148 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The correlation of the configurations of 1-serine, d-alanine and 

 1-cystine was established in 1908 by Fischer and Raske by means of 

 l-a-amino-y3-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. 



If l-serine has the configuration represented, d-alanine and 1-cystine 

 will have the groups attached to the asymmetric atom arranged in the 

 same order as in 1-serine : 



CH, CHj,OH CH 2 S S CH 2 



I J I I 



NH 2 C H < NH 2 C H > NH 2 C H NH 2 C H 



COOH COOH COOH COOH 



d-alanine 1-serine 1-cystine 



The foundation for the configuration of these amino acids will be 

 obtained if a-amino-$-chloropropionic acid can be converted into 

 aspartic acid, the configuration of which is known from its relationship 

 to malic acid. 



Fischer and Flatau have commenced this synthesis by preparing 

 methylsuccinic acid from d-alanine : 



d-alanine - 1-a-bromopropionic acid > 1-methylsuccinic acid. 



If no "Walden inversion" occurs the 1-bromopropionic acid and 

 1-methylsuccinic acid will have the same configuration. 



Kay has shown a correlation between methylisoserine and bromo- 

 methyl-lactic acid : 



NOBr 



d-o-methylisoserine > d--bromo-a-methyl-lactic acid. 



