The above discussions of the composition of typical alkaloids clearly indicate the extreme complexity of their molecular structure. 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, C5H4N4. All of the naturally occurring compounds 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 (CH3) group or an amino (NH2) group. The following structural formula represents the arrangement of the purine nucleus, the numbers being used to designate the nitrogen or carbon atoms to which the additional atoms, or groups, are attached in the more complex compounds of the group. In purine itself, the four hydrogen atoms are attached in the 2, 6, 7, and 8 positions.

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 reference to the structure of the purine bases, the term "oxy" is used to mean an oxygen atom attached by both its bonds to one of the carbons in the nucleus, instead of its customary use to mean the monovalent OH group replacing a hydrogen, as in the case of all other nomenclature of organic compounds. With this understanding, reference to the numbered nucleus formula above will make plain the structure of all of the purine bases which are included in the following list:

In order to make these structural relationships quite clear, the following formulas for uric acid and for caffeine are presented as typical examples:

Uric acid is found in the excrement of all animals; in the urine of mammals, and in the solid excrement of birds and reptiles. It is not known to occur in plants.

Xanthine and hypoxanthine occur in animal urine, and also in the tissues of both plants and animals.