THEORY OF PROTEIN IONIZATION 191 



and oppositely charged colloids. The phenomena can only be 

 interpreted to mean that solutions of these proteins contain oppo- 

 sitely charged but otherwise similar moieties, i.e., oppositely 

 charged protein ions. 



(iii) Pauli, Samec and Strauss also urge the objection that 

 under certain conditions (a low proportion of acid to protein) 

 H+ and CI' may not be bound equally in solutions of protein in 

 hydrochloric acid. This, however, does not really affect the 

 question of the mode of ionization of the protein molecule but 

 rather the question of the relative afl&nity of the nitrogen atom 

 for H+ and CI' ions. It is not inconceivable that some measure 

 of "Z witter ion" or doubly and oppositely charged ion formation 

 may occur in protein solutions (37). Yet this is not the normal, 

 but rather the exceptional form of ionization and it is especially 

 remarkable, and quite inconsistent with the view that proteins 

 normally dissociate into protein and non-protein ions, that the 

 inferiority of CI' binding to H+ binding is dependent solely upon 

 the proportion of HCl to protein and not at all upon the dilution 

 of the system, although the equivalent conductivity of protein 

 solutions is very decidedly influenced by dilution (Cf. section 1), 



Perhaps the most obvious objection to the hj^pothesis which 

 I have advanced is that a form of ionization involving the break- 

 ing of a double bond between carbon and nitrogen is very unusual 

 and in fact without precedent in other fields of chemistry. It 

 must be remembered, however, that as the experiments of Gom- 

 berg (7) have so beautifully and decisively shown, the precise 

 point within a molecule at which ionization may occur is de- 

 termined by the strains to which the molecule is subjected, and 

 that when the strain is unusually great the break involved in 

 ionization may occur at points which resist the tension due to 

 strains of normal magnitude. Thus ethane, H3C — CH3, is a sub- 

 stance of remarkable stability, and not only ethane, but tetra- 

 phenyl ethane, (C6H5)2HC — CH(C6H5)2, and pentaphenyl ethane 

 (C6H5)3C — CH(C6H5)2, are stable substances. Yet all attempts to 

 prepare hexaphenylethane have failed for the reason that the 

 compound breaks in the middle, the bond between the carbons, 

 which we are accustomed to regard as one of the most stable 

 types of union, being ruptured by the strain imposed upon it by 

 the great weight of three phenyl groups at either end of the 

 molecule. The rupture of this bond is all the more remarkable 



