142 CHEMISTRY OF THE PROTEIDS CHAP. 



In proteids we meet with the combination 

 -CHNH- 



CONH- 



According to Schiff, only one of the hydrogen-atoms attached to the 

 nitrogen can be substituted, and the CONH 2 -group must be free ; but 

 E. Fischer has observed the biuret-reaction with carb-ethoxyl-di-glycyl- 

 glycin-ester and with triglycyl-glycin-carboxylic acid, compounds 

 in which a hydrogen-atom is substituted in both CONH 2 -groups, and 

 Levites l has shown that treatment with nitrous acid, which completely 

 destroys the CO . NH 2 - group, does not alter egg-albumin, casein, or 

 gelatine in such a way as to prevent the biuret-reaction, and 

 Scheermesser's 2 pepsin-glutin-peptone gives the biuret-reaction, although 

 it contains no amid-nitrogen, and therefore no CO . NH 2 groups. On 

 p. 95, when discussing desamino- albumin, Levites' view as to the 

 non-existence of CO . NH 2 -groups in the albumin-molecule is dealt 

 with. It will suffice now to point out, as was done above, the 

 possibility of hydrolysing the albumin after its treatment with nitrous 

 acid into dissociation-products, which must contain NH 2 -groups. 



On the other hand, no biuret-reaction is obtained with glycyl- 

 glycin and other dipeptids, although one might expect it. It would 

 appear that CONH 2 and CH 2 NH 2 are only active when the CONH 2 

 group is terminal, otherwise several groups are necessary. The 

 conditions determining the biuret-reaction are therefore not as yet 

 cleared up. 



The oldest synthetically prepared substances giving the biuret- 

 reaction are fully discussed in Chapter III., p. 115. 



Schiff found that a-asparagin (onion-red) and methyl-a-asparagin 

 (red-violet), belonging to the oxamide type and glycocoll-amide (glycin- 

 amide or amino-acetamide) ; further /3-asparagin and homo-asparagin 

 (blue-violet), belonging to the malon-amide type, give the biuret- 

 reaction. 



1 S. Levites, Zeitschr. f. physiol. Chem. 43. 202 (1904). 

 2 Scheermesser, ibid. 41. 68 (1904). 



