THE CHEMICAL CONSTITUTION OF ITS UNITS 49 



and therefore the SH group was in the /3-position. Further proof was 

 given by Friedmann by the synthesis of bromophenylmercapturic acid 

 from cysteine. p-Bromodiazobenzene chloride was combined with 

 cysteine ; this compound, when decomposed by dilute soda, gave bromo- 

 phenylcysteine, which, on acylation, was converted into bromophenyl- 

 mercapturic acid : 



CH 2 SH CH 2 .S.N 2 C 6 H 4 Br CH 2 . S . C 6 H 4 Br CH 2 . S . C 6 H 4 Br 



CH.NH 2 -> CH.NH 2 -> CH . NH 2 -> CH.NH.CO.CH 3 



COOH COOH COOH COOH 



Cysteine Bromodiazobenzene- Bromophenylcysteine Bromophenylmer- 



cysteine capturic acid 



It was first observed by Suter, in 1895, that a-thiolactic acid was 

 formed by the hydrolysis of proteins, and it was constantly obtained by 

 Friedmann. It was always regarded as a secondary product, but its 

 formation from cystine could not be explained, as cystine is a yS-thiopro- 

 pionic acid. 



In 1904 Morner found that pyruvic acid was a constant product of 

 hydrolysis of proteins, and that this compound gave a-thiolactic acid 

 with hydrogen sulphide. Its formation was thus explained, but it was 

 curious that hair, which is very rich in sulphur, gave less pyruvic acid 

 than horn, which is less rich, and that caseinogen, which contains very 

 little sulphur, also gave it. Morner therefore supposed that there was 

 another sulphur-containing compound in the protein molecule, which 

 supposition was strengthened by Neuberg and Mayer's statement that 

 stone cystine differed from protein cystine in many of its physical pro- 

 perties. Morner's subsequent work on the decomposition of stone 

 cystine, when he obtained a-thiolactic acid, ammonia and alanine 

 helped to support this idea ; he regarded the alanine as formed from 

 cystine and the a-thiolactic acid from the isomeric a-thio-/3-aminopro- 

 pionic acid, both of which he supposed were present in the stone cystine 

 in equal quantities. 



Fischer and Suzuki soon afterwards showed that Neuberg and 

 Mayer's stone cystine contained tyrosine, and that its different behaviour 

 to protein cystine was due to the presence of this compound. Rothera 

 also could find no difference between stone cystine and protein cystine, 

 and further, Gabriel's synthesis of isocysteine or a-thio-/3-aminopropionic 

 acid and isocystine, which had quite different properties to cystine, 

 though the two were much alike in many of their reactions, proved 

 that stone cystine and protein cystine must be identical substances. 



Finally, it has been shown by Friedmann that a-thiolactic acid, ammonia 

 PT. i. 4 



