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Tannins
- 3. • Tannins are naturally occurring complex organic compounds possessing nitrogen free polyphenols of high molecular weight. • They form colloidal solution with water giving acid reactions. • They also precipitate proteins and alkaloids.
- 4. • The astringent in nature of tannins is due to the fact that they can precipitate proteins and render them resistant to enzymatic attack. When applied on a wound or injury, tannins form a protective coating so as to prevent external irritation and thus promote healing.
- 5. Properties of tannins: • Tannins are freely soluble in water, alcohol, glycerol, and acetone and dilute alkalies. • They are sparingly soluble in chloroform, ethyl acetate and other organic solvents. • They have an astringent taste. • They combine with alkaloids to form tannates, most of which are insoluble in water.
- 6. Classification of tannins: 1. Hydrolysable tannins 2. Condensed tannins 3. Pseudotannins.
- 7. 1. Hydrolysable tannins: • These tannins are hydrolyzed by acids, or enzyme and produce gallic acid and ellagic acid. • Chemically, these are esters of phenolic acid like gallic acid and ellagic acid. The tannins derived from gallic acid are known as gallitannins and from that of ellagic acid are known as ellagitannins. • The gallic acid is found in rhubarb, clove and ellagic acid is found in eucalyptus leave and myrobalans and pomegranate bark. • These tannins treated with ferric chloride to produced blue or black colour.
- 9. 2. Condensed tannins: • These tannins are resistant to hydrolysis and they derived from the flavonols, catechins and flavan-3, 4-diols. • On treatment with acids or enzymes they are decomposed into phlobaphenes(give characteristic red color to many drugs). • On dry distillation condensed tannin produce catechol. These tannins are called as catechol tannins. • These tannins are found in cinchona bark, male fern, areca seeds, tea leaves and wild cherry bark, bahera fruits, Amla, etc. they produce green colour with ferric chlorides.
- 11. 3. Pseudotannins: • They are phenolic compounds of lower molecular weight and do not show the goldbeater’s test. They are found in catechu and nux- vomica, etc. (i) Catechins: Catechu, cocoa, etc (especially in dead tissues and dying cells) (ii) Ipecacuanhic acid: Ipecacuanha (iii) Chlorogenic acid: Coffee and nux-vomica
- 12. CHEMICAL TESTS 1. Goldbeater’s skin test: (main test) • Goldbeater’s skin is a membrane produced from the intestine of Ox. It behaves just like untanned animal hide. • A piece of goldbeaters skin previously soaked in 2% hydrochloric acid • washed with distilled water is placed in a solution of tannin for 5 minutes. • then washed with distilled water and transferred to 1% ferrous sulphate solution. • change of the colour of the goldbeater’s skin to brown or black indicates the presence of tannin.
- 14. • Hydrolysable and condensed tannins both give the positive goldbeater’s test, whereas pseudotannins show very little colour or negative test.
- 15. Test for Catechin (Matchstick Test): Catechin test is the modification of the well-known phloroglucinol test for lignin. Matchstick contains lignin. Dip a matchstick in the dilute extract of the drug, dry, moisten it with concentrated hydrochloric acid, and warm it near a flame. Catechin in the presence of acid produces phloroglucinol which stains the lignified wood pink or red.
- 16. ISOLATION 1. Both hydrolysable and condensed tannins are highly soluble in water and alcohol but insoluble in organic solvents such as ether, chloroform, and benzene. 2. Tannin compounds can be easily extracted by water or alcohol. The general method for the extraction of tannic acid from various galls is either with water-saturated ether, or with mixture of water, alcohol, and ether.
- 17. 3. In such cases, free acids such as Gallic and ellagic acid go along with ether, whereas true tannin gets extracted in water. If the drug consists of chlorophyll or pigment, it may be removed by ether. After extraction, the Aqueous and ethereal layers are separately concentrated, dried, and subjected to further isolation and purification using various separation techniques of chromatography.
- 18. MEDICINAL PROPERTIES AND USES 1. Tannins are medicinally significant due to their astringent properties. 2. Tannins are used in the treatment of haemorrhoids, minor burns, frostbite, as well as inflammation of gums. 3. Internally tannins are administered in cases of diarrhoea, intestinal catarrh, and in cases of heavy metal poisoning as an antidote.
- 19. MEDICINAL PROPERTIES AND USES 4. They are widely used in the leather industry for conversion of hide into leather, the process being known as tanning. 5. Tannins are also used for clarifying beer or wine, in photography or as a coagulant in rubber manufacture. 6. Tannins are used for the manufacture of gallic acid and pyrogallol, and sometimes as a reagent in analytical chemistry.