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Glycosides- Introduction, Classification, Chemical Properties, Identification Test

INTRODUCTION

Glycosides are the naturally occurring organic compounds found in plants and some Animals, which upon hydrolysis (either acid or enzymatic) gives one or more sugar (glycone) moiety and non sugar (aglycone) moiety. The non sugar or aglycone moiety is called genin.
 
The pharmacological activity of any glycoside is mainly due to the presence of genin part whereas glycone part facilitates the transportation of genin part to the site of action. Most frequently occurring sugar is β-D-glucose, although rhamnose, digitoxose, cymarose and other sugars are components of glycosides. When sugar part is glucose then it is known as glucoside. Other sugars may be developed during hydrolysis then term glycoside is applied.

CLASSIFICATION OF GLYCOSIDES

(a) On the Basis of the Linkage:
 
1. O-glycosides: In these glycosides the sugar part is linked with alcoholic or phenolic
hydroxyl or carboxyl group.
2. S-glycosides: In these glycosides the sugar is attached to a sulfur atom of aglycone such as in sinigrin.
3. N-glycosides: In these glycosides the sugar linked with nitrogen atom of (-NH2,-NH-) amino group of aglycone like in nucleosides DNA, RNA.
 
 
(b) According to aglycone part:
 
1. If aglycone part alcohol: This group called alcoholic group like Salicin.
2. If aglycone part aldehyde: This group called aldehydic group like glucovanillin.
3. If aglycone part phenol, it is called phenolic group like arbutin.
4. If aglycone part cyanone, it is called cyanogenic or cyanophoric or cyanoside like
amygdalin.
 
(c) According to glycone part:
 
1. Glucose: Glucoside group like in Sennoside.
2. Rhamnose: Rhamnoside like in frangulin.
3. Digitoxose: Digitoxoside like in digoxin.
4. Glucose and Rhammnose: Glucorhamnoside – glucofrangulin.
5. Rhamnose and glucose: Rhamnoglucoside – Rutin
 

PHYSICO-CHEMICAL PROPERTIES

• They are colorless, amorphous, solid, non-volatile compounds.
• They give positive test with Molisch and Fehling’s solution test.
• They have solubility in water but insoluble in organic solvents.
• Most of them possess bitter taste but some exceptions are populin, glycyrrhizin,
stevioside.
• They are odorless compounds except saponin (glycyrrhizin).
• Glycosides gets hydrolyzed by mineral acids and temperature or by enzymes like
 
(a) Emolsin: Bitter almond seeds.
 
(b) Myrosin or Myrosinase: Black mustard seeds.
 
(c) Rhamnase: Glycosides containing rhamnose as sugar part.
 
 

TESTS FOR IDENTIFICATION

General Test:
 
Test A: In this test 200 mg drug is extracted with 5 ml sulphuric acid (dilute), warm it on
waterbath and filter. Take the acidic extract and neutralize it by adding sodium hydroxide solution (5 percent). After neutralization Fehling solution A and B (0.1 ml) is added and heated it on waterbath for few minutes (2-3 minutes).
 
Test B: Here 200 mg drug extracted with water (5ml), boil it then add water in same
volume as used for sodium hydroxide in the previous test. After that add Fehling solution A and B (0.1ml) and heat it on waterbath for 2 to 3 minutes. 

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