ALKALOIDS

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. 

Alkaloids can be classified;

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