our body and one of the main types
of nutrients. In living systems carbohydrates mainly convert to glucose which is also called as
blood sugar and uses this sugar as energy source. The excess of glucose stores in the form
ofstarch (in plants) and glycogen (in animal) for when it is needed. Depending on their
chemical structures, carbohydrates can be simple or complex compounds.
Simple carbohydrates include sugars found naturally in foods like dairy products,
vegetables and fruits. While the complex carbohydrates include starchy vegetables,
cereals, legumes and whole grain breads. They are organic molecules with the general
formula of CH2O and constitute only 1 - 2 percent of cell mass as they provide the raw
fuel for cellular energy production. They mainly classified according to molecular size
and solubility.
In general, the smaller carbohydrates are more soluble than the larger ones. For example,
monosaccharides like glucose, fructose, galactose, deoxyribose, and ribose are soluble in water
as they contain a single unit of sugar. Out of these carbohydrates, glucose is a hexose sugar in
our blood, and fructose is mainly present in sweetens fruits, however galactose is found in milk
and it is a isomer of glucose.
1. Sugar with five carbon atoms are called as pentose sugar like deoxyribose and ribose
present in nucleic acids.
2. Monosaccharides are bonded together through glycosidic linkage to form disaccharides
like sucrose, maltose and lactose.
3. Sucrose composed of glucose and fructose unit while Lactose formed by the combination
of glucose and galactose units. Similarly two units of glucose bonded together to form
maltose sugar.
4. Polysaccharides are composed of 10-1000 units of monosaccharides bonded through
glycosidic linkage like starch and cellulose.
5. Polysaccharides are long, chain-like polymers which are insoluble in water and generally
act as storage food in living systems. Carbohydrates can be representing in open chain
form as cyclic form called as Haworth projection.
Epimer Definition
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Monosaccharides are generally represented in open chain form. Sometimes their chemical and
physical properties cannot be explained by using open chain form and for that purpose they can
represent in cyclic forms called as Haworth projections.
Monosaccharide like glucose, fructose, mannose and galactose can show different isomerism.
For example, glucose and fructose are functional isomer of each other as glucose contains
aldehyde group and fructose contains ketonic group in molecule. They can also
show stereoisomerismdue to the presence of chiral carbon atoms. Those stereoisomers which
are differing in its configuration at only one chiral carbon atom are called asEpimers.
For example, glucose and galactose are Epimers of each other, as they differ in only in the
position of hydroxyl group at C4.
Both cyclic forms of glucose, -glucopyranose and -glucopyranose are also a type of Epimers
only which are differ from each other at anomeric carbon atom and also known as anomers of each
other.
Epimers of Glucose
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Glucose is one of the most abundant aldohexose on earth which is also named as dextrose. It
involves in the formation of many oligosaccharides and polysaccharides like starch, cellulose
and glycogen.
1. The molecular formula for glucose is C6H12O6 and with molecular mass 180.16
g/mol. Each glucose units is composed of one aldehyde group with four
secondary alcoholic and one primary alcoholic group.
2. Since glucose is an optically active molecule, therefore it can show optical
isomers and exist as Enantiomers known as L-(-) glucose and D(-) glucose. Here +ve
and ve sign indicates its optical rotation, i.e. dextrorotatory and levorotatory.
3. However D & L are not related with their optical rotation but they indicate their
configurations. Hence D-configuration can be dextrorotatory or levorotatory and
vice versa. The D and L configuration assigned on the basis of configuration of
glyceraldehyde molecule which contains one chiral atom.
4. If the OH group of glyceraldehyde molecule lies towards right side, it called
as D- configuration and if it is on left side, it will be L-configuration.
Similarly both configurations of glucose are assigned on the basis of last chiral carbon atom that
is C5. D and L configurations of glucose are as follows.
Because of the presence of chiral carbon atoms in glucose molecules, it can show
stereoisomerism like Enantiomers and diastereomers. Enantiomers are stereoisomer which
is non-superimposable mirror image of each other; hence they cannot be superimposable on
each other by rotating it in the plane of the page. The number of enantiomer depends on the
chiral carbon atoms in molecules.
For example, there are four chiral carbon atoms in glucose molecule, as an optically active
molecule can show 2n enantiomer where n is the chiral carbon atom. Hence there are total sixteen
stereoisomers, out of that eight are Enantiomers of other eight. There is no meso form of glucose.
Diastereomers or diastereoisomers are stereoisomers which are not enantiomer and have
different configurations at one or more chiral carbon atoms (stereocenters). They are not
mirror images of each other. Because of one stereocenter, two different configurations arise and
increases the number of stereoisomers by a factor of two. For example : D-threose and Derythrose are an example of diastereomers.
1. Epimers are diastereoisomers which differ from each other at only
one
stereocenter.
2. For example,
-D-glucopyranose and
-D-mannopyranose are Epimers of
each other as they differ only in the stereochemistry of hydroxyl group at the
C-2 position.
3. In
the
-D-glucopyranose, the hydroxyl group is equatorial and lies in
"plane"
of the ring while in
position i.e. lies up from the
"plane"
-D-mannopyranose, it is at axial
of the ring.
Similarly glucose, galactose and mannose are Epimers of each other as glucose and mannose are
differ in the position of hydroxyl group at C-2 position whereas glucose differs from galactose at
C-4 position.
The cyclic structures of glucose are also epimers of each other. Initially the open chain structure
was used to explain properties of glucose. The open chain structure of glucose was assigned by
Bayer on the basis of some evidence like,
Glucose forms sorbitol, a hexahydric alcohol on reduction with sodium amalgam.
It forms n-hexane and 2-iodohexane with red phosphorus and HI which proves that there
are six carbon atom bonded in a straight chain.
The formation of oximes with hydroxyl amine and cyanohydrine with hydrogen cyanide
proves the presence of aldehyde group in molecule.
Mild oxidation of glucose with bromine water forms gluconic acid and with strong
oxidizing agent it forms saccharic acid. It proves the primary and secondary alcoholic group in
molecule.
Although the open chain structure explains many properties of glucose, yet it fails to explain
some of the following facts.
Glucose does not form additional products with sodium hydrogen sulfite and does not
respond to Schiff base.
D (+) glucose exists in two stereoisomeric forms -glucose and -glucose.
Both -glucose and -glucose undergo mutarotation in aqueous solution. The specific
rotation of aqueous solution of -D(+)glucose falls gradually from +111 to 52.5 and
that of -D(+)-glucose increases from +19.2 to 52.5. The spontaneous change in
specific rotation of an optically active compound with time to an equilibrium value is
called as mutarotation.
The cyclic structure of glucose involves the hemi-acetal formation between C5-OH of glucose
combines with C-1-aldehyde group. Because of that C-1 position becomes chiral and show two
possible arrangements of H and OH group around it.
An anomer is also a type of epimers which differs only in its configuration at the hemi-acetal or
hemiketal carbon or anomeric carbon. The cyclic forms of glucose named as -glucopyranose
and -glucopyranose which can show in Haworth projection and epimers of each other.
Oligosaccharides
REFERENCE :
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