742 CHARGE OF ISOELECTRIC PROTEIN BY IONS 



a recent book^) and the gelatin was allowed to solidify at 15°C. 

 The p.D. between the solid gelatin and the supernatant LaCls solu- 

 tion was then measured with two identical saturated KCl-calomel 

 electrodes connected with a Compton electrometer as previously 

 described.2 Table I gives the observed p.d. The LaCls solutions 

 used varied between m/24 and m/98,304. 



The gelatin was nearly but not entirely isoelectric, since it had a 

 positive charge of about 15 millivolts. This means that its pH was 

 not 4.7 but probably about 4.6 or possibly slightly less. 



Table I shows that the p.d. rises upon the addition of increasing 

 quantities of LaCls from 15 to 23.5 millivolts at a concentration of 

 m/6,144 LaCls. When the concentration of LaCls rises beyond this 

 point, the p.d. decreases again until it becomes 5.0 at a concentration 

 of m/24 LaCls. LaCls acts, therefore, upon the p.d. of isoelectic 

 gelatin in a similar way to HCl. In the case of HCl it has been 

 proven that a salt is formed between gelatin (or any other isoelectric 

 protein) in which the H ion becomes part of a complex positively 

 charged protein ion while the CI is the anion. It is natural to assume 

 that the reaction is similar to that between NH3 and HCl where the 

 salt NH4CI is formed, inasmuch as the protein contains NH2 or NH 

 groups in which the N is still able to attract and hold electrostatically 

 another H ion. The correctness of this view is supported by the 

 fact that when we add HCl to a 1 per cent solution of gelatin and 

 measure the CI potential of the solution, the CI potential is the same 

 as when we add the same amount of HCl to the same quantity of 

 water (without gelatin), while the H potential is considerably lower 

 in the gelatin solution than in the pure aqueous solution. 



When we add increasing concentrations of HCl to solid isoelectric 

 gelatin, gelatin chloride is formed and the ionization of the protein 

 leads to the establishment of a Donnan equilibrium between the 

 solid gelatin and the surrounding aqueous solution, and the unequal 

 distribution of the oppositely charged ions inside and outside the 

 gel is responsible for the p.d.^ The theory of membrane equilibria 



^ Loeb, J., Proteins and the theory of colloidal behavior, New York and London, 

 1922, 154. 



•'' Loeb, J., Proteins and the theory of colloidal behavior, New York and London, 

 1922, 120 ff; /. Gen. Physiol, 1920-21, iii, 667; 1921-22, iv, 351, 463. 



