750 



CHARGE OF ISOELECTRIC PROTEIN BY IONS 



of the same pH as the gelatin solution (but without gelatin), was 

 also measured after osmotic equilibrium was established. The 

 figures for the p.d. are always under the figures for the pH of the 

 gelatin-salt solutions. The result is very striking. The gelatin is 

 always positively charged when the gelatin is dissolved in m/768 

 LaCls regardless of the pH. Even when the pH of the solution 

 is on the alkaline side of the isoelectric point the gelatin is positively 

 charged in the presence of m/768 LaCls. No such result is observed 

 when the salt added is NaCl, CaCl2, or Na2S04. In this case the 

 P.D. across the membrane was zero at pH 4.7 and 4.8 and the gelatin 

 became negative when the pH became 5.0 or above. This shows 

 that the positive charge of isoelectric gelatin in the presence of 



TABLE VII. 



m/768 Lacis. l^^ "":,;:';: 



I P.D., miUivolls 



M/5i2CaCi2. 1 P^" ■■.;■■■;; 



I P.D., miutvolts 



M/256NaCl. I^^" " ' V.;.' ",■ " 

 I P.D., mmivoUs 



M/5i2Na2S04.r^" ■■■.;;.■■;; 



I P.D., mtlhvoUs 



Gelatin-acid salts. 



3.66 



+8.5 

 3.62 



+8.5 

 3.66 



+9.5 

 3.63 



+6.0 



3.98 

 +7.0 



4.02 

 +5.5 



3.98 

 + 7.0 



4.06 

 +4.0 



4.32 



+5.5 

 4.44 



+2.5 

 4.49 



+2.5 

 4.41 



+ 1.0 



Isoelec- 

 tric 

 gelatin. 



4.72 

 +3.5 

 4.72 





 4.80 





 4.83 







Metal 

 gelatinates. 



4.82 

 +3.0 



5.02 

 -1.0 



5.19 

 -3.0 



5.16 

 -2.0 



5.3 

 +2.0 



5.43 

 -2.5 



5.68 

 -6.0 



5.82 

 -3.0 



LaCls of pH 4.7 cannot be due to the change in pH but must be due 

 to some other cause of ionization of gelatin such as the formation of 

 a complex gelatin-La cation. 



The pH for Na4Fe(CN)6 could not be measured electrometrically. 



In view of all these results we must draw the conclusion that salts 

 with trivalent (and probably also tetravalent) cations cause iso- 

 electric gelatin and gelatin on the alkaline side of the isoelectric 

 point (but near this point) to assume a positive charge owing to 

 the formation of positively charged protein ions, probably of the 

 type gelatin-La. This explains the reversal of the charge of gelatin 

 by trivalent cations observed in anomalous osmosis and kindred 

 phenomena. Tetravalent anions confer a negative charge on iso- 

 electric gelatin and the mechanism is probably similar. Changes of 



