PROTEINS 



117 



rate of coagulation. This is due in part to the decrease in the concen- 

 tration of the free acid, which results from the disturbance of the equilib- 

 rium between the protein and acid and also in part to the direct influence 

 which the salts exert upon the protein. The presence of neutral salts 

 may under certain circumstances facilitate the coagulation of proteins 

 by heat. 



The temperature at which egg-white is coagulated causes a difference 

 in the appearance of the coagulum. 1 Coagulated egg-white which has 

 been immersed in water at a low temperature and then gradually heated 

 to the coagulating temperature is more translucent and has a bluish 

 color, whereas egg-white which has been immersed in water heated to a 

 temperature above the coagulating temperature is creamy white in 

 color. They also possess different digestibilities. 



x - s 



EXPERIMENTS ON COAGULATED* PROTEIN 



Ordinary coagulated egg-white may be used in the following tests : 



1. Solubility. Try the solubility of small pieces of coagulated protein in 

 each of the ordinary solvents (see page 22). 



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 con- 

 centrated potassium hydroxide solution. If the proper dilution of copper sul- 

 phate solution is now added the white coagulated protein, as well as the protein 

 solution, will assume the characteristic purplish-violet color. 



5. Glyoxylic Acid Reaction (Hopkins-Cole). Conduct this test according to 

 the modification given on page 98. 



Secondary Protein Derivatives 



These derivatives result from a more profound cleavage of the protein 

 molecule than that which occurs in the formation of the primary deriva- 

 tives. The class included proteases, peptones, and peptides. 



PROTEOSES AND PEPTONES 



Proteoses are intermediate products in the digestion of proteins by 

 proteoly tic enzymes, as well as in the decomposition of proteins by hy- 

 drolysis and the putrefaction of proteins through the action of bacteria. 

 Proteoses are called albumoses by some writers, but it seems more logical 

 to reserve the term albumose for the proteose of albumin. 



Peptones are formed after the proteoses and it has been customary to 



1 Frank: Journal of Biological Chemistry, 9, 463, 1911. 



