Glycosides are a group of organic compounds that yield one or more sugars upon hydrolysis. Structurally, a glycoside is composed of a sugar part, known as the glycone, and a non-sugar part, referred to as the aglycone or genin. These two parts are linked through a glycosidic bond, which typically involves the anomeric carbon of the sugar.
Classification of Glycosides
Glycosides can be classified based on various criteria, including the type of sugar, the nature of the aglycone, and the type of glycosidic linkage. The main classifications are:
1. By Sugar Component (Glycone)
O-glycosides: The sugar is linked to the aglycone via an oxygen atom.
N-glycosides: The sugar is linked to the aglycone via a nitrogen atom.
C-glycosides: The sugar is linked to the aglycone via a carbon atom.
S-glycosides: The sugar is linked to the aglycone via a sulfur atom.
2. By Aglycone Component
Phenolic glycosides: The aglycone is a phenol.
Alcoholic glycosides: The aglycone is an alcohol.
Aldehyde glycosides: The aglycone is an aldehyde.
Anthraquinone glycosides: The aglycone is an anthraquinone.
Steroidal glycosides (cardiac glycosides): The aglycone is a steroid.
Saponin glycosides: The aglycone is a triterpenoid or steroid saponin.
3. By Glycosidic Linkage
α-glycosides: The glycosidic bond involves the alpha anomer of the sugar.
β-glycosides: The glycosidic bond involves the beta anomer of the sugar.
Properties of Glycosides
Glycosides possess a range of physical and chemical properties, influenced by both the glycone and the aglycone components. Common properties include:
1. Solubility: Glycosides are generally soluble in water due to the presence of the hydrophilic sugar moiety. Their solubility in organic solvents varies depending on the nature of the aglycone.
2. Stability: Glycosides are relatively stable under normal conditions but can be hydrolyzed by acids, bases, or enzymes (glycosidases) to yield the sugar and aglycone.
3. Optical Activity: Many glycosides exhibit optical activity due to the chiral centers in the sugar moiety.
4. Taste: Glycosides can have various tastes, including bitter (e.g., cardiac glycosides), sweet (e.g., glycyrrhizin), and tasteless.
5. Biological Activity: Glycosides often have significant biological and pharmacological activities, such as cardiac glycosides used in heart disease treatment and saponins used for their detergent properties.
Tests for Identification
Several tests can be used to identify glycosides, based on their chemical and physical properties. Common tests include:
1. Hydrolysis Test: Hydrolyze the glycoside with an acid or enzyme to release the sugar and aglycone. Identification of the sugar can be done using chromatographic techniques.
2. Legal’s Test: Specifically used for the identification of cardiac glycosides.
Procedure: Dissolve the glycoside in pyridine, add sodium nitroprusside, and then add a few drops of sodium hydroxide.
Positive Result: Appearance of a red color indicates the presence of cardiac glycosides.
3. Keller-Kiliani Test: Also used for detecting cardiac glycosides.
Procedure: Treat the sample with glacial acetic acid containing ferric chloride, then add concentrated sulfuric acid.
Positive Result: Formation of a reddish-brown color at the interface and a blue-green color in the acetic acid layer.
4. Borntrager’s Test: Used for the detection of anthraquinone glycosides.
Procedure: Boil the glycoside with dilute sulfuric acid, extract with an organic solvent, and then add ammonia.
Positive Result: A pink or red color in the ammoniacal layer indicates anthraquinone glycosides.
5. Baljet Test: Used for the identification of cardiac glycosides.
Procedure: Treat the glycoside with sodium picrate solution.
Positive Result: Appearance of an orange-yellow color.
6. Liebermann’s Test: Used for steroidal glycosides.
Procedure: Dissolve the glycoside in chloroform, add acetic anhydride, and then add concentrated sulfuric acid.
Positive Result: A blue-green color indicates steroidal glycosides.
7. Foam Test: Used for saponin glycosides.
Procedure: Shake the aqueous solution of the glycoside vigorously.
Positive Result: Persistent foam indicates the presence of saponins.
These tests provide a combination of chemical reactions and observations that help in the identification and confirmation of glycosides in various samples.
