16 CHEMICAL STATICS 



A number of other peptids have been isolated by various 

 observers (42) (48) from among the products of the incomplete 

 hydrolysis of proteins. 



6. The Analysis and Characterization of Proteins by the Deter- 

 mination of the Chemical Groups Characteristic of the Different 

 Ammo-acids. The hydrolysis of the proteins is accompanied 

 by a very marked increase in the total number of free amino 

 groups. This is due to the fact, now incontestibly established, 

 that the various amino-acid radicals of the protein molecule 

 are attached to one another in an end to end or catenary linkage, 

 through the union of the amino group of one amino-acid with the 

 carboxyl group of the adjacent acid, in accordance with the 

 general equation: 



HaNRiCOOH+H.HNRaCOOH = HaNRiCOHNR^COOH+HaO. 



The reversion of this reaction, in hydrolysis, leads, of course, to 

 the transformation of an imino or potentially imino group into 

 a free amino group and the series of such transformations which 

 constitutes the process of the hydrolysis of a protein leads to the 

 appearance of a large number of free amino groups which were 

 not present as such in the unhydrolysed protein molecule. 



Free amino groups in the aliphatic series have the well-known 

 property of reacting with nitrous acid with the liberation of 

 nitrogen, in accordance with the equation: 



RNH 2 + HN0 2 = ROH + N 2 + H 2 O. 



Very ingenious advantage has been taken of this fact by D. D. 

 Van Slyke (69) (70) (2) in the method which he has devised and 

 which is now very widely employed for the determination of the 

 distribution and partition of nitrogen within the protein molecule. 



This method consists essentially in the following process. The 

 protein having been in the first place subjected to complete 

 hydrolysis, the ammonia in the mixture of products (derived 

 from "amid" nitrogen in the protein molecule) is first removed 

 by vacuum distillation and separately determined. The residual 

 mixture of products is then treated with phosphotungstic acid, 

 which results in the precipitation of the diamino-acids, namely 

 cystin, arginin, lysin and histidin. A determination of sulphur 

 yields a measure of the cystin-content. Arginin has the property 

 of yielding one-half of its nitrogen in the form of ammonia on 



