Tannins: Hydrolysable vs Condensed

• Tannins are complex organic, non-nitrogenous plant products
• They are derivatives of polyhydroxy benzoic acid (polyphenols)
• Tannins have the ability to precipitate proteins
• They have a high molecular weight (500 to > 20000)
• Historically, tannin containing drugs are related to their tanning property, meaning their ability to transform fresh hides into imputrescible leather

Properties and Tests

• Tannins are soluble in water, dilute alkalis, alcohol, glycerol, and acetone
• They are generally only sparingly soluble in other organic solvents
• Tannin solutions precipitate heavy metals, alkaloids, glycosides, and gelatin
• Gallitannins and ellagitannins give blue-black precipitates with ferric salts
• Condensed tannins give brownish-green precipitates with ferric salts

Chemical Tests

• Goldbeater's Skin Test:
  • Soak a small piece of goldbeater's skin in 2% hydrochloric acid
  • Next, rinse with distilled water and place in the solution to be tested for 5 minutes
  • Then, 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, behaving similarly to an untanned hide
• Phenazone Test:
  • Add 0.5 g of sodium acid phosphate to 5 ml of aqueous solution of a tannin-containing drug
  • Warm, cool, and filter the resulting solution
  • Add 2% phenazone solution to the filtrate
  • This precipitates all tannins as a bulky, colored precipitate
• Gelatin Test:
  • Add a little 10% sodium chloride to a 1% gelatin solution
  • When a 1% solution of tannin is added to the gelatin solution, it results in the precipitation of gelatin
• Bromine Water:
  • Only condensed tannins give a precipitate with bromine water

Uses of Tannins

• Industrially in leather tanning and ink manufacture
• Medicinally as an astringent, hemostatic, antidiarrheal, and as an antidote in alkaloid and heavy metal toxicity
• In labs as a reagent for detection of proteins, alkaloids, and heavy metals due to their precipitating properties

Classification

• Based on the goldbeaters skin test, tannins are classified as either True Tannins (positive tanning test) or Pseudotannins (negative tanning test)

True Tannins

• True tannins can be divided into Hydrolysable tannins and Non-Hydrolysable (or Condensed) tannins

Hydrolysable Tannins

• These consist of esters of sugars and phenolic acid molecules
• They can be broken down by mineral acids or tannase
• The phenolic acid in hydrolysable tannins can be either gallic acid or ellagic acid
• Gallotannins: Tannins where the phenolic acid component is gallic acid
  • Examples include rhubarb, gall, and clove
• Ellagitannins: Tannins where the phenolic acid component is ellagic acid
  • Examples include pomegranate and eucalyptus
• On dry distillation, hydrolysable tannins convert to pyrogallol derivatives
• Hydrolysable tannins are soluble in water and produce a blue color in solution with ferric chloride

Condensed Tannins

• Consist of flavan-3-ol units linked together by carbon-carbon bonds
• The bonds usually occur at the 4, 6, or 8 positions
• Condensed tannins occur due to polymerization (condensation) reactions between flavonoids
• Polymers may include up to 50 monomer units
• Treatment with acids or enzymes converts condensed tannins into red insoluble compounds known as phlobaphenes
• Phlobaphenes give the characteristic red colour to drugs such as red cinnamon bark
• On dry distillation, condensed tannins yield catechol derivatives
• These tannins are soluble in water and produce a green color with ferric chloride

Pseudotannins

• Pseudotannins are compounds of lower molecular weight than true tannins
• They do not respond to the goldbeater's skin test
• Examples of drugs containing Pseudotannins:
  • Gallic acid: Rhubarb
  • Catechins: Guarana, Cocoa
  • Chlorogenic acid: Mate, Coffee
  • Ipecacuanhic acid: ipecacuanha

Hydrolysable Tannin Containing Drugs

• Myrobalan: Mature dried fruits of Terminalia chebula (Combretaceae)
  • Contains about 30% hydrolysable tannins, including chebulinic acid, chubulagic acid, and D-galloyl glucose
  • Contains free tannic acid, gallic acid, ellagic acid, and resin myrobalanin
  • Due to antiseptic and healing properties of tannins, myrobalan is used externally in chronic ulcers, wounds, and piles, and as a stomachic
  • Fine powder of myrobalan is used in dental preparations
• Nutgalls: Excrescence obtained from the young twigs of the Dyer's Oak (Quercus infectoria Olivier, Fagaceae)
  • This excrescence is caused by the puncture of ovums of insect Cynips tinctoria or Adleria gallaetinctoriae Olivier (Cynipidae), and is obtained principally from Asiatic Turkey
  • Contains about 50%-70% tannin mainly gallotannic acid, the official tannic acid
  • Nutgall is used in tanning and dyeing, in the manufacture of ink, and medicinally as a local astringent in ointments and suppositories
• Pomegranate Rind: Dried pericarp of the fruit of Punica granatum (Punicaceae
  • Used in India as a herbal remedy for non-specific diarrhea and is highly astringent
  • Contains about 28% of tannin (ellagitannins) and coloring matters
  • Distinguished from the root bark, which contains alkaloids

Drugs with Condensed Tannins

• Pale Catechu (Gambier): A dried aqueous extract produced from the leaves and young twigs of Uncaria gambier Roxburgh (Rubiaceae) native to Malaya, and cultivated in Indonesia, Malaysia, Sumatra, Borneo, and Singapore
  • Contains 7% to 30% of pseudotannin catechin and 22% to 55% of a phlobatannin catechutannic acid
  • Medicinally used as a local astringent and as a general astringent in cases of diarrhea
  • Gambier is used in dyeing and tanning industries. Is used for tanning animal hides to convert to leather
• Black Catechu: Dried aqueous extract prepared from the heartwood of Acacia catechu Willdenow (Fabaceae/Leguminosae), which is native to India
  • Resembles pale catechu or gambier in its composition
  • Contains about 2%-12% of catechin and about 25% to 33% of phlobatannin catechutannic acid
  • Used in medicine as an astringent to cure mouth troubles, throat diseases, and diarrhoea
  • It increases appetite and used for dyeing fabrics brown or black

Explore the characteristic properties, types, and sources of tannins, differentiating between hydrolysable and condensed tannins. Learn about black and pale catechu, their production, and the compounds they contain. Test your knowledge of tannin classifications and occurrences in various drugs and natural sources.