366 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(2) Xanthoproteic. 



On heating a portion of the protein solution with concentrated 

 nitric acid, a yellow colour is formed ; the colour changes to orange 

 on adding ammonia or soda in excess to the cooled solution. 



This reaction is most probably due to the formation of nitro 

 compounds with the aromatic units contained in the protein, namely, 

 tyrosine, phenylalanine. 



(3) Millon's. 



On adding Millon's reagent to some of the protein solution, a 

 white precipitate is formed ; it becomes red on heating. 



This reaction is due to the presence of tyrosine in the protein. 

 (Compare hydroxy derivatives of benzene.) 



(4) Sulphur. 



A drop of lead acetate solution is added to some of the protein solu- 

 tion and sufficient caustic soda to redissolve the precipitate which is 

 first formed A brown coloration, sometimes black, occurs on boiling. 



This reaction is due to the separation of hydrogen sulphide from 

 the cystine unit, which gives lead sulphide with the lead acetate, 



(5) Adamkiewicz' or Glyoxylic Acid. 



If excess of glacial acetic acid be added to the solution and con- 

 centrated sulphuric acid be run underneath it, on standing, or on 

 gently shaking, a reddish-violet colour appears at the junction of the 

 fluids which gradually spreads throughout the solution. 



It has been shown by Hopkins and Cole that this reaction is due 

 to the presence of glyoxylic acid in the glacial acetic acid ; it is there- 

 fore better to use a solution of glyoxylic acid instead of glacial acetic 

 which (if kept in the dark) may not contain this substance. 



A little glyoxylic acid solution is added to some of the protein solu- 

 tion and concentrated sulphuric acid is run in as before to the bottom 

 of the test tube. The reddish-violet ring as above described slowly 

 forms. 



This reaction is due to the presence of tryptophan in the protein 

 molecule. Some substance is formed from the glyoxylic acid which 

 reacts with the tryptophan. 



A similar colour is produced on adding commercial sulphuric acid to 

 a protein solution containing a minimal quantity of formaldehyde. This 

 reaction is brought about by the presence of oxidising agents in the sulphuric 

 acid which act upon the formaldehyde (Rosenheim). It is not due to the 

 formation of glyoxylic acid by aldol condensation of formaldehyde and oxida- 

 tion (I)akin). 



It should be noted that this reaction has been used for many years for 

 detecting formaldehyde added to milk as a preservative. 



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