Carbohydrates include both sugars and polymers of sugars. Carbohydrates can be :
Monosaccharides – Glucose , Fructose, Galactose.
Oligosaccharide – sucrose, lactose, and maltose.
Polysaccharides – cellulose and chitin
They are simplest carbohydrates, by polymerization, they form complex carbohydrates. They can be :
Monosaccharides have asymmetric carbon centers except for Dihydroxyacetone. The presence of chiral carbon atom makes them optically active and form two forms :
- D – isomer
- L – isomer
Number of stereoisomers = 2n
n = No. of the chiral center
When the hydroxyl group of the reference carbon is on the right side of the projection formula it is D – isomer
and when the -OH group is on the left side of the reference carbon in the projection formula then it is L – isomer.
When two sugar differs only in the configuration around one carbon atom then they are known as epimers.
- D-glucose and D-mannose, which differ only in the stereochemistry at C-2.
- D-glucose and D-galactose differ only at C-4.
In aqueous solution,
All the monosaccharides with five or more carbon atoms backbone and aldotetrose occur predominantly as cyclic (ring) structure. These rings are formed as a result of the reaction between alcohols and aldehydes or ketones to form derivative called Hemiacetal or Hemiketals.
Hemiacetal and Hemiketals contain an additional asymmetric carbon atom and thus can exist in 2 stereoisomeric forms.
In solution, the D-glucose exist as an intramolecular hemiacetal, which has free -OH group at carbon-5. The C-5 react with the aldehydic carbon-1, and rendering the latter carbon asymmetric and produce two stereoisomers α and β.
The systematic name for the two ring forms of D-glucose:
They are called Pyransose because they resemble the six-membered ring compound Pyran.
Aldohexoses also exist in cyclic form having 5 member rings called furanoses (as they resemble with five member ring compound furanose).
Aldophyronose is more stable than Aldofurnose.
The hemiacetal or hemiketal carbon atom is called anomeric carbon and the isomeric forms of monosaccharides that differ only in their configuration about hemiacetal or hemiketal carbon atom is called Anomers.
The α and β anomers of D-glucose interconvert in aqueous solution by the process of muta rotation.
In ketohexoses compounds the -OH group at (or C-6) react with keto group at C-2 forming a furanose (or Pyranose) ring containing a hemiketal linkage. They also occur in α and β anomeric form.
Hexose derivative in various organism
In glucosamine, galactosamine, and mannosamine the hydroxyl at C-2 of the parent compound is replaced with an amino group.
The substitution of a hydrogen to the hydroxyl group at C-6 of L-galactose or L-mannose produce L-fucose or L-Rhamnose respectively. These deoxy sugar found in plant polysaccharides and in the complex oligosaccharide components of glycoproteins and glycolipids.
Oxidation of Carbonyl(aldehydic carbon) form:
Aldoses → Aldonic acids
- Glucose → Gluconic acid
Oxidation of Carbon at the other end of the carbon chain form:
Aldoses → Uronic acid
- Glucose → Glucuronic acid
- Galactose → Galacturonic acid
- Mannose → Mannuronic acid
Monosaccharides are reducing agents
Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric ( Fe3+ ) or cupric ( Cu2+ ) ion.
Fehling’s Reaction is a qulitaitve test for the presence of reducing sugar.
This test was used to detect and measure the elevated glucose level in blood and urine in the diagnostic of diabetes mellitus but nowadays, enzyme Glucose oxidase is used for measuring blood glucose.