THE CHEMICAL CONSTITUTION OF THE UNITS 107 



C0 2 



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

 more fully described by him and Stoop 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 S - 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 148) 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-aminopropion- 

 ic acid were also present in the protein molecule together with the 

 di-y5-thio-a-aminopropionic acid, or cystine. The investigation of their 

 constitution was therefore taken up by Friedmann in 1904, who suc- 

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

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



By the action of nitrous acid in hydrochloric acid solution on 

 bromophenylcystine, prepared by Baumann's method, chlorobromo- 

 phenylthiopropionic acid was obtained, which, on reduction, gave 

 bromophenylthiolactic acid. This was identical with the substance 

 prepared from /3-iodopropionic acid and sodium bromophenylmer- 

 captan : 



C 8 H 4 BrSNa + I . CH 2 Nal + C 9 H 4 Br . S . CH a 

 CH a CH, 



COOH COOH 



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- 



