CHAPTER II 

 THE PREPARATION OF PURE PROTEINS 



1. The Proteins as Chemical Individuals. — With the excep- 

 tion of casein and the protainins and those proteins such as 

 hsemoglobin, fibrin, egg albumin and certain of the vegetable 

 proteins which may be prepared in crystalline condition, it can- 

 not be positively affirmed that any of the proteins which have 

 hitherto been isolated in a "pure" condition are, in reality, 

 chemical individuals and not mixtures, tolerably constant in 

 composition, of two or more proteins or of one protein with the 

 products of its partial hydrolysis or with other colloidal sub- 

 stances. Our methods of isolation and purification are admittedly 

 inadequate, and the most diverse opinions exist regarding the 

 appropriate criteria of the purity of any given protein. The 

 reason for this latter fact is probably to be sought in the imper- 

 fection of our knowledge of the properties, chemical, physical, 

 and physico-chemical, of the proteins as a class; lack of knowledge 

 implying, of course, the lack of a basis for comparison and of a 

 standard for calibration. 



The proteins are, as a rule, non-crystallizable or only crystal- 

 lizable with difficulty and under such conditions as to involve 

 contamination with a variable and usually indeterminate pro- 

 portion of impurities, while repeated re-crystallization usually 

 leads to a more or less sensible alteration in the properties of the 

 protein, which may be attributable, in the majority of cases, to 

 a partial hydrolysis. The salts which the proteins form with the 

 inorganic bases and acids are, as a rule, either soluble or else, if 

 insoluble, of such a nature (for example, the protein salts of the 

 heavy metals) that the inorganic constituent cannot be removed 

 again from the molecule without altering its properties. 



A limited number of the proteins are insoluble in distilled water 

 when uncombined with bases or acids, and these, of course, 

 afford exceptionally favorable opportunities for isolation and puri- 

 fication. Precipitation of these proteins in the free condition can 



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