HISTORICAL INTRODUCTION 9 



"react with acids and alkalies to form salts, but the reactions are not 

 precise, an indefinite number of salts of the form (B)BHA being formed 

 where the value of n is determined by conditions of temperature and 

 concentration, and of inertia due to electrification of internal surfaces 

 within the solution." 



There are two elements in this view which should be separated. 

 The suggestion that the electrical charges of the micellae are not 

 "a definite multiple of a fixed quantity" harmonizes with the 

 results to be given later. The other suggestion, however, "that 

 the reactions are not precise" seems to be contradicted by the 

 stoichiometrical facts to be enumerated in the fourth chapter. 



When the methods of measuring the hydrogen ion concentra- 

 tion had been developed by H. Friedenthal and by S^rensen it 

 became possible to determine the isoelectric point of genuine 

 proteins. This was first done by Michaelis and his collaborators 

 in 1910. Michaelis used the same method of migration of the 

 particles in an electrical field which had been used by Hardy. 

 The isoelectric point is, according to Michaelis, that hydrogen 

 ion concentration at which the particles migrate neither to the 

 anode nor to the cathode. The following figures give the 

 hydrogen ion concentrations defining the isoelectric points of 

 different proteins as determined by Michaelis. 1 



Genuine serum albumin 2 X 10~ 5 N 



Genuine serum globulin 4 X 10~ 6 N 



Oxyhemoglobin 1.8 X 10~ 7 N 



Gelatin 2 X 10~ 5 N 



Casein 2 X 10~ 6 N 



According to S^rensen the isoelectric point of crystalline egg 

 albumin is near that of serum albumin (namely, at a pH of 4.8). 2 



We shall denote in this book the hydrogen ion concentration 

 by S^rensen's logarithmic symbol pH; e.g., the concentration 

 2 X 10~ 5 N = 10~ 4 - 7 N is written merely pH 4.7, the minus sign 

 being omitted. 



If we assume that the ultimate units of a protein solution are 

 as a rule isolated protein molecules or ions which react stoichi- 



1 MICHAELIS, L., "Die Wasserstoffionenkonzentration," p. 54 ff, Berlin, 

 1914. 



2 S0RENSEN, S. P. L., Studies on proteins: Compt. rend. trav. Lab. Carls- 

 berg, vol. 12, Copenhagen, 1915-17. 



