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Purine alkaloids
médecine générale : pharmacologie
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Introduction
Purines nucleus is a heterocyclic nucleus consisting of
pyrimidine ring fused to 5-membered imidazole ring
known as xanthine.
Amphoteric Character
Purines unlike other alkaloids do not give positive
results with general tests of alkaloids; instead
murexide test (Purple Color) is used in its
identification.
Purines are present as methylated compound, which
are:-
1) Caffeine (1, 3, 7-tri methylxanthine).
2) Theophylline (1, 3,-dimethylxanthine).
3) Theo bromine (3, 7-dimethylxanthine).
Generally the pharmacological activities of
the Purine Alkaloids
1) Stimulation of the CNS.
2) Diuretic effect.
3) Increase gastric acid secretion.
4) Relaxation of the bronchial smooth muscle
(theophylline).
5) Positive inotropic and chronotropic effect on the
heart.
The most important plants in this group
are:-
1) Coffee ( Coffea arabica of the family Rubiaceae)
contain about 1-2% of caffeine.
2) Tea ( Camellia sinensis of the family Theaceae)
contain about 1-4% of caffeine.
3) Cola ( Cola nitida of the family Sterouliaceae)
contain about3% of caffeine.
Isolation Of Caffeine From Tea
AIM: - To isolate the caffeine alkaloid from tea.
Equipments and reagent:-
Large beaker and small beaker
Water bath
Muslin
Centrifuge test tubes
Evaporating dish
Boiling water
Separatory funnel
Basic lead acetate
Sulphuric acid
Decolorized charcoal
Chloroform
Hot 60C° ethanol
50 gm. of coarsely powder tea
250 ml of water and boil gently for
15 min.
Strain the resulting hot extract
through muslin
Wash the mass remains on the
muslin with 250 ml of boiling
water and express again.
Add solution of basic lead acetate
to the filtrate until no more
precipitate is formed.
Heat the mixture to boiling and
place rapidly in a centrifuge test
tubes for 5min.
Collect the supernatant result,
heated to boiling.
Add dilute Sulphuric acid until
precipitation ceases.
1
2
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4
5
6 Add 1gm of decolorized charcoal
to the filtrate then evaporated over
a low Bunsen flame with frequent
stirring until a volume of 75-100 ml
is reached.
7
8
####### 8) Approximately 1gm of decolorized charcoal is added to the filtrate which
####### is then evaporated over a low Bunsen flame with frequent stirring until a
####### volume of 75-100 ml is reached.
####### 9) Evaporation of the large volume of the liquid may be carried out in
####### portions in a smaller evaporating dish.
####### 10) The solution is filtered hot, returning the first portion collected back
####### through the filter until a clear filtrate is obtained.
####### 11) Cool, transfer to a separatory funnel and shake out with three successive
####### 10 ml portion of chloroform.
####### 12) Transfer the combined chloroform extract to a small evaporating dish.
####### 13)Carefully evaporate the chloroform on steam bath in the hood.
####### 14)Scrap out the residue, transfer to a small beaker and dissolve in the
####### smallest quantity of hot 60 c ethanol necessary to affect the solution.
####### 15) Allow the solution to stand overnight.
####### Results:-
####### Pure crystals with a white color will be obtained.
####### Discussion:-
####### Purines differ from other alkaloids in that they are soluble in hot water,
####### which is used in it is extraction, and this is the cause why all the time you
####### should heat the mixture ( to prevent the precipitation of caffeine in cold
####### water).
####### Addition of basic lead acetate is to precipitate the tannins and other
####### unwanted materials.
####### Addition of dilute Sulphuric acid is to take the excess of lead acetate.
####### The centrifugation is used to isolate the decant containing the unwanted
####### material from the desired supernatant.
####### Addition of charcoal is for the decolorization, so that the crystals will take
####### its normal white color.
####### Use of chloroform is to extract the caffeine from other component of the
####### mixture.
####### Use of hot ethanol is for the recrystallization process.
Purine alkaloids
Course: médecine générale : pharmacologie
University: Université Sidi Mohamed Ben Abdellah de Fès
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