THE CHEMICAL CONSTITUTION OF ITS UNITS 71 



to be the actual synthesis of a natural compound, but there seems no 

 reason to doubt the possibility of separating synthetical asparagine 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 separation. 



From inactive cystine Neuberg and Mayer separated d-cystine, using 

 Aspergillus niger instead of Penicillium glaucum , which gave no result 

 with this amino acid. 



Not only moulds, but also yeasts can be employed in the separation 

 of optically active compounds as was shown by F. Ehrlich in 1906, who 

 obtained in this way 1-alanine, d-leucine, 1-valine. Further, amino acids, 

 other than those which occur in nature, can be separated by moulds and 

 yeasts into their components, e.g., n-aminocaproic acid, methylethyl- 

 aminoacetic acid. 



It was first shown by E. Fischer, in 1894, that enzymes were specific 

 in their action ; thus maltase acts only upon a-glucosides and emulsin 

 only upon /@-glucosides. Later, he found that trypsin acted " asymmetri- 

 cally " upon inactive polypeptides, e.g.^ alanyl-leucine was hydrolysed in 

 such a way that only the compound composed of d-alanine and 1-leucine, 

 the natural isomers, was split up into its constituents, whereas the com- 

 pound composed of 1-alanine and d-leucine was unattacked. Again, 

 inactive leucine ester was found by Warburg to be only partially hydro- 

 lysed by trypsin ; he obtained 1-leucine and d-leucine ester. 



Kossel and Dakin, in 1904, found that d-arginine was hydrolysed by 

 the enzyme arginase into d-ornithine and urea ; and by using this enzyme 

 Riesser, in 1906, separated dl-arginine, which he had prepared by heating 

 d-arginine with sulphuric acid to 160-180 C. into 1-arginine, d-ornithine 

 and urea, the racemic compound being hydrolysed asymmetrically by 

 the enzyme. 1-Ornithine can be prepared from the 1-arginine by hydro- 

 lysis with baryta. 



The biological method thus only serves for the preparation of that 

 isomer which does not occur in nature, since the mould or yeast or 

 enzyme destroys the naturally occurring form, leaving the other isomer 

 untouched, or according to Marckwald and Mackenzie, it acts upon the 

 natural isomer more rapidly than upon the other. The method therefore 

 does not lead to the synthesis of the naturally occurring amino acid. 



