THEORY OF PROTEIN IONIZATION 189 



difference between the velocity of OH' and that of Na+ is not 

 so great as that between H+ and CI') towards the cathode. This 

 accords with the experimental observations of Hardy. The 

 velocities measured by Harcjy, therefore, do not afford a meas- 

 ure of the migration-velocities of protein ions under a uniform 

 potential gradient. 



5. Objections to the above Theory of Protein Ionization. — 

 Several objections to the hypothesis of protein ionization which 

 has been presented above have been urged by Pauli, Samec and 

 Strauss (22). These objections may be summarized as follows: 



(i) The hypothesis involves the splitting in solution of a poly- 

 amino-acid into parts which are not amino-acids. Enzymatic 

 splitting of a protein, if it took place in the same way, would not 

 yield amino-acid end-products. This objection is really analo- 

 gous to that which was originally advanced against Arrhenius' 

 theory of the electrolytic dissociation of inorganic substances. 

 It will be remembered that at the time that that hypothesis was 

 first advanced, it was argued that the ionization of KCl into 

 K+ and CI' would imply the presence of free potassium and chlo- 

 rine in the solution, whereas the characteristic properties of these 

 substances are not displayed by a solution of KCl. The answer 

 to this was simply that the properties of an ion are not to be 

 estimated in terms of those of a molecule and furthermore the 

 ability of ions in a solution to display properties independent of 

 those of the oppositely charged ion is limited by electrostatic 

 forces. We may well suppose that protein ions, like other ions 

 in solution, cannot react with other substances independently 

 of the corresponding ion of opposite charge. In considering the 

 reaction between proteins and water which leads to the decompo- 

 sition of the polyamino-acid chains and is accelerated by enzymes 

 we must consider the two ions of the protein molecule together, 

 and not each ion separately. Undoubtedly the hydrolytic de- 

 composition of a protein must involve the interaction of the 

 protein ions with accompanying transfer of a labile hydrogen 

 atom from the cation to the anion in accordance with the scheme : 



H 



I 

 _C = N- -1- HO.H= -C-OH -h N- 

 I II I 



OH OH 



