98 



A. I. RINGER 



The Biuret Reaction. When protein is treated with a strong sodium 

 hydroxid solution and then a few drops of a very dilute copper sulphate 

 solution is added, a beautiful violet blue color develops. This reaction 

 is due to the presence of the Biuret group. 



CHNH 



COOH 



CH 2 NIL, 



CO 

 \ 

 NH 



/ 



CH 2 



Biuret group. 



COOH 

 2 Glycocoll Glycyl-glycin 



All proteins and polypeptids give this reaction. 



The Xanthoproteic Reaction. When a protein is boiled with strong 

 nitric acid, a yellow solution is formed, which after making alkaline with 

 sodium hydroxid, turns red brown, and with ammonia, turns orange red. 

 This reaction is due to the presence of the benzene group. 



The Sulphur-lead Reaction. When protein is heated in a solution 

 of sodium hydroxid in the presence of lead acetate, a black color is pro- 

 duced, due to the presence of sulphur in the protein molecule. This re- 

 action in protein is produced by cyst in. 



The Molisch Reaction. When a few drops of an alcoholic solution of 

 a-napthol is added to a protein solution and this mixture stratified upon 

 concentrated sulphuric, acid, a beautiful violet mixture is formed at the 

 point of contact. This reaction is not given by the proteins themselves 

 but by the carbohydrate radical which is frequently bound to certain pro- 

 teins (glucoproteins). 



The Adamkiewicz-Hopkins-Cole Reaction is obtained when to a solu- 

 tion of protein a small amount of glyoxylic acid is added, and the mix- 

 cure stratified upon concentrated sulphuric acid. A beautiful violet blue 

 color develops at the point of contact. This reaction is given by the amino 

 acid tryptophan, and proteins which do not contain this amino acid, like 

 gelatin, zein, protamins, etc., do not give it. The presence of sodium' or 

 potassium nitrate will interfere with this reaction. 



The Triketohydrinden Hydrat Reaction (Ninhydrm). When a small 

 amount of 0.1 per cent of triketohydrinden hydrate is added to a dilute 

 protein solution, and the mixture boiled for a minute or two and then 

 allowed to cool, a beautiful blue color will develop. This is characteristic 



