Download 2-2.1 tannins - PharmaStreet

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 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.
Classification of tannins
• based on Goldbeaters skin test
positive
tanning test
True tannin
negative
tanning test
Psedutannin
True tannins
Hydrolysable
tannins
Non Hydrolysable
tannins
Hydrolysable tannins
• These are ester of sugars and phenolic acid molecules
• Mineral acids
• Tannase
Phenolic acid
Sugar
part
• 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
• 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.
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: ipecacuanha
OH
COOH
HO
O
OH
HO
OH
OH
OH
OH
Catechin
Gallic acid
HOOC
OH
O
HO
O
OH
Chlorogenic acid
OH
OH
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
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.
THANK YOU
-PHARMA STREET