THE VEGETABLE BASES 159 



The above discussions of the composition of typical alkaloids 

 clearly indicate the extreme complexity of their molecular^tructure. 

 It is generally supposed that they are formed by the decomposition 

 of proteins. But they are developed in only a few particular 

 species of plants and are always present in these plants in fairly 

 constant quantities. Hence, it appears that, in these species, the 

 production of alkaloids is in some way definitely connected with 

 protein metabolism; but it is certain that this is not a common 

 relationship, as it is manifested by such a limited number of species 

 of plants, and there is absolutely no knowledge as to its character 

 and functions. Some authorities prefer to regard the alkaloids 

 as waste-products of protein metabolism; but here, again, it is 

 difficult to understand why such products should result in certain 

 species of plants and not in others. 



THE PURINE BASES 



This is a group of compounds, widely distributed in both plant 

 and animal tissues, all of which are derivatives of the compound 

 known as purine, CsH^N^ All of the naturally occurring com- 

 pounds of this group may be regarded as derived from purine, 

 either by the addition of oxygen atoms, or by the replacing of one 

 or more of its hydrogen atoms with a methyl (CHs) group or an 

 amino (NH2) group. The following structural formula repre- 

 sents the arrangement of the purine nucleus, the numbers being 

 used to designate the nitrogen or carbon atoms to which the addi- 

 tional atoms, or groups, are attached in the more complex com- 

 pounds of the group. In purine itself, the four hydrogen atoms are 

 attached in the 2, 6, 7, and 8 positions. 



. 5 7 



-^C C N\- 



II 4 



3 N 



The double bonds, in each case except those between the 4 and 5 

 carbon atoms, are easily broken apart and readjusted, so that other 

 atoms or groups can be attached to any atom in the nucleus except 

 the 4 and 5 carbon atoms. In all of the statements with refer- 

 ence to the structure of the purine bases, the term " oxy " is used 



