374 



NITROGEN BASES 



they provide a certain method of recognizing substances which 

 do not give any characteristic colour reactions. 



An additional advantage of the method Hes in the fact 

 that the reagents employed (auric or platinic chloride, etc.) 

 being substances of high molecular weight produce crystalline 

 derivatives whose weight is very considerably greater than 

 that of the substance which is being isolated, and thus ponder- 

 able quantities of substance may be obtained from compara- 

 tively small amounts of material. 



PURINE BASES. 

 Under this heading are included such substances as caffeine, 

 theobromine, xanthine, guanine, etc., which are called purine 

 bases because they are all derivatives of the same substance, 

 purine, whose formula is given below : — 



I N = 6 CH 



2 CH 5 C— 7 NH 



\ 

 8CH 



3N — 4C— 9N 

 Purine 



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- 

 3N 



4C^ 



qN 



\ 



-N I 



\ 

 CH6 



/ 

 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. 444). 



