There are three types of alkaloids in pharmacognosy based on their biochemical nature and type of nitrogen.
Alkaloids are nitrogenous organic molecules with complex ring structures, that have pharmacological effects on humans and animals. These base compounds naturally occur in seed-bearing plants.
Generally, alkaloids are derivatives of amino acids and sometimes be poisonous as well. This is because plants produce alkaloids as a product of the detoxification of toxic substances that are produced by metabolic reactions. Apart from this, plants produce alkaloids as a nutrient reserve and as a protection against grazing animals.
Although alkaloids occur in plants, fungi, microbes, etc, the major portion comes from the dicots, especially from the members of the families- Apocynaceae, Fabaceae, and Papaveraceae,
There are over 27000 known alkaloids and 21000 of them come from flowering plants.
Properties of Alkaloids
- Alkaloids are colorless, basic, crystalline solids with a heterocyclic structure and definite melting point.
- They taste bitter and are thus used as a defense mechanism.
- Some alkaloids are in a liquid state such as nicotine.
- Betanidine, berberine, and sanguinarine are colored alkaloids.
- Alkaloids are mostly present as isomers.
- While one isomer may have physiological significance the other may not have any notable physiological importance.
- Basic alkaloids are insoluble in water but soluble in organic solvents.
- However, strychnine hydrochloride which is acidic is more soluble in water than strychnine the base.
- Alkaloids are derivatives of amino acids such as anthranilic acid, histidine, lysine, ornithine, phenylalanine, tyrosine, tryptophan, etc.
Chemical Tests for Alkaloids
- Dragendorff’s Test produces an orangish red color.
- Mayer’s Test forms a creamy-white precipitant.
- Hager’s Test produces a crystalline yellow precipitate.
- Wagner’s Test forms a reddish-brown precipitate.
- Tannic Acid forms a buff-colored precipitate.
Types of Alkaloids in Pharmacognosy
Alkaloids are classified based on their precursors, chemical structures, or the biological pathway used for their synthesis. The types of alkaloids in pharmacognosy based on their chemical structure are,
- Pyridine- Piperidine
- Tropane
- Quinoline
- Isoquinoline
- Indole
- Imidazole
- Steroid
- Alkaloidal amines
- Purine
Based on their chemical structure alone, alkaloids are classified into three main classes
- True alkaloids: Derivative of amino acids with a heterocyclic ring
- Proto alkaloids: Derivatives of amino acids but the nitrogen atom is not part of the heterocyclic ring.
- Pseudo alkaloids are non-amino acid derivatives, connected to amino acid pathways.
True Alkaloids
- True alkaloids are amino acid derivatives.
- They are biologically reactive compounds even in smaller doses. Eg., cocaine, dopamine, morphine, and quinine.
- They appear white and have a bitter taste.
- Nicotine is the exception which appears brown.
- True alkaloids form water-soluble salts with crystalline form.
Proto Alkaloids
- Proto alkaloids are mainly derived from tryptophan and tyrosine.
- They have a closed ring but the nitrogen atom is not part of the ring.
- It is a small portion of alkaloids.
- Stachydrine and 4-hydroxystachydrine obtained from Boscia anguistifolia are examples of proto-alkaloids. They have a pyridine nucleus and are used for treating pain, mental illness, and neuralgia.
- Other Examples of proto alkaloids are, hordenine, mescaline, and yohimbine.
Pseudo Alkaloids
- Pseudo-alkaloids have a basic carbon skeleton and are connected to amino acid pathways.
- They are derivatives of precursors and postcursors of amino acids after transamination and amination processes.
- They are also formed from non-amino acid precursors by the insertion of nitrogen atoms.
- Pseudo alkaloids can be derived from terpenoids, steroidal, acetate, or phenylalanine.
- Examples of pseudo-alkaloids include caffeine, capsaicin, conine, ephedrine, theobromine, solanidine, etc.
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