48 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



These reactions also showed how taurine might originate in the body 

 from cystine. 



At about the same time Neuberg, by treating cystine with nitric 

 acid, obtained isethionic acid, which pointed to the correctness of Fried- 

 mann's formula ; it at any rate showed that the sulphur and nitrogen 

 atoms were attached to different carbon atoms. In the reaction the SH 

 group was oxidised to the SO 3 H group, and the NH 2 group was con- 

 verted into the OH group by nitrous acid formed in the oxidation : 



CH 2 .SH CH 2 .SO 3 H CH 2 .SO 3 H 



CH.NH 2 _> CH.NH 2 _> CH 2 OH + CO 2 

 COOH COOH 



The synthesis of cystine by Erlenmeyer jun. in 1903, which was 

 more fully described by him and Storp in 1904, showed that Friedmann's 

 formula was correct : 



Benzoylserine was heated with phosphorus pentasulphide, and the 

 product after hydrolysis with hydrochloric acid, gave cysteine which 

 was converted by oxidation into cystine : 



CH 2 OH CH 2 SH CH 2 SH CH 2 . S-S . CH 2 



CH . NH . COC 6 H 5 -> CH . NH . COC 6 H 5 -> CH . NH 2 -> CH . NH 2 CH . NH 2 



COOH COOH COOH COOH COOH 



Benzoylserine Benzoylcysteine Cysteine Cystine 



Another synthesis of cystine was described by Fischer and Raske 

 in 1908 (see page 75) which is similar to that of Erlenmeyer. 



The formation of bromophenylmercapturic acid from bromobenzene 

 and cystine in the organism, if it had the formula given it by Baumann 

 now seemed scarcely possible, unless an isomeric a-thio-/3-aminopropionic 

 acid were also present in the protein molecule together with the di-/3- 

 thio-a-aminopropionic acid or cystine. The investigation of their con- 

 stitution was therefore taken up by Friedmann in 1904, who succeeded 

 in showing that they were also derived from /3-thio-a-aminopropionic 

 acid and not from the isomeric a-thio-/3-aminopropionic acid. 



By the action of nitrous acid in hydrochloric acid solution on bromo- 

 phenylcystine, prepared by Baumann's method, chlorobromophenyl- 

 thiopropionic acid was obtained, which, on reduction, gave bromophenyl- 

 thiolactic acid. This was identical with the substance prepared from 

 /3-iodopropionic acid and sodium bromophenylmercaptan : 



C 6 H 4 BrSNa + I . CH 2 = Nal + C 6 H 4 Br . S . CH 2 

 CH 2 CH 2 



COOH COOH 



