There are three main types of alkaloids:

Colchicine is an example of a proto-alkaloid.

Pseudo-alkaloids can be derived from;

• Terpenoids or
• Purines

The basic unit in the biogenesis of the true alkaloids are AMINO ACIDS. The non-nitrogen containing rings or side chains are derived from TERPENE units and / or ACETATE, while METHIONINE is responsible for the addition of methyl groups to nitrogen atoms.

Alkaloids are highly reactive substances with biological activity in low doses.

DEFINITION

1. Contains nitrogen - usually derived from an amino acid.
2. Bitter tasting, generally white solids (exception - nicotine is a brown liquid).
3. They give a precipitate with heavy metal iodides.
   • Most alkaloids are precipitated from neutral or slightly acidic solution by Mayer's reagent (potassiomercuric iodide solution). Cream coloured precipitate.
   • Dragendorff's reagent (solution of potassium bismuth iodide) gives orange coloured precipitate with alkaloids.
   • Caffeine, a purine derivative, does not precipitate like most alkaloids.
4. Alkaloids are basic - they form water soluble salts. Most alkaloids are well-defined crystalline substances which unite with acids to form salts. In plants, they may exist
   • in the free state,
   • as salts or
   • as N-oxides.
5. Occur in a limited number of plants. Nucleic acid exists in all plants, whereas, morphine exists in only one plant species.

• in terms of their BIOLOGICAL activity,
• CHEMICAL structure (nucleus containing nitrogen),
• BIOSYNTHETIC pathway (the way they are produced in the plant).