267 



This substance, which is also the mother substance of uric 

 acid, does not occur in nature, but has been synthesized by 

 Fischer. 



By writing the formula somewhat differently, as follows : — 

 2CH N I 



^ \ 



3 N CH 6 



\ / 



4C=C5 



I I 



Q N NH 7 



\/ 

 CH 



8 



it will be seen that it is composed of two rings, the upper one, 

 which is six membered, being a so-called pyrimidine ring, 

 while the lower one, which is five membered, is an imidazol or 

 glyoxaline ring, the same as occurs in histidine (see p. 306). 



The relationship between purine, xanthine, theobromine 

 and caffeine is best understood from the following considera- 

 tions. 



Xanthine may be regarded as purine with the addition of 



two atoms of oxygen attached to the carbon atoms numbered 



2 and 6 ; and it is accordingly called 2 : 6 dioxypurine, and is 



given the formula : — 



NH— CO 



I I 

 CO C— NH 



I II ^ 

 NH— C N 



Xanthine or 2 : 6 dioxypurine 

 From this compound theobromine and caffeine are derived 

 by replacing two and three atoms of hydrogen respectively 

 by methyl groups, as may be seen from the following 

 formulae : — 



