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Tannins • Complex organic, non-nitrogenous plant products. • Derivatives of polyhydroxy benzoic acid (polyphenols). • Ability to precipitate proteins • Having high molecular weight (500 to > 20000). • Historically, tannin containing drugs are related to their tanning property i.e. their ability to transform fresh hides into imputrescible leather. • The consequence of tanning is the formation of bonds between the collagen fibers in the hide, which imparts resistance to water, heat, and abrasion. • This capability of tannins to combine with macromolecules explains why they precipitate cellulose, pectins, and proteins • It also explains their characteristic astringency and tartness: by precipitating the glycoproteins contained in saliva, tannins make the latter lose its lubricating power. Classification of tannins • based on Goldbeaters skin test positive tanning test True tannin negative tanning test Psedutannin True tannins Hydrolysable tannins Non Hydrolysable tannins Hydrolysable tannins • These are ester of sugars and phenolic acid molecules • Mineral acids • Tannase Phenolic acid Sugar part • Phenolic acid either • Gallic acid or Ellagic acid Gallotannins e.g: • rhubarb • Gall • Clove Ellagitannins e.g: • pomegranate • eucalyptus • on dry distillation are converted to pyrogallol derivatives. • The hydrolysable tannins are soluble in water and their solution produces blue color with ferric chloride. Condensed tannins • Proanthocyanidins • They consist of flavan-3-ol units linked together by carbon-carbon bonds • most often 4 8 or 4 6 • Condensed tannins occur due to polymerization (condensation) reactions between flavonoids. • The polymers may include up to 50 monomer units. • On treatment with acids or enzymes condensed tannins are converted into red insoluble compounds known as phlobaphenes. • Phlobaphenes give the characteristic red colour to many drugs such as red cinnamon bark. • On dry distillation, they yield catechol derivatives. • Condensed tannins are also soluble in water and produces green color with ferric chloride. Pseudotannins • They are compounds of lower molecular weight than true tannins and they do not respond to the goldbeater's skin test. • Examples of drugs containing Pseudotannins are: Gallic acid: Rhubarb Catechins: Guarana, Cocoa Chlorogenic acid: Mate, Coffee Ipecacuanhic acid: ipecacuanha OH COOH HO O OH HO OH OH OH OH Catechin Gallic acid HOOC OH O HO O OH Chlorogenic acid OH OH Properties and tests of tannins • Tannins are soluble in water, dilute alkalis, alcohol, glycerol and acetone, but generally only sparingly soluble in other organic solvents. • Solutions precipitate heavy metals, alkaloids, glycosides and gelatin. • With ferric salts, gallitannins and ellagitannins give blue-black precipitates and condensed tannins brownish-green ones. 1. Goldbeater's skin test Soak a small piece of goldbeater's skin in 2% hydrochloric acid; rinse with distilled water and place in the solution to be tested for 5 min. Wash with distilled water and transfer to a 1% solution of ferrous sulphate. A brown or black colour on the skin denotes the presence of tannins. Goldbeater's skin is a membrane prepared from the intestine of the ox and behaves similarly to an untanned hide. • 2- Phenazone Test: • To 5 ml of aqueous solution of tannin containing drug, add 0.5 g of sodium acid phosphate. Warm the solution, cool and filter. Add 2 % phenazone solution to the filtrate. All tannins are precipitated as bulky, colored precipitate. • 3- Gelatin Test: • To a 1 % gelatin solution, add little 10 % sodium chloride. If a 1 % solution of tannin is added to the gelatin solution, tannins cause precipitation of gelatin from solution. • 4- Bromine water: • only condensed tannin gives ppt. with bromine water Uses of Tannins • Industrially; in leather tanning, ink manufacture. • Medicinally; astringent, haemostatic, antidiarrheal, antidote in alkaloid and heavy metals toxicity. • In labs; reagent for detection of proteins, alkaloids and heavy metals because of their precipitating properties. THANK YOU -PHARMA STREET