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).

04/09/23 1Dr. Harish Kakrani & Purvi Kakrani

04/09/23 2Dr. Harish Kakrani & Purvi Kakrani

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.

04/09/23 3Dr. Harish Kakrani & Purvi Kakrani

Types of hydrolysable tannin:

A- GallitanninOn 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.

Gallic acid

COOH

OH

OH

HO

OH

OH

HO

Pyrogallol

OH

OH

OH

HO

OH

COOHHO

HOOC

Hexahydroxydiphenic acid

O

OO

O

OH

OH

OH

HO

Ellagic Acid

04/09/23 4Dr. Harish Kakrani & Purvi Kakrani

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.Catechin:Flavan-3-ol and leucocyanidin:

flavan-3,4-diol.04/09/23 5Dr. Harish Kakrani & Purvi

Kakrani

OHO

OH

OH

OH

OH

(+) Catechin (catechol)

OHO

OH

OH

OH

OH

OH

Flavan-3,4-diol structure

OHO

OH

OH

OH

OH

OHO

OH

OH

OH

OH

A dimeric structure04/09/23 6Dr. Harish Kakrani & Purvi Kakrani

Pseudotannins 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: ipecacuanha04/09/23 7Dr. Harish Kakrani & Purvi Kakrani

Gallic acid

COOH

OH

OH

HO

Catechin

OHO

OH

OH

OH

OH

Chlorogenic acid

O

O

OH

HO

OHHOOC

OH

OH

04/09/23 8Dr. Harish Kakrani & Purvi Kakrani

Identification of tannins

1- tannin + FeCl3 → bluish black colour with hydrolysable tannin and brownish black colour with catechol.

2- Gelatin test: solution of 1% tannin precipitate gelatin from its solution at conc. 1%.

3- Gold beater's skin test: soak a small piece of Gold beater's skin membrane in 2% HCl, rinse with H2O, place the piece for 5 minutes in the tannin solution to be tested, wash with H2O, transfer to 1% FeSO4 solution, a brown colour is formed.

4- Bromine water: only condensed tannin gives ppt. with bromine water.

04/09/23 9Dr. Harish Kakrani & Purvi Kakrani

Quantitative determination of tannin

1- Hide powder method:The difference in the dry weight of the

extract before and after treatment with hide powder is taken as a measure for tannin content.

2- Copper acetate method:Gravimetric method using copper acetate

as precipitant.04/09/23 10Dr. Harish Kakrani & Purvi

Kakrani

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.

04/09/23 11Dr. Harish Kakrani & Purvi Kakrani

Tannin containing drugs Galls

(Blue Galls, Aleppo Galls, Turkish Galls)Origin: dried excrescence

(pathological outgrowth) resulting from the deposition of the eggs of Cynips gallae tinctoria F. Cynipidae on the young twigs of Quercus infectoria F. Fagaceae. Collected before the escape of the insect and known as blue galls.

Morphology: subspherical, 12-20 mm in diameter, external surface is bluish green to olive green in colour, 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.

04/09/23 13Dr. Harish Kakrani & Purvi Kakrani