82 PHYSIOLOGICAL CHEMISTRY 



view of some of the important facts relating to the amino acids 

 we will turn our attention to the general properties and reac- 

 tions of the proteins themselves. 



General Protein Reactions 



The general protein tests may be divided into two groups, 

 the color tests and the precipitation tests. 



Color Tests. The protein color tests are by no means specific 

 for proteins. They are tests which are given by certain group- 

 ings or certain constituents generally found in the -protein mole- 

 cule. If the grouping upon which a test depends is absent 

 from a particular protein, that protein will not respond to the 

 test in question. Thus a single positive test should not be 

 taken as evidence of the presence of a protein, but should be 

 confirmed by some other test. 



Biuret Test. If concentrated sodium hydroxide is added to 

 a protein solution, and then a few drops of very dilute copper 

 sulphate solution, a violet color appears either at room tem- 

 perature or on slight warming. This test is named from the 

 fact that it is given by a substance biuret which is made from 

 two molecules of urea with loss of NH 3 



NH 2 



NH 2 C = 



/ / 



2 CO -> HN 



\ \ 



NH 2 C = 



NH 2 



Biuret 



This substance does not occur as a constituent of the protein 

 molecule. The test depends on the presence of two amid 

 groups CO NH 2 united either directly or by a carbon or 

 nitrogen atom. One of the amino groups may be substituted 

 as CO NHR but not both of them. If the NH 2 of the amid 



