Glycoside

• These are the organic compounds from plants or animal sources which on enzymatic or acid hydrolysis give one or more sugar moieties along with nonsugar moiety.

  • • • Sugar moiety is known as glycone and the non sugar moiety is known as aglycone or genin.

      • Chemically, they are the acetals or sugar ethers, formed by interaction of hydroxyl group each of non-sugar and sugar moiety, with a loss of water molecule.

      • The hydroxyl group of aglycone may be alcoholic or phenolic and in some cases from amines also.

      • The sugars involved in glycosides are of different types, but most commonly, it is β - D - glucose. The other sugars found are galactose, mannose, rhamnose, digitoxose, cymarose, etc.

      • The linkage between glycone and aglycone is called glycosidic linkage

Classification of Glycoside

The classification of glycosides is based either on the chemical nature of aglycone part or therapeutic activity exhibited by the same

According to chemical nature of aglycone moiety, they are grouped into:

• 1. Anthraquinone or anthracene glycosides Examples: Senna leaf, Aloe, Cascara, Rhubarb

  2. Sterols or cardiac glycosides, Examples: Digitalis, Thevitia, Indian Squill

  3. Saponin glycosides, Examples: Dioscorea, Glycyrrhiza, Shatavari, momordica

  4. Cyanogenetic or cyanophoric glycosides, Examples: Bitter almond, wild cherry bark

  5. Isothiocynate glycosides, Examples: Mustard

  6. Flavanoids Flavonol glycosides, Examples: Buck wheat, Gingko, Milk Thistle

  7. Coumarin and Furanocoumarin glycosides, Examples: Ammi, Psoralea, Visnaga.

  8. Aldehyde glycosides, Examples: Vanilla, Anantmul

  9. Phenol glycosides, Examples: Bearberry

  10. Steroidal glycolkaloids, Examples: Solanum

  11. Glycosidal bitters or miscellaneous glycosides. Examples: Gentian, Kalmegh, Quassia.

According to the linkage across glycone and aglycone part, they are grouped into:

• • 1. C-glycosides (when sugar moiety is linked to carbon atom) : eg. Aloe and cochineal

Glycone -OH + HC-aglycone========>glycone - C - aglycone + H₂0

2. O - glycosides (when sugar moiety is attached to oxygen atom) : eg. Rhubarb, senna

Glycone -OH + HO-aglycone========>glycone - O - aglycone + H₂0

3. S - glycosides (when sugar is linked to sulphur atom) : eg Black Mustard

Glycone -OH + HS-aglycone========>glycone - S - aglycone + H₂0

4. N - glycosides (when sugar is linked to Nitrogen atom) :

Glycone -OH + HN-aglycone========>glycone - N - aglycone + H₂0

Physical and Chemical Properties of Glycosides

• • • The glycosides contain sugars but still the physical, chemical and therapeutic properties are dictated by aglycone part.

    • The sugar moiety facilitates absorption of glycosides and helps in transportation of aglycone portion at the site of action.

    • Glycosides are crystalline or amorphous substances

    • Soluble in water and dilute alcohol with an exception of resin glycosides, but insoluble in organic solvent like chloroform or ether.

    • The aglycone moiety is soluble in non-polar solvents like benzene or ether.

    • Glycosides are easily hydrolysed by water, mineral acids and enzymes.

    • They show optical activity, normally with laevo rotatory effects.

    • Glycosides do not reduce Fehling's solution until they are hydrolysed.

    • They are believed to participate in growth regulation and protection of plant.

References:

1. Trease D. & Evans W.C.: Text Book of Pharmacognosy: W.B. Saunders.

2. Tyler V. E. Brady L. R. & Robbers J. E.: Pharmacognosy; Lea Feibger, USA.

3. Wallis T. E.; Text Book of Pharmacognosy; CBS Publishers, Delhi.

4. Kokate C. K., Purohit A. P. & Gokhale S. B.: Pharmacognosy; Nirali Publications, Pune.

5. Harbone J. B.: Phytochemical Methods: A guide to modern techniques Analysis: Chapman & Hall, London.

6. Bruneton J.: Pharmacognosy, Phytochemistry, Medicinal Plants: Intercept Limited.