iv THE LINKING OF AMINO-ACIDS 141 



The grouping 



_CO | NH CH CO | NH CH CO | NH- 



X X 



allows us to explain the following properties exhibited by albumins : 



1. The fact that all albumins greatly resemble one another, 

 although they are built up of such diverse dissociation-products. 



Now we understand why the albumins dissociate in so uniform a 

 manner, even if the most divergent reagents are used, and why dissocia- 

 tion always takes place at the already existing 'locus minoris re- 

 sistentiae' and why all albumins can be dissociated by trypsin, 

 while each of the polysaccharids requires its own specific carbo- 

 hydrate-ferment. 4 



2. The Biuret-Readion. The red colour which biuret and similar 

 substances exhibit, when they are treated with sodium hydrate and 

 copper sulphate, depends, according to Schiff, 1 on the formation of a 

 copper-potassium-biuret compound having this constitution : 



OH OH 



C NH 2 - C* NH 2 C. 

 NH/ > 



X C NH 2 K K NH 2 C X 



ii | I ii 



OH OH 0. 



This substance Schiff isolated in the form of long, red, needle-like 

 crystals. According to Goto, 2 protones give the biuret-reaction with- 

 out the addition of an alkali, and Henze 3 states that the copper-con- 

 taining hsemocyanin does not require the addition of a copper salt. 

 Instead of copper salts one may also use nickel salts (see p. 6). 



This reaction Schiff believes to be given by all compounds in 

 which two CON H 2 - groups are linked either to a carbon-atom or to 

 a nitrogen -atom or directly to one another, and which therefore 

 correspond to one of the three following types : 



,CONH 2 /CONH 9 CONH 9 



HN< H 9 C< || 



N CONH 2 X CONH 2 CONH 2 



Biuret. Malonamide. Oxamide. 



One of the CON H 2 - groups may also be replaced by a CH 9 NH 2 

 group or a CSNH 2 -group. 



1 H. Schiff, Ber. d. deutsch. chem. Ges. 29. 1. 298 (1896) ; Liebig'sAnnalen, 299. 236 

 (1897) ; 319. 300 (1901). 2 M. Goto, Zeitschr. f. physiol. Chem. 37. 94 (1902). 



3 M. Henze, ibid. 33. 370 (1901). 



