PRODUCTS OF DECOMPOSITION 5 



pended upon the fractional crystallization either of the free 

 ammo-acids or of their salts. They were not even approximately 

 quantitative and the isolation and identification of a given amino- 

 acid could only be effected with certainty when that acid was 

 present in relatively large amounts. Until 1890 only mono- 

 amino-acids were known, with certainty, to occur among the 

 products of protein hydrolysis. Then Drechsel (13) discovered 

 lysin and lysatinin, Hedin (29) isolated arginin and Kossel (34) 

 isolated histidin from among the dissociation products of proteins. 



The attainment of our present relatively extensive knowledge 

 of the nature and yield of the products of the hydrolysis of pro- 

 teins is an achievement of the past twenty years, and we owe it 

 in the first place to the labors of Emil Fischer and of Kossel and 

 their pupils. 



In 1900 Kossel and Kutscher (39) (40) succeeded in working 

 out a method for the quantitative separation and estimation of 

 the diamino-acids lysin, arginin and histidin which Kossel (35) 

 calls the hexone bases.* The method depends in principle upon 

 the precipitation of arginin and histidin in the form of their 

 silver salts, and of lysin first by phosphotungstic acid and then. 

 by picric acid. A partial but not quantitative separation of the 

 diamino-acids from the monoamino-acids in a protein digest, 

 can be procured by precipitation of the former with phospho- 

 tungstic acid. 



In 1901 Emil Fischer (15) introduced a new method of sepa- 

 rating and estimating the monoamino-acids, which, with modi- 

 fications suggested by experience, is the one to which we owe the 

 greater part of our present knowledge of these products of protein 

 hydrolysis. The hydrolysis is carried out, as a rule, by boiling 

 with hydrochloric acid.f The amino-acids which result are then 

 converted into their esters by dissolving them in alcohol and 

 esterifying by saturation of the solution with dry hydrochloric 

 acid gas. The mixed esters thus obtained are separated by 



* Owing to the fact that they each contain six atoms of carbon. An 

 especial interest attaches to these substances, since, unlike the monoamino 

 acids, they are predominantly basic in character, and their relation to the 

 hexoses suggests their possible importance in carbohydrate metabolism. 



t N. Zelinsky (74) has suggested the employment of formic acid as a hy- 

 drolysing agent. According to this investigator the hydrolysis by formic acid 

 is much more rapid than that which is brought about by boiling the proteins 

 with hydrochloric acid. 



