ii PRIMARY DISSOCIATION-PRODUCTS 43 



Pauly gives in support of his view the fact established by Pinner 

 and Schwarz l that imido-azol derivatives are very susceptible to alkaline 

 oxidising agents, while they resist acid oxidising media. The property 

 possessed by the imide-group, of acting as a weak base as well as a 

 weak acid, explains also its power of forming coloured compounds with 

 diazonium salts, as do, e.g. cyclopentadien, pyrrol, and imido-azol. 

 HC CH HC CH N CH 



II II II II II II 



HC, X!H HC V ,CH HC, ,CH 



CH 2 NH NH 



Cyclopentadien. Pyrrol. Imido-azol. 



A solution of histidin in sodium carbonate gives with diazobenzene 

 sulphanilic acid, in acid solutions, a pure orange, and in alkaline solu- 

 tions a deep cherry-red colour. As no other tissue constituent gives 

 this reaction, apart from tyrosin, the presence of histidin may always 

 be readily ascertained in mixtures of albuminous dissociation-products 

 and also in the native albumins, whenever by Millon's reaction the 

 absence of tyrosin has been ascertained (see p. 10). 



Pauly, taking into account the close relationship of arginin and 

 histidin, gave, provisionally, the following formula for histidin : 



CH N v CH NH V 



C HN' CH 2 



I I 



CH 2 CH 2 



CH.NH 2 OH.NH 2 



COOH COOH 



Histidin. Arginin. 



Pauly's view that histidin is a-amino-/3-imido-azol propionic acid, 

 Knoop and Windhaus have proved to be correct by synthesis, and 

 therefore histidin should be placed amongst the ring compounds. 

 The position of the NH 2 -group is still uncertain. Herzog's discovery 

 of a violet biuret reaction points to a terminal . CONH 2 group. 

 Arginin does not give a biuret reaction. When albumins or peptones 

 are digested tryptically or ereptically, 2 the originally intense biuret- 

 reaction of the albumoses and peptones diminishes so much as to be 

 only demonstrable on taking the greatest care in performing the 



1 Pinner and Schwarz, Ber. d. deutsch. chem. Gesellsch. 35. 2448 (1902). 



2 0. Cohnheim, Zeitschr. f. physiol. Chem. 33. 451 (1901), 35. 134 (1902); K. 

 Mays, ibid. 38. 428 (1903). 



