PART II. 



THE GENERAL CHEMICAL CHARACTERS OF 

 THE PROTEINS. 



SECTION XII. THE QUALITATIVE REACTIONS OF THE 



PROTEINS. 



PROTEINS may be precipitated from solutions by the following re- 

 agents : 



I. By various mineral acids. Nitric acid is very often employed. 

 If allowed to flow into a protein solution a white ring forms at the 

 junction of the acid and the solution. This delicate reaction is 

 generally known as Heller s test. 



Metaphosphoric acid is a precipitant of the proteins, but not the 

 ortho- or pyro-acids. 



II. Ferrocyanic acid is a good precipitant. A mixture of 

 potassium ferrocyanide and acetic acid is generally employed. 



III. The salts of the heavy metals precipitate proteins. This 

 reaction will be discussed in greater detail later. 



IV. The ordinary alkaloidal reagents precipitate proteins, e.g. t 

 phosphotungstic acid, phosphomolybdic acid, potassium mercuric 

 iodide, potassium bismuth iodide, tannic acid, picric acid- These 

 reagents should be added to the slightly acidified solution. 



V. Trichloracetic and sulphonylsalicylic acids are good precipi- 

 tants. 



VI. Also uranyl acetate. The precipitate thus produced is 

 soluble in acids. 



VII. Nucleic acid and protamines. 



The following are the chief colour reactions for proteins. Not 

 every protein gives a positive result when treated with the reagents, 

 as the various colours are due to certain specific groups contained in 

 the molecule, which are not common to all proteins. Nevertheless, 

 every protein will give positive reactions in a large number of the 

 tests. 



I. Biuret Reaction. The protein is first treated with sodium 

 hydroxide solution, and then copper sulphate in very dilute solution 

 is added drop by drop ; a reddish violet to violet-blue colour will be 

 produced. This is due, according to SchifT, to the presence of the 

 following groups : 



X CO.NH 2 ,CO.NH 2 CO-NH 2 | | 



NHX C H 2 / | NH,_C-CO-NH-C- 



\CO.NH 2 \CO.NH 2 CO-NH 2 | | 



II. The Xanthoproteic Reaction. Proteins give on boiling with 

 strong nitric acid yellow flakes or a yellow solution, which, on 



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