Figure 15.—Albrecht Kossel (1853-1927) received the Nobel Prize in Medicine and Physiology in 1910 for his work on nucleic substances, which contain a high proportion of phosphorus. The chemical bonds of this phosphorus in the molecules of nucleic substances were determined in later work. (Photo courtesy National Library of Medicine, Washington, D.C.)
From these findings, together with what Oswald Schmiedeberg (1838-1921) had established concerning the presence of four phosphate groups in the molecule (1899), Robert Feulgen (1884-1955) constructed the following scheme of a nucleic acid. Feulgen’s formula of 1918 is:
Phosphoric acid—Carbohydrate—Guanine
Phosphoric acid—Carbohydrate—Cytosine
Phosphoric acid—Carbohydrate—Thymine
Phosphoric acid—Carbohydrate—Adenine
Of the four basic components on the right, thymine occurs in the nucleic acid from the thymus gland. Yeast contains uracil instead. The difference between these two bases is one methyl group: thymine is a 5-methyluracil. In all of these basic substances, the structure of urea
is involved, and they form pairs of oxidized and reduced states: