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 CH 2 OH 



+ HN0 2 =1 + N 2 + H 2 



COOH COOH 



the amount of nitrogen evolved being double that contained in the 

 amino acid. 



We can ascertain the following particulars : 



I. Amide nitrogen (ammonia). 



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



I b lysine j difference 

 xx. , and 



I /in non- Arginine evolves half its 



V Cystine nitrogen I c arginine arginine contains f of its N.I amino N as NHg by boiling 



\^ histidine histidine f of its N |, form. with alkali. Histidine 



by difference. 



III. 



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-Mliller 

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

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



