THE CHEMICAL CONSTITUTION OF ITS UNITS 67 



The formation of indole by the putrefaction of proteins was observed 

 by Kiihne and by Nencki in 1874, that of skatole by Brieger in 1877, of 

 skatolecarboxylic acid by E. and H. Salkowski in 1880, and of skatole- 

 acetic acid by Nencki in 1889. Nencki regarded these substances as 

 originating from a skatoleaminoacetic acid in the protein in a manner 

 similar to that by which phenol, cresol, oxyphenylacetic acid and 

 oxyphenylpropionic acid arose from tyrosine, namely : 

 C . CH 3 C . CH 3 



C 6 H/^C.CH(NH a )COOH -> C 6 HC . CH 2 COOH 



NH NH 



Skatoleaminoacetic acid Skatoleacetic acid 



C . CH 3 C . CH 3 C . H 



. COOH _> C 



NH NH NH 



Skatolecarboxylic acid Skatole Indole 



Tryptophane was found by Hopkins and Cole to have the empirical 

 formula C 11 H 12 N 2 O 2 and to yield large amounts of indole and skatole 

 when heated, and when subjected ; to putrefaction by bacteria the above- 

 mentioned four products resulted. As under anaerobic conditions a 

 large yield of skatoleacetic acid was obtained, and as skatole was the 

 principal product when it was fused with potash, Hopkins and Cole 

 regarded their substance as skatoleaminoacetic acid rather than the 

 isomeric indoleaminopropionic acid. 



The constitution of indole and skatole had been proved by synthe- 

 sis, but that of the other two compounds had not been determined, and 

 Nencki's formulae for them were accepted. Investigations by Ellinger 

 and Gentzen in 1903, who found that in the large intestine indole was 

 formed in large amounts from tryptophane, but skatole only in small 

 amounts, and that skatole only gave traces of indole under the same 

 conditions led Ellinger to doubt the accuracy of Nencki's formulae for 

 skatoleacetic and skatolecarboxylic acids, more especially as Wislicenus 

 and Arnold's skatolecarboxylic acid, which was synthesised from pro- 

 pionyl formic acid phenylhydrazone had the formula 



C.CH 3 



C 6 H/V}.COOH 

 NH 



and was not identical with the putrefaction product, which might equally 

 well possess the constitution of 



C.CH 2 .COOH 



5* 



NH 



