40 THE PROTEINS. [CH. II. 



5. In performing the test on a solid substance, like fibrin, or keratin, a 

 small amount of the material should be heated with a few cc. of the reduced 

 oxalic acid and an equal volume of strong sulphuric acid. The mixture is 

 shaken, and as the protein dissolves in the strong acid both the fluid and the 

 solid particles assume a purple colour. 



6. The substance in the protein molecule that is responsible for the 

 reaction is tryptophane (indol-amino-propionic-acid) C 11 H 12 N 2 O 2 . 



NH 2 

 C CH 8 .CH COOH 



7. A similar reaction can be obtained by using a very dilute (1:250) 

 solution of formaldehyde containing a trace of an oxidising reagent like ferric 

 chloride. The authors of the original reaction regarded this test (Rosenheim's) 

 as being different from the glyoxylic test, though the whole question is still 

 confused. 



8. The author has shown that many substances, especially aldehydes, 

 react with tryptophane to yield coloured products in the presence of strong 

 HC1 or H 2 SO 4 . Most of these reactions only succeed in the presence of an 

 oxidising reagent, and are possibly due to a reaction with some oxidation pro- 



of tryptophane. 



24. The biuret reaction (Piotrowski's reaction). Treat about 3 

 cc. of the solution with I cc. of 40 per cent, sodium hydroxide. Add 

 a single drop of a I per cent, solution of copper sulphate. A violet 

 or pink colour is produced. 



NOTES. i . The reaction is of especial importance in testing for proteoses 

 and peptones, which give a rose colour. It is generally stated that other pro- 

 teins give a violet colour, but usually egg-albumin gives a distinct rose tint. 



2. It is important to avoid an excess of copper sulphate, the blue copper 

 colour obscuring the violet or rose tint. 



3. The test cannot be applied in the presence of a large amount of 

 magnesium sulphate, owing to the precipitation of magnesium hydroxide by 

 the alkali. 



4. If the solution contains much ammonium sulphate it must be treated 

 with a large excess of strong sodium hydroxide, as directed in Ex. 57. 



5. The reaction is given by nearly all substances containing two 



O H 

 II I 



