THE CHEMISTRY OF! THE PROTEINS QI 
discover another method. This consisted in the preparation 
of the formyl derivative by heating the amino acid with 
anhydrous formic acid at 100° 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 Jately 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 f-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-v-caproic 
acid, valine, and a-amino-v-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-v-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 
