THE CHEMISTRY OF THE PROTEINS 91 



discover another method. This consisted in the preparation 

 of the formyl derivative by heating the amino acid with 

 anhydrous formic acid at ioo° C. ; the separation of the inactive 

 compound into its optically active isomers was again effected 

 by means of the brucine salts ; and finally the hydrolysis of the 

 formyl compound was performed by heating for one and a half 

 hours with ten times its quantity of 10 per cent, hydrochloric 

 acid. Large amounts of active leucine and valine have thus 

 been obtained. The optically active components of the latter 

 compound, a-aminoisovalerianic acid, were first prepared by 

 this method. 



Quite lately Fischer and Jacobs have separated serine into 

 its two optical isomers ; neither the benzoyl derivative nor the 

 formyl derivative gave the desired result, but they were success- 

 ful when they used the />-nitrobenzoyl compound. Still more 

 recently, the optically active forms of isoserine and diamino- 

 propionic acid have been prepared. 



It might be here mentioned that the biological method of 

 separating the optically active compounds of the amino acids 

 has recently been carried out by F. Ehrlich, who employed 

 yeast in his experiments instead of the usual moulds. 



In order to obtain the optically active amino acids, the 

 racemic compound must first be synthesised. Two methods are 

 generally employed for this purpose : 



(1) The action of ammonia on the halogen fatty acid. 



(2) The combination of aldehydes with hydrogen cyanide 

 and ammonia and the subsequent hydrolysis of the amino- 

 cyanohydrin. 



Fischer has made use of both these methods : by the first 

 method he prepared a-aminobutyric acid, a-amino-^-caproic 

 acid, valine, and a-amino-zj-valerianic acid. The latter two 

 amino acids were also synthesised by the second or Strecker's 

 method, by which isobutyl-a-aminoacetic acid or leucine was 

 prepared from isovaleraldehyde. The identity of the benzoyl 

 compound with that from natural leucine, as obtained from 

 proteins and for a long time erroneously supposed to be 

 a-amino-;z-caproic acid, definitely proved the constitution of this 

 amino acid, which had already been pointed out by E. Schulze 

 and Likiernik in 1891, but had not been generally recognised. 



Fischer has extended the first method by introducing a new 

 mode of preparing the a-halogen fatty acid. This consists in 



