THE CHEMICAL CONSTITUTION OF ITS UNITS 73 



these derivatives, but, by making the p-nitrobenzoyl compound, Fischer 

 and Jacobs obtained d- and 1-serine. Isoserine and diaminopropionic 

 acid must also be added to the list of optically active amino acids 

 separated by Fischer and his pupils. 



By combining a-aminophenylacetic acid with d-camphorsulphonic 

 acid Betti and Mayer in 1908 separated it into its isomers. This seems 

 to be the first case in which the basic function of an amino acid has 

 been requisitioned for purposes of separation ; in all the above cases, 

 the acidic function, by combination with optically active bases, has been 

 made use of. 



The separation of serine into its optical antipodes was of the 

 greatest importance, since it has enabled Fischer to correlate together 

 the configuration of d-alanine, 1-serine and 1-cystine, and also to connect 

 them with d-glucose. 



It was first observed by Walden, in 1896, that a change of configura- 

 tion took place in the conversion of the malic acids into the chlorosuc- 

 cinic acids and vice versa. His results were collected together in 1897 

 and expressed in the following scheme, in which was included Tilden 

 and Marshall's observations on aspartic acid : 



PC1 5 



l-chlorosuccinic acid < ~, v 



f KOH, NH 3 



1 -aspartic acid 



PCI 



1-malic acid 



d-malic acid 



t 



d-chlorosuccinic acid 



KOH, NR 3 



Walden concluded that potassium hydrate and phosphorus penta- 

 chloride acted optically normally, i.e., without alteration of the con- 

 figuration, but that silver oxide, and therefore also nitrous acid and 

 nitrosyl chloride, acted optically abnormally, but as to which of these 

 reactions was really the normal one he was not able to decide. The 

 conclusion was remarkable, since the action of silver oxide takes place 

 in aqueous solutions at a low temperature and the effect of potash in 

 producing racemisation is well known. Still more curious is the sup- 

 position that nitrous acid and nitrosyl chloride act optically abnormally. 

 A similar change in rotation was observed in 1905 by Fischer and 



