Introduction about Alkaloids
- Alkaloids comprise of a chemically diverse group of natural compounds with pharmaceutical importance.
- The term alkaloid is derived from the word “alkali-like”.
- The knowledge of alkaloids and their biological activities dates back to the start of civilization.
- Alkaloids are present in 20% of the plant species.
- The first alkaloid to be identified was morphine from the Opium Poppy by Serturner in 1806.
- Since then nearly 20000 different alkaloids have been isolated mostly from plants.
- Alkaloids are secondary metabolites.
- They are nitrogen-containing organic substances that are basic (alkaline) in nature and are generally synthesized from amino acid precursors and possess pharmaceutical properties.
- They have potent physiological effects on mammalian systems as well as other organisms.
Classification of alkaloids
The chemical classification of alkaloids is based on the type of ring structure.
1. The True alkaloids are toxic in nature, contain nitrogen in a heterocyclic ring, which is derived from the amino acid precursor, are basic or alkaline, and occur as salts of organic acids in plants. Most true alkaloids are derived from amino acids such as phenylalanine, tyrosine, tryptophan, ornithine, and lysine e.g. Morphine, Atropine.
2. The proto-alkaloids or amino alkaloids are simple amines in which nitrogen is not present in a heterocyclic ring. They are alkaline and derived from amino acids e.g. Colchicine, Ephedrine.
3. The pseudo-alkaloids include steroidal and terpenoid alkaloids and purines. They are not derived from amino acids and don’t possess the properties of alkaloids but give standard qualitative tests of alkaloids e.g. Caffeine, Theobromine.
Properties of Alkaloids
1. Most of the alkaloids are colorless, crystalline solids. However, some alkaloids are volatile, colorless liquids eg. Nicotine, Coniine. Some alkaloids are colored, like berberine (yellow) and sanguinarine (orange).
2. Most of the alkaloids dissolve poorly in water but are readily soluble in organic solvents like chloroform, diethyl ether, etc. Some of the pseudoalkaloids and proto alkaloids show higher solubility in water. Colchicine is soluble in alkaline water, acid, or water. Caffeine is freely soluble in water.
3. Alkaloids are chemically alkaline (basic in reaction) due to the presence of lone pair of electrons on nitrogen. As they are alkaline, they form salts with acids.
Role of alkaloids in plants
The alkaloids are poisonous in nature, but when used in small quantities they exert a physiological effect on humans and animals so are pharmaceutically important. The possible role of alkaloids in plants are-
1. Defence – Alkaloids serve as defensive agents and protect plants from grazing animals, herbivores, and insects.
2. Reserve Substance – Alkaloids serve as reserve substances capable of supplying nitrogen to the plant.
3. End product of detoxification – It is possible that alkaloids are the end product of the detoxification mechanism in plants thereby keeping a check on the formation of harmful substances within the plant.