THE CHEMICAL CONSTITUTION OF THE UNITS 139 



E. THE OPTICALLY ACTIVE AMINO ACIDS. 

 I. Separation of Racemic Mixture. 



With the exception of glycine, all the amino acids contain an asym- 

 metric carbon atom, and they are therefore capable of existence in two 

 optically active forms. In one of these forms they are present in the 

 protein molecule, and the synthesis of a naturally occurring amino acid 

 is only completed when the synthesised compound has been separated 

 into its optically active components. 



Three methods are known, all due to Pasteur, by which an inactive 

 mixture can be separated into its optically active isomers : 



1. The crystallisation and mechanical separation of the two 

 isomers. 



2. The action of micro-organisms moulds, yeasts which destroy 

 the one isomer more rapidly than the other. This is known as the 

 biological method. 



3. The fractional crystallisation of the salts of these compounds 

 with optically active bases or acids. 



By all these methods the optically active forms of the amino acids 

 have been prepared. 



By the first method Piutti, in 1887, obtained dextro- and laevo- 

 aspartic acid. He re-crystallised natural asparagine and separated it 

 into dextro- and laevo-asparagine, from which compounds he prepared 

 both the dextro- and the laevo-aspartic acids. As, however, Piutti 

 used asparagine from vetch seedlings, this separation cannot strictly be 

 said to be the actual synthesis of a natural compound, but there 

 seems no reason to doubt the possibility of separating synthetical as- 

 paragine by this means. 



Glutamic acid, the homologue of aspartic acid, according to Menozzi 

 and Appiani, on recrystallisation from water, can be obtained in its two 

 enantiomorphous forms. 



By the second method Schulze and Bosshard prepared d-leucine 

 and 1-glutamic acid, and Engel prepared d-aspartic acid. Menozzi 

 and Appiani also separated glutamic acid by this method. In all 

 these cases the mould Penicillium glaucum was used to effect the sepa- 

 ration. 



From inactive cystine Neuberg and Mayer separated d-cystine, 

 using Aspergillus niger instead of Penicillium glaucum, which gave no 

 result with this amino acid. Fischer found that Aspergillus niger, when 

 cultivated on alanine, destroyed 10 per cent, of the natural isomer. 



