THE CHEMICAL CONSTITUTION OF ITS UNITS 61 



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

 relation to arginine : 



CH N 



) 



C NH 

 CH 2 

 CH.NH 2 



COOH 

 Histidine 



CH 2 NH 



\C-NH 2 

 CH 2 NH 

 CH 2 

 CH.NH 2 



COOH 



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 histidine 

 by the action of nitrous acid, they obtained /3-imidazole-propionic acid. 1 

 This compound was identical with the synthetical product prepared 

 from glyoxylpropionic acid, ammonia and formaldehyde : 



CHO NH, 



CO 

 CH 2 

 CH 2 

 COOH 



NH, 



H 



\CH 



o 



CH NH 



;CH 



CH 2 

 CH 9 



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 histidine is /3- 

 imidazole-a-amino-propionic acid : 



CH NH 



CH NH 



NH 



N 



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

 the hydrochloride of #-imidazole-propionic acid. 



