io THE CHEMICAL CONSTITUTION OF THE PROTEINS 



hydrolysis is finished. Further hydrolysis is required if the biuret test 

 is positive. Many proteins are hydrolysed with great slowness, as has 

 been pointed out by Osborne and Jones who, by making use of Van 

 Slyke's method for estimating amino groups, find that hydrolysis with 

 concentrated hydrochloric may only be complete after two to five days. 

 The length of time of hydrolysis is therefore of primary importance 

 for a complete analysis. 



Hugounenq, who has employed hydrofluoric acid, finds that the 

 results depend on the strength of the acid ; the stronger the acid the 

 greater is the amount of complex polypeptides. Many hours' boiling 

 with dilute acid are required to effect complete hydrolysis. 



Abderhalden, Medigreceanu and Pincussohn have compared the 

 hydrolysis by acids and alkalies. Alkali seems to produce the most 

 complete hydrolysis. Abderhalden and Brahm found that a resistant 

 body formed from silk could only be completely hydrolysed by alkali. 

 The main difference in the results of hydrolysis by acids and alkalies 

 consists in the products ; the optically active amino acids are com- 

 pletely racemised by the alkali, whereas acids only produce partial 

 racemisation. Arginine is destroyed by boiling with alkali and con- 

 verted into ornithine and ammonia. Cystine is also decomposed by 

 alkali. 



The Isolation and Estimation of the Units. 



The amino acids which compose the protein molecule can be 

 divided into two main groups : 



A. The monoamino acids, including proline and oxyproline. 



Tyrosine and cystine are usually treated independently. 



B. The diamino acids, including histidine. 



The three compounds in this group were formerly called the hexone bases on account 

 of their basic properties and the fact that each of them contains six carbon atoms. 



The remaining unit, tryptophane, is almost completely destroyed 

 by hydrolysis by acids ; it can only be isolated after hydrolysis by 

 trypsin. 



Ammonia is estimated by hydrolysing the protein with concentrated 

 hydrochloric acid, removing the great excess of acid by evaporation 

 under reduced pressure, adding excess of magnesia, distilling off 

 the ammonia in vacua and collecting it in excess of standard acid. 

 This operation is usually carried out in the determination of the 

 distribution of the total nitrogen amongst the two main groups. The 



