42 CHEMISTRY OF THE PROTEIDS CHAP. 



Kossel 1 described the method given above, namely, that of precipi- 

 tating it with silver sulphate and barium hydrate, and then purifying 

 it with mercuric sulphate. 



The constitution of histidin has been studied by Herzog, 2 

 Frankel, 3 Pauly, 4 and Knoop and Windaus. 5 Histidin possesses two 

 hydrogen atoms replaceable by metals, one of which is in a carboxyl 

 group, COOH, and the second attached to an imide radical, NH ; it 

 contains neither methoxyl nor methyl joined to nitrogen, nor a 

 guanidin remainder ; it is optically active, and therefore contains an 

 asymmetric carbon atom ; on being boiled with alkaline permanganate 

 solution it yields hydrocyanic acid, carbonic acid, and ammonia ; it 

 resists oxidation by means of sulphuric acid + permanganate solution 

 (Herzog), and by means of dilute nitric acid (Frankel), and therefore 

 it cannot be a dihydropyrimidin ring, as held by Frankel (Pauly) ; 

 on being boiled with barium hydrate solution it does not form 

 ammonia ; it gives a violet biuret-reaction (Herzog) and Ehrlich's 

 diazo-reaction (Pauly) (see p. 10 and below) ; it replaces C0 2 from the 

 carbonates of silver and copper, and gives off C0 2 on being heated 

 above its melting-point, and therefore contains the carboxyl group, 

 COOH (Frankel) \ when acted upon by sodium hypobromite it 

 liberates one atomic nitrogen atom, while sodium nitrite replaces 1 

 nitrogen by 1 oxygen (Frankel) ; on being heated with lime it gives 

 rise to ammonia and a substance which gives the pyrrol reaction ; 

 with ammonia and chlorine water it further gives Weidel's pyrimidin 

 reaction. The firmness with which histidin resists oxidation by means 

 of sulphuric acid + permanganate shows that two nitrogen atoms must 

 be linked up in a ring-like form, a fact which led Frankel to believe 

 that histidin was dihydropyrimidin, 



y CH 2X N CH 



HC/ ' X N || || 



II II HC X /CH 



HC \ / CH V 



NH NH 



Dihydropyrimidin. Tmido-azol. 



and Pauly to assume the existence of an imido-azol or glyoxal ring, 

 because in such a ring the hydrogen atom of a CH 2 or of an NH-group 

 may be replaced by a metal. 



1 A. Kossel and F. Kutscher, Zeitschr. f. physiol. C/iem. 31. 165 (1900) ; A. Kossel 

 and A. J. Patten, ibid. 38. 39 (1903). ' 2 R. 0. Herzog, ibid. 37. 248 (1902). 



3 S. Frankel, Sitzungsber, d. Akad. d. Wissensch. in Wien, mathem.-naturw. Kl. 

 112. Abt. II.* Marz 1903. 4 H. Pauly, Zeitschr. f. physiol. Ghem. 42. 508 (1904). 



5 F. Knoop and A. Windhaus, Hof, mister' s Beitrage, 1, 144 (1905). 



