CORRECTNESS OF THE CHEMICAL VIEWPOINT 33 



shown by W. Kossel 1 and by Langmuir 2 that this idea of Werner 

 is in perfect harmony with the electronic conception of molecular 

 compounds, and we shall give later in this book a direct 

 proof that it holds for proteins. We can, therefore, say that on 

 the acid side of its isoelectric point the protein particle is able 

 to add acid to its NH 2 groups in the following form: 



p r> __NH 2 HCl 



+ HC1 = K 

 L iX - 



COOH I* v COOH 



which dissociates electrolytically into a protein cation and an 

 anion. 





COOH A ^ COOH 



While our symbol indicates only one NH 2 group in the mole- 

 cule, it is probable that more than one NH 2 or NH group is 

 capable of adding an acid molecule. 



The simplification in the general chemistry of proteins implied 

 in these experiments is considerable. We only need to remember 

 that on the alkaline side of its isoelectric point the protein 

 behaves as if it were a fatty acid, only one or more COOH 

 groups existing in a chemically active form; while on the acid 

 side of its isoelectric point we may again disregard the enormous 

 protein molecule and go on the assumption that the protein 

 consists only of one or a number of NH 2 groups, each capable of 

 adding the hydrogen ion of an acid. 



It is possible though not proven that the difference in the 

 behavior of the proteins on the two opposite sides of the isoelectric 

 point is accompanied by an intramolecular change in the protein 

 molecule, and that the protein anion in a metal proteinate may 

 be considered an isomer of the protein cation in protein-acid salt. 

 Such a possibility is suggested by the behavior of indicators the 

 electrolytic dissociation of which is accompanied by an intra- 

 molecular change. 



When we mix a metal gelatinate, e.g., sodium gelatinate, with 

 another salt, e.g., MgSO 4 , the Na of the metal gelatinate can be 



1 KOSSEL, W., Ann. d. Physik, vol. 49, p. 229, 1916. 



2 LANGMUIR, I., /. Am. Chem. Soc., vol. 41, p. 868, 1919, 



3 



