Introduction

• • The term 'alkaloid' or 'Pflanzenikalien' was coined by Meissner, a German pharmacist, in 1819.

  • The term is derived from the word 'alkali-like' and hence, they resemble some of the characters of naturally occurring complex amines.

  • These are the organic products of natural or synthetic origin which are basic in nature and contain one or more nitrogen atoms, normally of heterocyclic nature, and possess specific physiological actions on human or animal body, when used in small quantities.

True alkaloid

• • The true alkaloids are toxic in nature, contain heterocyclic nitrogen which is derived from amino acids and always basic in nature.

  • In plants true alkaloids are present in salts of organic acid form.

Proto-alkaloid or Amino alkaloid

• • The 'proto alkaloids' or 'amino alkaloids' are simple amines in which the nitrogen is not in a heterocyclic ring.

  • Some times, Proto-alkaloid are considered as biological amines. Alkaloids are formed from amino acid and thus basic in nature.

    • • • • • • Eg. mescaline, N, N-dimethyl tryptamine, colchicine, and ephedrine.

Pseudoalkaloid

• • The term 'pseudoalkaloids' includes mainly steroidal and terpenoid alkaloids and purines.

  • They are not derived from amino acids. They will not shows many characteristics like alkaloid, but give the standard qualitative tests for alkaloids.

    • • • • • • The examples of pseudoalkaloids are conessine and caffeine .

Physical Properties of Alkaloid

• All the alkaloids are

  • • • • Colourless,

        • Crystalline solids with a sharp melting point

        • Decomposition range except few.

        • Some alkaloids are amorphous gums,

        • Some are liquid and volatile in nature. eg. coniine, sparteine, nicotine etc.

        • Some alkaloids are coloured in nature, e.g. betanidin is red, berberine is yellow

• The free bases of alkaloids are soluble in organic non-polar, immiscible solvents.

• The salts of most alkaloids are soluble in water.

• In contrast, free bases are insoluble in water and their salts are also very sparingly soluble in organic solvents.

• Quaternary bases containing alkaloids are only soluble in water

• Some of the pseudoalkaloids and protoalkaloids show higher solubility in water.

  • • • • For example, colchicine is soluble in alkaline water, acid or water and caffeine (free base) is freely soluble in water.

Chemical Properties of alkaloid

• In alkaloid lone pair of electrons are present on nitrogen due to this alkaloid shows basic in reaction.

• Basicity will increased if adjacent functional groups are electron releasing.

• The alkaloid turns to be neutral or acidic when the adjacent functional groups are electron withdrawing like amide group.

• The alkaloids may contain one or more number of nitrogen and it may exist in the form as

  • • • • Primary (R - NH2), e.g. Mescaline;

        • Secondary amine (R2 - NH), e.g. Ephedrine;

        • Tertiary amine (R3N) e.g. Atropine: and

        • Quaternary ammonium compounds [R4N+Xl e.g. Tubocurarine chloride.

CHEMICAL TESTS FOR ALKALOIDS

1. Mayer's reagent containing potassium mercuric iodide solution giving cream coloured precipitate;

2. Dragendorff's reagent containing potassium bismuth iodide solution giving reddish brown precipitate;

3. Wagner's reagent containing iodine-potassium iodide solution yielding reddish brown precipitate.

4. Hagesr's reagent containing picric acid giving yellow coloured precipitates.

References:

1. Trease D. & Evans W.C.: Text Book of Pharmacognosy: W.B. Saunders.

2. Tyler V. E. Brady L. R. & Robbers J. E.: Pharmacognosy; Lea Feibger, USA.

3. Wallis T. E.; Text Book of Pharmacognosy; CBS Publishers, Delhi.

4. Kokate C. K., Purohit A. P. & Gokhale S. B.: Pharmacognosy; Nirali Publications, Pune.

5. Harbone J. B.: Phytochemical Methods: A guide to modern techniques Analysis: Chapman & Hall, London.

6. Bruneton J.: Pharmacognosy, Phytochemistry, Medicinal Plants: Intercept Limited.