Page 121 - Medicinal Plants_PharmD general

III. Tannins

Definition:

They are water soluble polyphenol compounds presents in plant extracts
having the following features:
1-They have high molecular weight ranging from 500-5000 amu
2-They possess 12-16 phenolic groups and 5-7 aromatic rings per 1000
relative molecular mass
3-They are able to combine with protein of animal hides prevent their
putrefaction and convert them into leather.
4-They give positive Gold beater's skin test

They are classified into two main groups:

1. Hydrolysable Tannins:

- These may be hydrolysed by acids or enzymes such as tannase.

- They are formed from several molecules of phenolic acids such as gallic

and ellagic acids which are united by ester linkage to a central glucose

residue

- Their solution turn blue with iron salts.

- They were formerly known as pyrogallol tannins since on dry distillation,

gallic acid and similar compounds are converted into pyrogallol.

OH OH OH
HO OH HO OH HO OH

Pyrogallol COOH COOC6H11O5

Gallic acid Glucoside of gallic acid

Examples of drugs containing hydrolysable tannins are:
Aleppo galls, hamamelis bark, clove, Uva ursi and pomegranate bark.

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