C NH 



CH 

 CH. 



CH . NH 2 

 COOH 



X 



NH 



126 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



imidazole ring in its composition. Pauly, therefore, gave it the constitu- 

 tion of imidazole-amino-propionic acid, at the same time showing its re- 

 lation to arginine : 



CH N CH 2 NH 



CH 2 

 CH 3 

 CH.NH 2 



COpH 



Histidine Arginine 



This assumption of Pauly's was confirmed by Knoop and Windaus, who 

 found that histidine is resistant to reduction by sodium and alcohol 

 whereas the pyrimidine ring is very unstable towards this reagent. On 

 reducing Frankel's oxydesaminohistidine, which is obtained from his- 

 tidine by the action of nitrous acid, they obtained /3-imidazole-propionic 

 acid. 1 This compound was identical with the synthetical product pre- 

 pared from glyoxylpropionic acid, ammonia and formaldehyde : 



CHO NH, 



CO 



I- 



CH 2 

 COOH 



NH, 



H 

 \CH 



o 



CH NH 





CH 



CH Q 



CH 2 

 COOH 



The presence of an imidazole ring in histidine was thus proved, and 

 it only remained to show the position of the amino group. Frankel 

 urged certain objections against the presence of an imidazole ring in 

 histidine, but Knoop and Windaus showed that these did not hold good. 

 Knoop has since obtained imidazole-glyoxylic acid, imidazole-carboxylic 

 acid, and imidazole from oxydesaminohistidine, and also imidazole- 

 acetic acid. The imidazole ring is therefore in the /3-position and his- 

 tidine is /3-imidazole-a-amino-propionic acid : 



1 Windaus and Vogt in 1908 showed that Frankel's chlorohistidine carboxylic acid was 

 the hydrochloride of /8-imidazole-propionic acid. 



