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There are three main types of alkaloids:
Colchicine is an example of a proto-alkaloid.
Pseudo-alkaloids can be derived from;
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.
Alkaloids can be classified;
- Contains nitrogen - usually derived from an amino acid.
- Bitter tasting, generally white solids (exception - nicotine is a brown liquid).
- 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.
- 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.
- 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).
Georges-Louis Friedli, PgDip., MSc., CH(t)., MACN., PhD.
Last Updated September 1996