iv THE BIURET-REACTION OF ALBUMINS 143 



E. Fisher, 1 in confirmation of SchifFs theory, obtained a biuret- 

 reaction with the following three substances, which are comparable to 

 glycin-amide : 



(1) carboxy-ethyl-glycyl-glycin-amide : 



C 2 H 5 COO . NH . CH 2 . CO NH . CH 2 . CO NH 2 . 



(2) carboxy-ethyl-glycyl-glycin-leucin-ester : 



C 2 H 5 COO . NHCH 2 CO NHCH 2 CO NH . CH(C 4 H 9 )CO . OC 2 H 5 . 



(3) carbonyl-diglycyl-glycin-amide : 



CO : [NH . CH 2 . CO . NHCH 2 . CO . NH 2 ] 2 . 



The free acid corresponding to carboxy-ethyl-glycyl-glycin-amide, 

 namely, COOH . NH . CH 3 . CO NH . CH 2 . CO . NH 2 , does not give 

 the biuret-reaction, probably owing to the presence of acid groups 

 in the molecule. This principle cannot, however, be generalised, as 

 a-asparagin (Schiff) does give a red reaction. The ordinary asparagin, 

 COOH . CH . (NH 2 )CH 2 . CO . NH 2 , which, according to Schiff, gives a 

 bluish-violet colour, Fischer says reacts so little as to hardly justify 

 the expression ' biuret.' The diaminoaspartic acid, NH 2 . CO . CH 

 (CH 2 ) . CH 2 . CO . NH 9 , gives, however, according to Fischer a strong 

 biuret-reaction. 



Schiff 2 has further discovered that the polyaspartic acids and 

 their derivatives also give the reaction. These bodies have the same 

 configuration as the peptids of Fischer, for they too are inline-com- 

 pounds, formed by the union of the amino-group of one molecule of 

 aspartic acid with the carboxyl-group of another molecule. 



The biuret-reaction is not a uniform one, for there are great 

 differences in the colour produced, as just pointed out, and also 

 in the ease with which it is obtained. According to Neumeister, 3 

 peptones can be demonstrated in dilutions of 1:100,000; they 

 give a pure red colour. Alb'umoses, according to Kiihne, 4 require 

 to be more concentrated, and give a reddish-violet colour; while 

 natural albumins give a violet colour, and require to be fairly 

 concentrated. The differences cannot be explained as due to differ- 

 ences in the size of the molecule or by similar factors, as Schiff 

 and E. Fischer have seen similar differences in their synthetic pro- 

 ducts. What factors bring about the colour changes is not known, 

 however. When dealing with the higher albumins a decision becomes 



1 E. Fischer and Fourneau ; E. Fischer and Qtto. See footnotes on p. 125 and 126. 



3 H. Schiff, Liebig'sAnnalen, 303. 183 (1898), 307. 231 (1899), 31O. 37 and 301 

 (1899). 



s R. Neumeister, Zeitschr. f. Biol. 26. 324 (1890). 



4 W. Kiihne, ibid. 29. 308 (1892). 



