PURINES 297 



Caffeine. Theophylline. Theobromine. 



These three compounds are not found in animals but are fairly 

 widely distributed in plants. Caffeine and theobromine are the active 

 constituents of tea, coffee and cocoa : they produce a stimulating effect 

 on the central nervous system and act as powerful diuretics. 



Caffeine. 



Caffeine is present to the extent of -8-1 7 per cent, in coffee beans, 

 I --8 per cent, in cocoa beans, 1-2 per cent, in kola nuts, 2-5 per cent, 

 in tea leaves; 2*5-5 per cent, is present in guarana, the roasted fruit 

 of Paullinia which is eaten in South America. 



Preparation. 



Caffeine is readily prepared from tea leaves by boiling about 250 gm. 

 with 500 c.c. of water for 15 minutes; the solution is filtered through cloth 

 into a basin and the leaves boiled again with 250 c.c. water and again filtered 

 off. The odour is due to a small quantity of essential oil and the solution con- 

 tains protein and tannin, which gives it the brown colour. The proteins and 

 tannin are removed by adding basic lead acetate as long as a precipitate is 

 formed. The precipitate is filtered off and washed with water and the solution 

 is treated with sulphuric acid or hydrogen sulphide to remove lead. The lead 

 sulphate or sulphide is filtered off and the solution evaporated with some 

 charcoal until its volume is 250-300 c.c. It is filtered from charcoal and 

 when cold extracted two or three times with chloroform. The chloroform is 

 distilled off and the caffeine, which remains, is recrystallised from boiling 

 water containing animal charcoal. From 1-2 gm. are obtained. 



Properties. 



Caffeine forms long silky needles containing one molecule of water of 

 crystallisation which it loses at 100. It melts at 233 and has a bitter taste. 

 It forms salts with mineral salts which are decomposed by water. 



Caffeine, on evaporation with chlorine water, leaves a reddish -brown 

 residue which becomes purple when treated with ammonia. 



Theobromine. 



Theobromine is present to the extent of i'5-2*4 per cent, in cocoa 

 beans ; smaller amounts are present in kola nuts and tea leaves ; it is not 

 present in coffee beans. 



Theobromine forms a crystalline powder which has a bitter taste, is 

 soluble with difficulty in hot water and alcohol but is easily soluble in am- 

 monia. It forms salts with mineral acids, which are decomposed by water, 

 and with silver nitrate and other metallic salts. 



Theophylline. 



Theophylline was discovered by Kossel in 1 888 in extracts of tea and has 

 been synthesised by Emil Fischer. 



It forms a white powder which melts at 264. 



Paraxanthine or i, 7-dimethylxanthine^j have been isolated from human 

 Heteroxanthine or y-methybcanthine I urine. They are products 

 I-methylxanthine [formed from caffeine in the 



Epiguanine or y-methylguanine J organism. 



