144 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



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-0 



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

 and showed it to be identical with that prepared by the action of 

 methyl iodide upon d-alanine. 



The change produced by nitrosyl bromide on d-alanine 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 

 thus acts optically normally on the ester and optically abnormally 

 upon the acid. 



Silver oxide behaves like nitrosyl bromide ; it acts normally on a 

 derivative, but abnormally on the acid. These changes 



-bromopropionic acid > d-lactic acid 



1-bromopropionyl-glycine > 1-lactic acid 



and hydrolysis 



KOH 

 1-bromopropionic acid > 1-lactic acid 



were observed. 



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 d-a-bromisovalerianic acid > 1-valine 



NOBr NH 3 

 d-valine > 1-bromisovalerianic acid > d- valine 



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



It seemed very improbable that a " Walden inversion " had occurred 

 twice, but the further experiments proved that this had actually 

 happened : 



