144 THE PROTEINS AND THE AMINO-ACIDS 



From the construction of these fragments of the protein molecule 

 we may infer the architecture of the whole, and without going so far 

 as to assume that no other types of linkage exist within the protein 

 molecule, an assumption which would very possibly be incorrect, we 

 may safely conclude that the COHN-linkage plays a very predomi- 

 nant part in building together the constituent parts of the protein. 



The tetrapeptide referred to above which has been isolated from 

 the products of the incomplete hydrolysis of silk fibroin is of especial 

 interest, because, had it not been identified as a tetrapeptide, it would 

 certainly have been included among the proteoses. It is precipitated 

 by phosphotungstic acid, readily dissolves in water, is insoluble in 

 alcohol, and coagulated by saturation of its aqueous solutions with 

 ammonium sulphate, or with sodium chloride. Its molecular weight, 

 determined by the cryoscopic (freezing-point) method, was 350. The 

 synthetic pentapeptide, 1-leucyl-triglycyl-l-tryosine, 'possesses very 

 similar properties, so that the proteoses are not necessarily exceedingly 

 complex substances, nor is excessive complexity necessary in order 

 that substances of this type may be coagulable by saturation of their 

 aqueous solutions with ammonium sulphate. 



THE ANALYSIS AND CHARACTERIZATION OF PROTEINS BY 



THE DETERMINATION OF THE AMINO-ACID RADICALS 



WHICH THEY CONTAIN. 



The hydrolysis of the proteins is accompanied by a very marked 

 increase in the total number of free amino-groups present in the solu- 

 tion. This is due to the fact that successive repetitions of the reaction : 



COHN h H 2 O = COOH + H 2 N 



result in the transformation of imino-groups into amino-groups, until 

 the final number of amino-groups formed corresponds \vith the total 

 number of amino-acid radicals out of which the protein is built up. 



Reference has been made above to the fact that 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 + HNO 2 = ROH + H 2 O + N 2 



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

 Slyke, in the method which he has devised, and which is now very 

 widely utilized 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 small proportion of ammonia which always occurs in the mixture of 

 products (derived from "amide" nitrogen in the protein molecule) is 

 first removed by vacuum-distillation and separately determined. The 



