2. ISOMERISM IN MONOSACCHARIDES
Compounds that possesses the same molecular formula & different structural
formula are called isomers and the phenomena is known as isomerism. Glucose ,
Fructose, Mannose , & Galactose all have same chemical formula but different
structural formula and therefore these are isomers of each other .
Isomerism in Monosaccharides are of several types
3. Stereoisomerism:
(stereo= having three dimensions)
Two or more monosaccharides having the same structure i.e. same linkage
between atoms but different from each other in configuration i.e. Arrangement of
atoms in space.
4. Enantiomers:
These are pairs of stereoisomers that are mirror images of each other in regard to
asymmetric c-atoms. An asymmetric C- atom or Chiral Carbon is a C-atom that is
attached to four different types of atoms or group of atoms.
D and L depends on the on last asymmetric carbon atom.
D Glucose = “D” refers right hand side of the hydroxyl (OH) side chain.
L Glucose = “L” refers left hand side of the hydroxyl (OH) side chain.
Most of the sugar is exist in D form.
5.
6. Anomers:
These are two isomers that differ in configuration around the anomeric carbon
atom i.e the carbon atom of the carbonyl group which is carbon No 1 and carbon
No 2 in ketoses. The two types of anomers are called alpha and beta anomers.
They are not mirror images of each other.
Type is based on the position of the C-1 OH
Alpha glycosidic bond
OH is below
- linkage between a C-1 OH and a C-4 OH
Beta glycosidic bond
OH is above
- linkage between a C-1 OH and a C-4 OH
7.
8. Epimers:
These are two isomers which differ in configuration around one specific carbon
atom other than carbon atom of carbonyl group. Glucose & Galactose differ from
each other only in the position of (OH) at C-4. They are called carbon-4 epimers.
Isomers in which orientation of –H and –OH is different at a particular carbon.
9. Optical Isomerism:
The enantiomeric carbon monosaccharides by virtue of their content asymmetric
carbon atoms can rotate the plane - polarized light either to right or to the left.
Those monosaccharides that rotate this light to right are called dextrorotatory and
are designated (+) type clockwise. Those rotating this light to left are called
levorotatory and are designated (-) type anticlockwise.
10.
11. Pyranose & Furanose Isomerism:
The ring structures of monosaccharides may be similar to either Pyran OR
Furan and accordingly monosaccharide is said to occur in pyranose or
furanose forms.
12. Fischer projection
The Fischer projection is a two-dimensional representation of a
three-dimensional organic molecule by projection.
Haworth projectionsHaworth projections
A Haworth projection is a common way of representing the cyclic
structure of monosaccharides with a simple three-dimensional
perspective.
13. CHEMICAL REACTIONS OF
MONOSACCHARIDES
Chemical properties depends upon the presence of various groups in monosaccharides
i.e. aldehyde ,ketone & hydroxyl groups.
1: Reaction with Hydrazines to form Osazones:
• Osazones which are actually phynylosazones which are yellowish crystalline
compounds and are produced as a result of heating sugar solutions with
phenylhydrazines.
• Glucose react with three molecules of phenylhydrazine to form glucasazone.
2: Reduction to form sugar alcohols:
• Both aldoses & ketoses may be reduced at their aldehyde and ketone groups to form
the corresponding poly hydroxy alcohols.
• Glucose → Sorbital, Mannose →Mannitol
• Galactose→ Galactitol (Dulcitol)
• Fructose → Sorbital & Mannitol
• Ribose→ Ribitol , Glyceraldehyde→ Glycerol
• Dihydroxyacetone → Glycerol
14. CHEMICAL REACTIONS OF
MONOSACCHARIDES
3. Oxidation to produce sugar acids:
• When oxidize under proper conditions the aldoses gives rise to the following three
types of sugar acids.
• Gluconic Acid (at C-1)
• Glucuronic Acid (at C-6)
• Glucaric Acid (at C-1 & C-6)
4: Reducing Action Of Sugar In Alkaline Solution:
• Sugars in alkaline solutions are very powerful reducing agents and the sugars are
oxidized to complex mixture of acids . e.g. Cu+, Ag+ etc.
