[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
is involved, and they form pairs of oxidized and reduced states:
(reduced) Adenine + (oxidized) Thymine (oxidized) Guanine + (reduced) Cytosine
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 || || \ | | | | || || CH O=C C--H O=C CH || || // | || | || N--C--N H--N--CH HN--CH
The carbohydrate is ribose or deoxyribose.
CHO CHO | | H--C--OH HO--C--H | | HO--C--H HO--C--H | | HO--C--H HO--C--H | | CH_{ 2}OH CH_{ 2}OH
H \(1)/-----O-----\(4) (5) C CH--CH_{ 2}OH / \(2) (3)/ HO CH_{ 2}--HC(OH)
The exact position of phosphoric acid was established after long work and verified by synthesis.[33]
A compound of adenine, ribose, and phosphoric acid was found in yeast, blood, and in skeletal muscle of mammals. From 100 grams of such muscle, 0.35-0.40 grams of this compound were isolated. If the muscle is at rest, the compound contains three molecules of phosphoric acid, linked through oxygen atoms. It was named adenosine triphosphate or adenyltriphosphoric acid,[34] usually abbreviated by the symbol ATP. It releases one phosphoric acid group very easily and goes over in the diphosphate, ADP, but it can also lose 2 P-groups as pyrophosphoric acid and leave the monophosphate, AMP.