Glycosides
Glycoside: is an organic compound, usually of plant origin, that is
composed of a sugar portion linked to a non-sugar moiety by glycosidic
bond. The sugar portion is called glycon, while the non-sugar portion is
called aglycon or genin; therefore glycosides yield one or more sugars
among the products upon hydrolysis.
The sugar component of glycosides may be mono, di, tri or
tetrasaccharides.
There are two diastereoisomers differ in configuration about the anomeric
carbon ( C-1) can exist α and β.
If the hydroxyl group on the anomeric carbon is down in relation to the
cyclic structure, it is α anomer.
If the hydroxyl is up, it is the β.
β forms primarily occur in plants, emulsin and most other natural
enzymes hydrolyze only the β varieties.
Sugars exist predominantly as cyclic hemiacetals (R-O-C-OH group),
while glycosides are usually mixed acetals (R-O-C-O-R) group..
Physical and chemical properties of glycosides
Because of the complexity of the structure of the naturally occurring
glycosides, no generalization are possible with regard to their stability. In
addition there are differences in their solubility properties. Many
glycosides are soluble in water or hydroalcoholic solutions because the
solubility properties of the sugar residues exert a considerable effect i.e.
sugar moiety increases water solubility.
The aglycon part is soluble in non-polar solvents like benzene and ether.
Glycosides can be hydrolyzed by heating with a dilute acid where by the
glycosidic linkages are cleaved.
Glycosides can be also hydrolyzed by appropriate enzymes, which are
usually found in the same plant, in separate compartments. There is a
specific enzyme for each glycosides to exert a hydrolytic action on it.
Classification of glycosides
1- According to the type of glycosidic linkage:
 α-glycosides (α sugar)
 β-glycosides (β sugar)
2- according to the chemical group of the aglycon involved in the
formation of glycoside linkage.
 Aglycone- O- Sugar
O-glycosides(OH group): eg. Senna
and rhubarb
 Aglycone- C- Sugar
C-glycosides (C- group): eg.
Cascaroside from cascara
 Aglycone- S- Sugar
S-glycosides (SH- group):
eg.sinigrin from black mustard
 AglyconeNSugar:
N-glycosides(NHgroup):
eg.glycoalkaloid.
N-glycosides
3- According to the chemical nature of the aglycon, the glycosides
containing drugs can be classified into:
1- Cardioactive group.
2- Anthraquinone group.
3- Saponin group.
4- Cyanophore group.
5- Isothiocyanate group.
6- Flavonol group.
7- Alcohol group.
8- Aldehyde group.
9- Phenol group.
4- According to the nature of the simple sugar component of the
glycoside:
1- Glucoside (the glycone is glucose).
2- Galactoside (the glycone is galactose).
3- Mannoside (the glycone is mannose).
4- Arabinoside (the glycone is arabinose).
Biosynthesis of glycosides
The principal pathway of glycoside formation involves the transfere of
uridylyl group from uridine triphosphate to a sugar 1-phosphate. Enzymes
catalyzing this reaction are referred to as uridylyl transferases (1). The
subsequent reaction, mediated by glycosyl transferases (2), involves the
transfer of sugar from uridine diphosphate to a suitable acceptor (
aglycon), thus forming glycoside.
1
UTP + sugar-1-P
UDP – sugar + acceptor
UDP – sugar + PPi
2
acceptor – sugar + UDP
(glycoside)
Extraction of glycosides
 Since glycosides are accompanied by specific hydrolyzing
enzymes, these enzymes must be inactivated by putting the plant in
boiling water or alcohol.
 Defatting or purification of the plant material in case of seeds.
 Treatment with lead acetate to precipitate tannins and other non
glycosidal impurities.
 Removal of any excess of lead acetate by passing hydrogen sulfide
H2S gas through the solution.
 The extract is filtered and concentrated to get crude glycoside.
 Purification of the crude glycosides by chromatography or
crystallization.