Tannins

Complex organic, non-nitrogenous plant products.

Ability to precipitate proteinsHaving 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.

These are ester of sugars and phenolic acid molecules

Mineral acidsTannase

Sugar part

Phenolic acid

Phenolic acid either Gallic acid or Ellagic acid

Gallotannins

Ellagitannins e.g: •rhubarb•Gall•Clove

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.

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.

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: RhubarbCatechins: Guarana, CocoaChlorogenic acid: Mate, CoffeeIpecacuanhic acid: ipecacuanha

Gallic acid

COOH

OH

OH

HO

Catechin

OHO

OH

OH

OH

OH

Chlorogenic acid

O

O

OH

HO

OHHOOC

OH

OH

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 testSoak 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

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.