What Is an Alkaloid? Classification of alkaloids

Vinblastine and Vincristine (Alkaloids)  obtained from Vinca rosea / Catharanthus roseus / Peri- winkle:
Morphine obtained from Papaver somniferumReserpine and Rescinnamine obtained from Rauwolfia serpentine
The alkaloids are defined as ‘basic nitrogenous plant products, mostly optically active and possessing nitrogen heterocyclic as their structural unit, with a pronounced physiological action. The term alkaloid was coined by W.Meibner. The first alkaloid to be synthesised was Coniine in1886.It was isolated in 1827.

Definition: An alkaloid is a cyclic organic compound containing nitrogen in a negative oxidation state which is of limited distribution among living organism”.
Example of true alkaloids are morphine first alkaloid), quinine,coniine (first alkaloid synthesised),reserpine etc.
Around 5000 alkaloids of all types are known to occur in 15% of all land plants and in more than 150 families. The important families are Apocyanaceae, Papaveraceae, Papilolnacea, Ranucunlaceae, Rubiaceae, Rutaceae, and Solanaceae.

Classification of alkaloids on the basis of their chemical nature is as follows:
  • Phenylalkaylamine ( Epherine, Pseudoephedrine, Taxine, Hordenine)
  • Pyrrolidines, Piperidines andpyridines
  • Tropane alkaloids (Hyoscamine, atropine and hyscine)
  • Quinoline alkaloids of Cinchona
  • Indole alkaloids (Apocyanaceae, Rubiaceae and Loganiaceae)
  • Purine alkaloids (Caffeine, Threobromine, methylated derivatives of Xanthine)
  • Tropolone alkaids (Colchicines)
  • Isoprenoid alkaloids
  • Diterpenoid alkaloids
In plants, alkaloids generally exist as salts of organic acids, glycosides of sugar etc.Alkaloids are soluble in water and extracted with alcohol. Various solvents are used for the separation of different classes of alkaloids and closely related products. Florescent and non -fluorescent alkaloids can be visualized by using TLC plates with or without fluorescent indication.
Alkaloids are biologically very useful. They are considered as reserve substances to supply nitrogen. They are also considered as poisonous substances to protect the plant itself from insects and animals for e.g. nicotine has insecticidal properties.
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