tannins

Tannins are "phenolic natural products that precipitate proteins from their aqueous solutions".

Classification:

1- Hydrolysable tannin (pyrogallol tannin):
Classified into gallitannin and ellagitannin

2- Condensed tannin (catechol tannin).
1- Hydrolysable Tannin
Members of this class consist of several molecules of phenolic acids united by ester linkage to a central glucose residue. Such esters are readily hydrolyzed by acids or enzymes so called hydrolysable tannins.
Types of hydrolysable tannin:

A- Gallitannin
On hydrolysis , it gives gallic acid and glucose.
Present in rhubarb, galls, clove, hamamelis.

B- Ellagitannin
On hydrolysis it gives ellagic acid and glucose
Present in pomegranate and eucalyptus.
2- Condensed tannin
On dry distillation they give catechol.
They differ from pyrogallol tannin where their structures are related to flavonoids and they lack sugar in their structures
Present in cinnamon, cinchona, tea
Pseudotannins
They are compounds of lower molecular weight than true tannins
Examples of drugs containing Pseudotannins are:
Gallic acid: Rhubarb
Catechins: Guarana, Cocoa
Chlorogenic acid: Mate, Coffee
Ipecacuanhic acid: ipecacuanha
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.