THE CHEMICAL CONSTITUTION OF THE UNITS 135 



ether employed in washing the precipitated protein. The reddish- 

 violet colour produced when proteins are heated with concentrated 

 hydrochloric acid is due to tryptophane and to furfural formed from 

 carbohydrate in the protein ; it is very marked when cane sugar or 

 furfural is added to a protein which does not give the reaction very 

 strongly. 



Reichl's reaction again a green to blue colour when proteins are 

 treated with an aldehyde such as benzaldehyde, a drop of ferric chloride 

 and concentrated hydrochloric acid is due also to the presence of 

 tryptophane in the protein. Heimrod and Levene have tested a large 

 number of aldehydes and have obtained positive results; Rohde, 

 Steensma and Bardachzi have tested the colour reaction with aromatic 

 aldehydes ; Ekenstein and Blanksma with various furfuraldehydes. 



It would appear from Osborne and Harris' results with the vege- 

 table proteins that the production of a violet colour requires the pre- 

 sence of carbohydrate for the formation of furfuraldehyde. 



The reaction with formaldehyde is used as a means of detecting 

 formaldehyde in milk; its cause was also explained by Hopkins and 

 Cole's discovery of tryptophane. Rosenheim at one time thought that 

 the Adamkiewicz reaction was due to formaldehyde and an oxidising 

 agent in the sulphuric acid, but Dakin has shown definitely that the 

 reaction is due to glyoxylic acid. 



The formation of indole by the putrefaction of proteins was ob- 

 served 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 skatoleacetic acid by Nencki in 1889. Nencki regarded these 

 substances as originating from skatoleaminoacetic acid in the protein 

 in a manner similar to that by which phenol, cresol, oxyphenylacetic 

 acid and oxyphenylpropionic acid originated from tyrosine, namely : 

 c . CH 3 c . CH S 



C 6 H 4 /"C . CH(NH 2 )COOH - C 6 H /C . CH 2 COOH - 



NH NH 



Skatoleaminoacetic acid Skatoleacetic acid 



C . CH 3 C . CH 3 C . H 



C 6 H 4<C . COOH -> C 6 H /CH - C 6 H 



NH NH NH 



Skatolecarboxylic acid Skatole Indole 



Tryptophane was found by Hopkins and Cole to have the empirical 

 formula C n 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- 



