90 SCIENCE PROGRESS 



i.e. consist of a mixture of equal parts of the dextro- and laevo- 

 rotatory varieties. The real synthesis of the natural compounds 

 is, therefore, only attained when the synthetical substances 

 have been separated into their two constituents. Two methods 

 may be used for this purpose : the biological method — 

 destroying the one isomer by means of micro-organisms, such 

 as Penicillium glaucum and Aspergillus niger — had been used 

 by Schulze and Bosshard to obtain leucine and glutamic acid, 

 and by Engel to obtain aspartic acid ; but the chemical method 

 of combining with optically active compounds to form salts 

 and fractionally crystallising these salts had not been employed 

 with any success. This was probably due to the small affinity 

 which the simple monoamino acids have for acids and bases. 

 The aminodicarboxylic acids, on the other hand, are moderately 

 strong acids, which combine with bases to form salts of this 

 kind, but the desired results were not achieved in the attempts 

 thus made to separate the optically active forms. 



Fischer found that by preparing the benzoyl derivatives of 

 the amino acids, their acidic character was greatly increased 

 and that they then formed with the optically active bases — 

 strychnine, brucine, and morphine — stable salts which were 

 soluble with greater difficulty than the amino acids and more 

 easily isolated. Hippuric acid, or benzoylglycine, has been 

 known for a long time ; the derivatives of the other amino acids 

 are similar in constitution. The ordinary process of benzoylating 

 by the Schotten-Baumann method in the presence of excess of 

 caustic soda gave poor yields of the benzoyl derivatives of the 

 amino acids, but by substituting sodium bicarbonate for the 

 caustic soda and using a large excess of benzoylchloride 

 Fischer obtained excellent results, and succeeded in preparing 

 the optical isomers of alanine, leucine, tyrosine, phenylalanine, 

 aspartic and glutamic acids, as also of a-aminobutyric acid and 

 a-amino-»-caproic acid. 



For the preparation of large quantities of optically active 

 amino acids, such as is required in the synthesis of the proteins, 

 the benzoyl method for the separation of the isomers was not 

 found to be convenient on account of the fact that the recon- 

 version into the amino acid was often carried out with difficulty, 

 since it required a long time and large quantities of acid 

 (ioo times the quantity of 10 per cent, hydrochloric acid) to 

 completely hydrolyse the compound. Fischer had therefore to 



