46 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



and on this account gave cystine the constitution of 



CH 2 (H 2 N) 



CS or C 3 H 5 NS0 2 



COOH 



Hoppe-Seyler, as cited by Baumann and Preusse, showed that the 

 nitrogen of cystine was separated off as ammonia by alkalies and not 

 as methylamine, as would be expected from this formula, and moreover 

 maintained that the formula was C 3 H 7 ] s iSO 2 . 



Baumann and Preusse's investigations in 1881 upon the fate of 

 bromobenzene in the animal body, though they were only indirect evi- 

 dence in regard to the constitution of cystine, were of great importance, 

 as they were carried out at the time when cystine was a very scarce 

 compound and only obtainable from calculi. They found that, when 

 bromobenzene was given to animals, it was excreted in the urine in 

 combination with a sulphur-containing compound, which combination had 

 the formula C n H 12 BrSNO 3 , and in this they were confirmed by Jaffe. 

 When boiled with hydrochloric acid, this compound was converted 

 into acetic acid and a substance C 9 H 10 BrNSO 2 , from the empirical 

 formula of which Baumann and Preusse supposed that it was cystine 

 C 3 H 7 NSO 2 , in which one of the hydrogen atoms was replaced by 

 C 6 H 4 Br. Their further experiments led them to the conclusion that it 

 was really a derivative of cystine. On decomposition by alkali, this 

 latter compound yielded bromophenylmercaptan, ammonia and another 

 substance, which they eventually recognised must be pyruvic acid. It 

 had been shown that Dewar and Gamgee's formula for cystine, which 

 was also based upon the formation of pyruvic acid, was not accurate, so 

 they proposed the following : 



/,NH 2 / NH 2 ^/NH.CO.CHj 



CH 3 . C SH CH 3 . C S(C 6 H 4 Br) CH 3 . C - S(C 6 H 4 Br) 



\COOH \COOH \COOH 



For cystine For bromophenylcystine For bromophenylmercapturic 



acid 



Baumann next found that cystine, on reduction with zinc and 

 hydrochloric acid, was converted into a new base, which he called 

 cysteine; this gave the same products on decomposition as cystine, 

 into which it was easily reconverted by oxidation. He therefore recog- 

 nised that these compounds were related to each other, as a mercaptan 

 is to a disulphide ; consequently the formula 



CH 3 .C SH 

 \COOH 



