74 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Warburg in the conversion of alanine into the corresponding halogen 

 fatty acid by nitrosyl bromide and in the reconversion of this compound 

 into alanine by the action of ammonia : 



NH 3 

 d-alanine < d-bromopropionic acid 



NOBr 



NOBr 



NH 3 

 1-bromopropionic acid % 1-alanine 



It was termed the " Walden inversion " by Fischer in 1907. 



A change in configuration occurs either by the action of ammonia 

 or by the action of nitrosyl bromide. By studying the conversion 

 by the action of ammonia under various conditions, Fischer was able 

 to show conclusively that this reagent behaved optically normally, 

 which result was confirmed by a later experiment upon optically active 

 trimethyl-a-propiobetaine (a-homobetaine), 



CH 3 . CH . CO 



(CH 3 ) 3 N - O 



which he prepared from trimethylamine and d-a-bromopropionic acid, 

 and showed was identical with that prepared by the action of methyl 

 iodide upon d-alanine. 



The change produced by nitrosyl bromide was found to be optically 

 abnormal ; the following reactions occurred : 



NOBr 



d-alanine > 1-bromopropionic acid 



NOBr 

 d-alanine ester > d-bromopropionic acid ester 



which were confirmed by similar observations upon 1-leucine ester, 

 1-phenylalanine ester and on 1-aspartic ester. The same reagent can 

 thus sometimes act optically normally and sometimes optically abnor- 

 mally upon such similar compounds as acid and ester. 



Phosphorus pentachloride most probably acts optically normally 

 since it yields products having the same configuration whether it acts 

 upon a hydroxy acid or its ester ; further evidence is, however, still 

 required. 



Silver oxide behaves like nitrosyl bromide, sometimes normally, 

 sometimes abnormally. The changes 



Ag 2 



1-bromopropionic acid - > d-lactic acid 



Ag 2 



1-bromopropionyl-glycine - > 1-lactic acid 



and hydrolysis 



KOH 

 1-bromopropionic acid > 1-lactic acid 



were observed. 



