THE PROTEINS 115 



coffee, and cocoa, namely caffeine, which is trimethyl dioxypurine, and 

 theobromine, which is dimethyl dioxypurine. From the structural 

 formulae given it will be seen that the purine radical contains two 

 nuclei. The nucleus 



N C 



c c 



N C 

 is spoken of as the pyrimidine nucleus, pyrimidine having the formula 



HO 5 CH 



The other is the radical which we have met with already in histidine, 

 a disintegration product of proteins, namely iminazol : 



HC NH 



II >H 

 HC N r 



Besides the purine bases proper, we meet among the disintegration 

 products of nucleic acid with a series of bases derived from the pyrimi- 

 dine ring. These are uracil, thymine, and cytosine. 

 URACIL is 2- 6- dioxy pyrimidine, 



NH CO 



CO CH 



I II 

 NH CH 



THYMINE is 5- methyl uracil, 



NH CO 



CO C.CH 3 



1 II 

 NH CH 



while CYTOSINE is 6-amino-2-o2ypyrimidine, 



NH C.NH 2 



CO CH 



I II 

 N =CH 



