6 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



study of the amino acids and their derivatives, introduced a new method 

 of isolating and separating the monoamino acids, which depended upon 

 the fractional distillation in vacuo of their esters, and which is now 

 commonly known as the ester method. This method, though not yet 

 really quantitative, has enabled us to obtain a knowledge of some 70 

 per cent, of the total products resulting by hydrolysis, and it has shown 

 us that phenylalanine, serine and alanine^ which were only known to 

 occur in a few, were present in all proteins, and that phenylalanine 

 in its distribution was the principal aromatic constituent, for it often 

 exceeds in amount that of tyrosine and occurs when this latter is 

 absent. Further, it has demonstrated the presence of two new com- 

 pounds, proline and oxyproline. In their isolation and estimation the 

 various units are therefore divided into two main groups : 



I. The Monoamino acids, including proline and oxyproline. 



II. The Diamino acids, including histidine. 



I. THE MONOAMINO ACIDS. 



The Isolation and Estimation of Tyrosine, Cystine and Diaminotrioxy- 



dodecanic Acid. 



Three of the amino acids, namely, tyrosine, cystine and diaminotrioxy- 

 dodecanic acid are characterised by their extremely slight solubility 

 in neutral aqueous solutions. They are therefore easily obtained after 

 hydrolysis by acids by neutralising and concentrating the solution, 

 when they crystallise out. 



The separation of cystine and tyrosine as they are obtained by 

 hydrolysis with hydrochloric acid was described by Morner in 1901. 

 The protein hair, keratin from horn, eggshells, etc. was boiled with 

 five times its quantity of 1 3 per cent, hydrochloric acid under a reflux 

 condenser on a water bath for six to seven days. The solution was 

 then decolorised with charcoal and evaporated in vacua, and the 

 residue dissolved in 60 to 70 per cent, alcohol. The two acids then 

 crystallised out on neutralising with soda, and were separated by 

 fractional crystallisation from ammonia ; if much tyrosine was present 

 it separated out first, but if cystine exceeded tyrosine in quantity this 

 compound crystallised out first ; the remainder was only separated with 

 difficulty. Embden separated the mixture of the two acids by means 

 of very dilute nitric acid, in which tyrosine is very easily soluble, but 

 cystine with difficulty. Their separation may also be effected by precipi- 

 tation with mercuric sulphate in 5 per cent, sulphuric acid solution in 

 which the mercury compound of tyrosine is soluble (Hopkins and Cole). 



