io6 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



and in conjunction with Suter's observation that a-thiolactic acid was 

 formed by the hydrolysis of horn, this formula for cystine was accepted. 

 The results obtained, however, scarcely justified this formula as pointed 

 out by Friedmann in 1902, who showed conclusively that the cystine, 

 obtained from proteins, had not this constitution. 



It had been found by Jochem in Hofmeister's laboratory that amino 

 acids, when treated with nitrous acid in hydrochloric acid solution, were 

 converted into the corresponding chloro-derivatives, and Friedmann, on 

 applying this reaction to cystine obtained dichlorodithiopropionic acid ; 

 this when reduced gave /3-thiopropionic acid, and on subsequent oxida- 

 1 tion /3-dithiopropionic acid, identical with the compound prepared from 

 /3-iodopropionic acid and potassium hydrogen sulphide. The sulphur 

 atoms in cystine and cysteine were therefore in the /9-position, and it 

 remained to show in which position the amino group was situated, 

 whether as 



/SH 



CH( CH a SH 



I \NH 2 

 CH 2 or CH . NH 2 



COOH COOH 



By oxidising cystine with bromine water, Friedmann obtained 

 cysteic acid, i.e., either 



/S0 3 H 



CH< CH 2 .S0 3 H 



I \NH 2 

 CH 2 or CH . NH 2 



COOH COOH 



which, when heated, was converted by loss of carbon dioxide into 

 taurine, which is only explainable by the second formula 



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 : 



