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 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
negative
tanning test
Psedutannin
True tannin
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True tannins
Hydrolysable
tannins
Non Hydrolysable
tannins
Hydrolysable tannins

These are ester of sugars and phenolic
acid molecules
Phenolic
acid
Sugar
part
Hydrolysed by
 Mineral acids
 Tannase
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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
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
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.
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
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: Cocoa
Chlorogenic acid: Coffee.
OH
COOH
HO
O
OH
HO
OH
OH
OH
OH
Catechin
Gallic acid
HOOC
OH
OH
O
HO
O
OH
OH
Chlorogenic acid
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
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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.
Tannin containing drugs
Galls
(Blue Galls, Aleppo Galls, Turkish Galls)
Origin: dried pathological outgrowth resulting from the deposition of the eggs of Cynips gallae
tinctoria F. Cynipidae on the young twigs of Quercus infectoria F. Fagaceae. and known as blue
galls.
Morphology: subspherical, 12-20 mm in diameter, external surface is bluish green to olive green
in color, showing blunt short projections. Internally it is buff in colour.
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Active constituents: 50-70% pyrogallol tannin: gallotannic acid, gallic acid and ellagic acid.
Uses: Medicinally: astringent, haemostatic used in treatment of hemorrhoids in form of supp. or
oint.
Industrially: 1- in manufacture of ink and paints
2- in tanning of leather.
Chemical test: FeCl3 → bluish black colour.
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