48 CHEMICAL CONSTITUENTS OF BODY AND FOOD. 



considerable change in the proteid molecule; inclol and skatol can 

 hardly be considered to be simple cleavage products of the proteid 

 molecule (see p. 29). 



Liebermanns test 1 is performed by precipitating the proteid by 

 alcohol, and then heating the washed precipitate with strong hydrochloric 

 acid. The result is a violet-blue colour. The reaction is not given by pure 

 peptone. 2 It is also not given by any of the aromatic putrefactive products 

 of proteid, nor by a large number of other cleavage products of proteid 

 which Pickering worked with. Its cause is therefore at present unknown. 



Krassers reaction? Alloxan in solution stains proteid matter a 

 brilliant red. It reacts in the same way with asparagin, aspartic acid, 

 and tyrosine. The reaction is probably connected with the presence 

 of amido groups. 



Piotrowski's reaction^ If a few drops of dilute copper sulphate 

 solution are added, and then excess of strong solution of caustic soda 

 and potash, a violet solution is the result. If ammonia is used instead, 

 a blue solution is formed. 



In the case of the proteoses and peptones, the result is a rose -red solu- 

 tion with potash 5 and a reddish violet with ammonia. As the same colour 

 is given by the decomposition product of urea called biuret, 6 the test 

 is often called the biuret reaction (2CON 2 H 4 - NH 3 = C 2 2 N 3 H 5 ). Biuret 

 yields, on decomposition, compounds which contain cyanogen ; for instance, 

 by heat it is split into ammonia and cyanuric acid, (CN) 3 H 3 3 . Biuret, 

 cyanuric acid, xanthine, hypoxanthine, sarcosine, hydrocyanic acid, all give 

 similar reactions to the proteids. Gne/da 7 considered it probable that the 

 biuret reaction was due to a cyanogen radicle, and that the cyanogen in 

 albumin and peptone is differently combined, corresponding to the similar 

 differences in cyanuric and hydrocyanic acid respectively. Pickering, 8 

 however, concludes, that the radicle in question is not ON but CONH. 



The related metals, nickel (Gnezda) and cobalt (Pickering) give correspond- 

 ing colour reactions, which may be summarised in the following table : 



Pickering found that when a cobalt salt has entered into the proteid 



1 Jahresb. it. d. Fortschr. d. Thier-Chcm., Wiesbaden, Bd. xvii. S. 8 ; Chem. Centr.-BL, 

 Leipzig, 1887, Nos. 18 and 25. 



Le Nobel, Jahresb. it. d. Fortschr. d. Thier-Chem., Wiesbaden, Bd. xvii. S. 3. 



Monalsh. d. Chem., Wien, Bd. vii. S. 673. 



Sitzungsb. d. Ic. AJcad. d. Wisscnsch., Wien, Bd. xxiv. S. 335. 



Brucke, Monatsh. d. Chem., Wien, Bd. iv. 



Wiedemann, Ann. d. Phys. u. Chem., Leipzig, Bd. Ixxiv. S. 67. 



7 Proc. Roy. Soc. London, 1889, vol. xlvii. p. 202. 



8 Journ. Physiol., Cambridge and London, 1893, vol. xiv, p. 347. 



