CHAPTER X. 

 PROTEINS. 



WE designate as proteins a group of nitrogenous colloids which 

 are the chief constituents of animals and plants. They consist en- 

 tirely or chiefly of substances which contain quantitatively: 



Per cent. 



C 50-55 



H 6.5- 7.3 



N 15-17.6 



19-24 



S 0.3-2.4 



One of the chief characteristics of most of the dissolved albumins 

 is their coagulability when heated. The effect of heat on undis- 

 solved proteins is shown by the loss of their capacity to swell; they 

 are "denatured." Hydrophile colloids become hydrophobe. 



A host of the most diverse substances are included under the 

 generic term "albumin." It includes water-soluble substances such 

 as egg and serum albumin, and substances soluble in saline solutions, 

 as globulin, vitellin, myosin and, finally, such substances as are solu- 

 ble neither in aqueous nor in saline solution, for example, fibrin. 

 We know that there exists in each plant and in each animal a distinct 

 serum albumin and a distinct serum globulin, etc. In the chapter on 

 "Immunity Reactions," we shall return to the species-native charac- 

 teristics (Artspezifitat) of proteins (see p. 194). We shall not speak 

 of these distinctions here, but we shall dwell, rather, upon the prop- 

 erties that the different proteins possess in common. 



Colloid research, in a negative way, by destroying a large number 

 of false conceptions, has been of great service to the chemistry of 

 proteins; and it is in a position to establish new principles, since 

 only a few proteins crystallize and, with others, common methods of 

 purification are unavailable. Absolutely misleading methods have 

 been relied upon to separate and distinguish proteins. It was form- 

 erly believed, e.g., that the coagulation temperature of different proteins 

 varied, but colloid investigations demonstrated that small quantities 

 of electrolytes could raise or depress it to a g'reat extent. By pre- 

 cipitation with copper sulphate, E. HARNACK believed that he had 

 obtained characteristic copper albuminates, and other observers that 

 they had obtained characteristic silver or calcium albuminates. 

 Colloid chemistry has shown that the different amounts of copper, 



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