"At Erlangen, Fischer studied the active principles of tea, coffee and cocoa, namely, caffeine and theobromine, and established the constitution of a series of compounds in this field, eventually synthesizing them.
The work, however, on which Fischer's fame chiefly rests, was his studies of the purines and the sugars. This work, carried out between 1882 and 1906 showed that various substances, little known at that time, such as adenine, xanthine, in vegetable substances, caffeine and, in animal excrement, uric acid and guanine, all belonged to one homogeneous family and could be derived from one another and that they corresponded to different hydroxyl and amino derivatives of the same fundamental system formed by a bicyclic nitrogenous structure into which the characteristic urea group entered. This parent substance, which at first he regarded as being hypothetical, he called purine in 1884, and he synthesized it in 1898. Numerous artificial derivatives, more or less analogous to the naturally-occurring substances, came from his laboratory between 1882 and 1896."