I I 2 PHYSIOLOGICAL CHEMISTRY. 



under definite conditions (see pp. 100 and 242). This characteristic 

 may be applied to separate different coagulable proteins from the same 

 solution by fractional coagulation. The coagulation temperature 

 frequently may serve in a measure to identify proteins in a manner 

 similar to the melting-point or boiling-point of many other organic 

 substances. The separation of proteins by fractional coagulation 

 is thus analogous to the separation of volatile substances by means 

 of fractional distillation. This method of separating proteins is 

 not a satisfactory one, however, inasmuch as proteins in solution 

 have different effects upon one another and also because of the 

 fact that the nature of the solvent causes a variation in the tem- 

 perature at which a given protein coagulates. The nature of the 

 process involved in the coagulation of proteins by heat is not 

 well understood, but it is probable that in addition to the altered 

 arrangement of the component atoms in the molecule, there is a 

 mild hydrolysis which is accompanied by the liberation of minute 

 amounts of hydrogen, nitrogen and sulphur. The presence of a 

 neutral salt or a trace of a mineral acid may facilitate the coagula- 

 tion of a protein solution (see page 100), whereas any appreciable 

 amount of acid or alkali will retard or entirely prevent such coagu- 

 lation. 



EXPERIMENTS ON COAGULATED PROTEIN. 



Ordinary coagulated egg-white may be used in the following 

 tests : 



1. Solubility. Try the solubility of small pieces of the coagu- 

 lated protein in each of the ordinary solvents (see page 23). 



2. Millon's Reaction. 



3. Xanthoproteic Reaction. Partly dissolve a medium-sized 

 piece of the protein in concentrated nitric acid. Cool the solution 

 and add an excess of ammonium hydroxide. Both the protein 

 solution and the undissolved protein will be colored orange. 



4. Biuret Test. Partly dissolve a medium-sized piece of the 

 protein in concentrated potassium hydroxide solution. If the proper 

 dilution of cupric sulphate solution is now added the white coagu- 

 lated protein, as well as the protein solution, will assume the char- 

 acteristic purplish-violet color. 



5. Hopkins-Cole Reaction. Conduct this test according to the 

 modification given on page 101. 



