THE CHEMICAL CONSTITUTION OF ITS UNITS 75 



It may be concluded that the " Walden inversion " is limited to the 

 reactions between nitrosyl bromide and the amino group and between 

 silver oxide and halogen fatty acid, and is dependent upon the presence 

 of the carboxyl group. 



All amino acids would be expected to behave in the same way as 

 leucine, etc., but valine was found by Fischer and Scheibler in 1908 to 

 behave differently: 



NOBr NH 3 

 1-valine > bromovalerianic acid > 1-valine 



that is, the same valine and not its optical antipode was obtained. 

 Either a " Walden inversion " has occurred twice, which is very im- 

 probable, or the change into bromovalerianic acid has taken place 

 without a change in the configuration. 1 It appears to be due to the 

 effect of the isopropyl group. In valine, 



(CH 3 ) 2 . CH . CH(NH 2 ) . COOH, 



it is attached directly to the asymmetric carbon atom ; in leucine, 



(CH 3 ) 2 . CH . CH 2 . CH(NH 2 ) . COOH, 



a methylene group is present between it and the CH(NH 2 ) group 

 which contains the asymmetric carbon atom. 



No " Walden inversion " was found to take place when amino acids 

 were converted into the corresponding hydroxy acid by the action of 

 nitrous acid. It was therefore possible to determine the relationship of 

 serine to glyceric acid : d-serine was converted by nitrous acid into a 

 glyceric acid which Neuberg and Silbermann regarded as 1-glyceric 

 acid on account of its relationship to 1-tartaric acid, but which Neuberg, 

 a little later, stated required confirmation. 



In 1907 and 1908 Fischer and Raske correlated together the con- 

 figurations of 1-serine, d-alanine and 1-cystine by means of l-a-amino-/3- 

 chloropropionic acid, 



CH 2 C1 . CH(NH 2 ) . COOH, 



which they obtained from 1-serine methyl ester by the action of phos- 

 phorus pentachloride and subsequent hydrolysis with hydrochloric acid. 

 By reducing it with sodium amalgam they obtained d-alanine, and by 

 treating it with barium hydrosulphide and oxidising the resulting cysteine, 

 by drawing a current of air through the solution, they obtained cystine. 



1 As this monograph is going through the press a more recent publication by Fischer 

 and Scheibler (Ber., 1908, 41, 2891) points to the probability of the " Walden inversion" 

 occurring twice. 



