-ribose,
which Emil Fischer had artificially made from arabinose and named ribose
to indicate this relationship (1891). The nitrogenous products of the
degradation were identical with substances previously prepared in the
long study of uric acid. In the course of this study, Emil Fischer
established uric acid and a number of its derivatives as having the
elementary skeleton of what he called "pure uric acid," abbreviated to
purine. Out of Adolf Baeyer's work on barbituric acid came the knowledge
of pyrimidine and its derivatives.

[Illustration: 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

NH_{2}
/
C=O
\
NH_{2}

is involved, and they form pairs of oxidized and reduced states:

PURINE PYRIMIDINE

(reduced) Adenine + (oxidized) Thymine
(oxidized) Guanine + (reduced) Cytosine

3N = CH4
| |
2H--C CH5
|| ||
1N--CH6

Pyrimidine

1N==CH6
| | H
| | 7/ N==C--NH_{2}
2H--C C--N | |
|| ||5 \ H--C C--NH
|| || \ || || \
|| || CH8 || || CH
|| || // || || //
3N--C--N N--C--N
4 9
Adenine
Purine

HN--C=O
| |
NH_{2}--C C--NH N==C--NH_{2} H--N--C=O
|| || \