154 ISOELECTRIC POINTS OF PROTEINS 



concentration near the second ionization constant of phosphoric acid 

 (1.6 X 10~^ = pH 6.8). But the strong buffer action, indicated by 

 the steepness of the titration curve, throughout the range investi- 

 gated cannot be attributed merely to the presence of phosphoric 

 acid nor to the other organic and inorganic weak acids that chemi- 

 cal analyses reveal. The increase in steepness of the titration curve 

 in the range, acid to pH 4.5 on the one hand and alkahne to pH 

 8.5 on the other, is largely due to the dissociation of the protein 

 compounds that exist in the potato and their recombination with 

 strong acids and bases with the retention of hydrogen and hydroxyl 

 ions. The formation of new compounds at these particular reactions 

 is suggested by a comparison of the titration curve (Fig. 3) with the 

 curve representing the solubility of tuberin in the juice of the potato 

 at different hydrogen ion concentrations (Fig. 2). Each change in 

 solubility is seen to affect the slope of the titration curve, but in 

 different ways. Thus, nearly three times as much acid was required 

 to redissolve tuberin as to precipitate it. Osborne has made a 

 similar observation on another globuHn, edestin.^^ 



The Isoelectric Point of Tuberin.— In an electric field the protein 

 in the juice of the potato migrated toward the anode. It bore, there- 

 fore, a negative charge. No change followed an increase in the alka- 

 linity of the juice. The direction of migration of the protein was, 

 however, reversed by the addition of acid. In Table III are collected 

 the results of many experiments. In all the direction of migration 

 of the protein changed at a slightly lower hydrogen ion concentration 

 than 10~^N. At that acidity the protein did not migrate in either 

 direction. It existed at its isoelectric point. At acidities greater 

 than the isoelectric point protein migrated to the cathode. Under 

 these circumstances tuberin apparently ionized as a base, and dis- 

 solved as an acid compound. 



A second change in direction of the migration of proten in very 

 acid juice occasionally occurred in our early experiments. This 

 change was probably apparent and must be explained as relative to 

 the increased passage of water at these acidities, since it was later 

 averted by increasing the buffer in the arms of the cataphoresis 

 apparatus. 



^^ Osborne, T. B., /. Am. Chem. Soc, 1902, xxiv, 39. 