• The reducing action of sugars i.e. benedict's solution and Barfoed’s solution have
been employed for both qualitative and semi qualitative determination of reducing
sugars especially glucose in urine.
15. 5: Action Of Acids:
• Monosaccharides are resistant to action of hot dilute mineral acids. Strong acids
remove water from sugar converting them to furfurals which condense with phenol to
give characteristics colored products.
• C5 H10 O5 → C5 H4 O2 + 3H2O
• Ribose → Furaldehyde → water removed
• This reaction is the basis of Molisch’s test for carbohydrates.
6: Action Of Basis:
• Dilute basis solution at low temperature can bring about rearrangement of groups
around anomeric C-Atom and its adjacent C-atoms . e.g. Glucose can be changed to
fructose and mannose.
16. 7: Formation Of Glycosides:
Glycosides are compounds in which a carbohydrate residue such as glucose is attached
by Acetyl linkage at anomeric carbon atom to an alcoholic residue called aglycone which
is non carbohydrate .
8: Formation Of esters:
• The OH group of sugar may be esterified to form esters such as phosphate acetates,
propionates etc.
• Sugar phosphate backbone form structural framework of nucleic acid (DNA & RNA ).
• Hexose + Phosphate = Hexose phosphate
• Ribose+ Phosphate = Ribose phosphate
17. 9: Formation Of Amino Sugars :
• A hydroxyl group of monosaccharides can be replaced by an amino group forming an
amino sugar .e.g.
• D – glycose amine = Hyaluronic Acid
• D – galactose amine = Chondroitin
• D – mannose amine = Mucoprotein
• Being derived from hexoses they are called hexose amine. Amino sugars are also
present in antibiotics . e.g. Erythromycin.
10: Fermentation:
• Breakdown of sugars by bacteria and yeast using a method of anaerobic respiration.
• Glucose→ Yeast or Bacteria→ 2 Ethanol+2 carbon dioxide
• C6 H12O6 → 2 (C2H5OH) + CO2
18. IMPORTANCE OF CARBOHYDRATES
1. Glucose:
Glucose is used to treat very low blood sugar level i.e. hypoglycaemia most often in people in
diabetes mellitus .Glucose is also used to provide carbohydrate calories to a person who cannot
eat because of illness trauma etc. ( primary source of energy ).
2. Component Of Cell Membrane: Carbohydrates are covalently linked to proteins (glycoproteins)
lipids (glycolipids) are parts of cell membrane and function as adhesion.
3. Pectin: is the heteropolysaccaride Pectin is used to reduced blood cholesterol level and
gastrointestinal disorders.
4. Agar: Microbial culture → nutrient agar is a nutrient medium most effectively used for the
cultivation of microorganism supporting growth of a wide range of non fastidious organism and
blood ager is most appropriate for human related organism in clinic practice.
5. Chondroitin Sulphate & Glucosamine Sulphate: For joints disorder treatments such as
osteoarthritis.
6. Dextran: It is used to treat hypovolemia or hypovolemic shock (decrease volume of circulating
blood plasma) that can result from surgery, trauma.
19. 7. Inulin: (Polymer of fructose) Inulin is used to measure kidney function by determining
glomerular filtration rate ( GFR) which is volume of fluid filtered from glomerular
capillaries into bowman capsule per unit time .
8. Heparin: Heparin act as an anticoagulant, prevent formation of clots .
9. Mannitol: It is osmotic diuretic used to reduce swelling and pressure inside the eye
all around the brain.
10. Lactose: Used as excipient in tablets.
11. Ribose & Deoxyribose: Form structure of hereditary material such as DNA &RNA.
12. Aminoglycosides: They are broad spectrum bactericidal antibiotics commonly
prescribe for children.
13. Hyaluronic Acid : Used in in topical serum, eye drops & injections. Also form
structural bases of joints, skin & aqueous humor of eye.
14. Gums: Used as foods as well as suspension preparations.
15. Glycogen & Starch: Energy storage → Glycogen (animals) Starch (Plants).
