36 THE ALBUMINS. 



such an event it is well to use a still more dilute solution of the 

 copper sulphate. Where larger amounts are present, it is neces- 

 sary to add more of the reagent. An excess of the neutral salts 

 which are often present when the test is employed does not inter- 

 fere with the reaction. With ammonium sulphate, however, it is 

 necessary to use a large quantity of the caustic alkali to bring out 

 the color. Should magnesium sulphate be present, a precipitate of 

 magnesium hydroxide results on adding the alkali, and is allowed 

 to settle. 



The resulting color, according to Schiff, is due to the formation 

 of biuret potassium cupric oxide : 



O OH 



II I 

 /C NH 2 Cu 



NH< 



XJ NH 2 K K NH 2 



O OH OH O 



In the place of sodium or potassium hydrate other substances 

 may also be used in the test, some of which are only feebly or 

 indeed scarcely alkaline, such as barium and calcium hydrate, the 

 carbonates of the alkalies, ammonia, magnesium oxide, trimethyl- 

 amin, coniin, piperidin, atropin, etc. Instead of copper salts, nickel 

 salts may also be used ; in that case no red color but an orange 

 yellow is obtained. 



The biuret reaction is dependent npon the presence in the albuminous molecule 

 of CH 2 .NH 2 



CO NH groups, and is also obtained with non-albuminous substances, in which 

 such radicles are present e. g., 



CH 2 NH 2 

 glycinamin : 



CH.NH 2 



CH 2 NH(CH 3 ) 

 sarcosinamin : 



CO NH 2 



CONH 2 

 ,NH 2 



V-'V^-L.-' 



CHJ 

 CH 2 



the diamin of asparaginic acid : 



)NH 2 , and notably the so-called base of 

 Curtius : NH 2 .(CH 2 .CO.NH) 6 .CH 2 .CO 2 .C 2 H 5 . 



Hofmeister has pointed out that there is excellent evidence to 

 support the belief that in the albuminous molecule the component 

 anhydrides of the a-amino acids are united to each other by 

 CO NH CH= groups, and it appears that the same atomic 

 grouping which in the albumins forms the basis of the biuret reac- 

 tion is also the point of attack in their cleavage by the digestive 



