TYPES OF UNION 17 



boiling with alkali. The quantity of ammonia developed on 

 boiling the precipitate with alkali therefore affords a measure of 

 the content of arginin. The total nitrogen in the precipitate 

 is now determined and from it is subtracted the proportion of 

 nitrogen which is contributed by the cystin and arginin content. 

 The residual nitrogen is derived from lysin (= x) and histidin 

 (= y). On treatment with nitrous acid lysin yields a volume 

 of free nitrogen corresponding to the whole of its nitrogen 

 content {= x), while histidin yields a volume of free nitrogen 

 which corresponds to two-thirds of its nitrogen content (= f y). 

 The amino-nitrogen content of the precipitate is therefore deter- 

 mined by the nitrogen yield on treatment with nitrous acid and 

 after subtraction of the amino-nitrogen contents of arginin 

 (= one-fourth of its total nitrogen) and of cystin (= the whole 

 of its nitrogen content), the residual amino nitrogen evidently 

 represents the whole of the lysin nitrogen plus three-fourths 

 of the histidin nitrogen. But the determination of the total 

 nitrogen in the precipitate and the subtraction therefrom of the 

 cystin and arginin nitrogen has already given us a measure of 

 the total nitrogen yielded by the lysin and histidin. Subtract- 

 ing, therefore, the amino-nitrogen yield of these amino-acids 

 the difference evidently corresponds to one-third of the histidin 

 nitrogen, from which the contents of histidin and lysin may 

 readily be computed. 



t In the filtrate, after the separation of the diamino-acid precipi- 

 tate, the total nitrogen and the amino-nitrogen are separately 

 determined. The difference yields a measure of the nitrogen 

 contained in p^Trollidine (prolin and oxyprolin) or indol (trypto- 

 phane) rings. 



During the hydrolysis by hydrochloric acid a small amount 

 of a very deeply colored precipitate separates out. The nitro- 

 gen content of this precipitate is the so-called "melanin" or 

 "humin" nitrogen. According to Gortner and Blish (27) this 

 is derived from a portion of the tryptophane and, in the presence 

 of a sufficiency of carbohydrate, the yield of melanin nitrogen 

 is a quantitative measure of the tryptophane content of the 

 protein. 



7. Tjrpes of Union in the Protein Molecule. — Following the 

 recognition of the fact that the proteins are complexes built up 

 by the union of amino-acids, the question of the mode of union 



