THE CHEMICAL CONSTITUTION OF ITS UNITS 47 

 was really that of cysteine, and that of cystine was 



^NHa ^NH 2 

 CH 3 . C . S S . C . CH 3 



COOH 



the former bromophenylcystine being bromophenylcysteine, etc. 



The actual formation of pyruvic acid from various mercapturic acids 

 upon which these formulae for cysteine and cystine were founded, was 

 only shown later by Baumann's pupils, Konigs, Brenzinger and Schmitz, 

 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- 

 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 /3-position, and it 

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

 whether as 



/SH 



CH< CH 2 SH 



I \NH 2 

 CH 2 or CH . NH 2 



COOH COOH 



By oxidising cystine with bromine water, Friedmann obtained 

 cysteic acid, t'.e. t either 



/S0 3 H 



CH/ CH 2 . SO,H 



I \NH 2 | 



CH 2 or CH . NH 2 



COOH COOH 



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

 taurine, which is only explainable by the second formula 



CH 2 . S0 3 H CH 2 . SO 3 H 



CH.NH 2 -> CH 2 .NH 2 



COOH 



Cysteic acid Taurine 



