DISTRIBUTION OF NITROGEN 85 



ANALYSIS OF PROTEINS BY THE DISTRIBUTION OF 

 THE VARIOUS KINDS OF NITROGEN. 



Since a complete analysis of a protein is still an impossibility, 

 owing to the unsatisfactory methods for isolating and estimating the 

 several mono-amino acids, the proteins cannot yet be differentiated by 

 means of their chemical composition. , 



It has been proved by Osborne, Leavenworth and Brautlecht [1908] 

 that the methods for estimating the ammonia content and the di-amino 

 acid (the three hexone bases) content of a protein are almost perfect. 

 We can therefore differentiate proteins by their content in these four 

 products. 



A further differentiation of the units composing the protein mole- 

 cule into those containing amino groups and those containing nitrogen 

 in heterocyclic combination was made by Van Slyke [1910, 1911], 



This subdivision is possible, since nitrous acid reacts only with 

 amino groups with liberation of nitrogen : 

 CH 2 .NH 2 



-t 

 COOH COOH 



the amount of nitrogen evolved being double that contained in the 

 amino acid. 



We can ascertain the following particulars : 



[. Ami'de nitrogen (ammonia). 



+ HNO 2 =| + N 2 + H 2 O 



C( 



Di-amino nitrogen ( a cystine \ contain only amino N. Sulphur content gives cystine. Lysine by 



I b lysine J difference 

 and 



1 /in non- Arginine evolves half its 



Cystine nitrogen I c arginine arginine contains f of its N-j amino N as NHj, by boiling 



\d histidine histidine f of its N (form. with alkali. Histidine 



by difference. 



Mono-amino nitrogen 



.Non-amino nitrogen 

 b 



glycine, phenylalanine, aspartic acid, tryptophan 



alanine, tyrosine, glutamic acid. 



valine, , 



leucine, 



isoleucine. 



proline 



oxyproline 



tryptophan () 



i.e., seven data out of the possible eighteen. 



An eighth value is obtainable by determining the mono-aminodi- 

 carboxylic acid content as was shown by Andersen and Roed-Miiller 

 [1915]. This determination depends upon the fact that in a mixture 

 of mono-amino acids, the mono-aminomonocarboxylic acids have a 



