CHAPTER I 

 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



1. The Chemical Homogeneity of the Protein Group. — The 



chemical similarity between the properties and behavior of 'the 

 various protein bodies, and the evidence thus afforded of their 

 close chemical relationship, early led protein chemists to ascribe 

 to the proteins a common and characteristic chemical structure, 

 and to endeavor to anticipate the nature of this structure. 



The difficulty of securing a quantitative separation of protein 

 bodies is eloquent of their chemical similarity; one by one the 

 standard methods of separation brought forward by the physio- 

 logical chemist, such as fractional heat-coagulation, fractional 

 precipitation by salts, etc., have been shown to possess but a 

 qualitative value; the separation is seldom or never complete, 

 and the critical temperatures, concentrations, etc., at which pre- 

 cipitations occur are seldom well-defined points, but appear, 

 rather, to represent portions of a continuous curve of solubility. 

 The close relationship between the various protein bodies is 

 furthermore evinced by the similarity of their physical properties, 

 and by the almost universal applicability, within the protein 

 group, of such typical color or other reactions as the proteins 

 display. The general physico-chemical characteristics of the vari- 

 ous proteins resemble one another, as we shall see in subsequent 

 chapters, quite as closely as their purely physical or purely chemi- 

 cal characteristics. One of the most striking of these physico- 

 chemical characteristics is their digestibility by the proteolytic 

 enzymes. All true proteins and a large number of the poly- 

 amino-acids are hydrolysable by trypsin, while the overwhelming 

 majority of the proteins are also hydrolysable by pepsin. The 

 chemical relationship between the various proteins would there- 

 fore appear to be even more intimate than that between the 

 various disaccharides, since the enzymes which accelerate the 

 hydrolysis of these are, as a rule, specific, the hydrolysis of each 



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