258 
CHEMISTRY: E. J. CORN 
Proc. N. a. S. 
Solubility oj Tuberin in Acid Solution. — Not only was tuberin dissolved 
by salt solutions, in the absence of acids, but it was also dissolved by acid 
in the absence of salt. In solutions of lower hydrogen ion concentration 
than 10"^ N, the dialyzed tuberin compound was only slightly soluble;* 
less so than in salt solutions. At greater hydrogen ion concentrations 
than 10"^ N, tuberin was largely dissolved in the systems to which no 
sodium chloride had been added. Its solubility in solutions of this de- 
gree of acidity was, however, decreased by sodium chloride. Indeed in 
the presence of sufficient salt the tuberin was nearly completely precipi- 
tated. The precipitation of many other globulins besides tuberin from 
acid solution by neutral salts has been observed. Osborne^ studied the 
precipitation of edestin under these circumstances, and Hardy^ that of 
serum globulin. This phenomenon is also apparently quite general. 
Solubility of Tuberin at pH 4.3 Independent of Salt. — Between the sys- 
tems of such acidity that sodium chloride decreased the solubility of tuberin 
and the more nearly neutral ones in which sodium chloride increased solu- 
bility, there existed a narrow range where sodium chloride had very little 
effect upon the solubility of tuberin. This range coincided with a slightly 
lower hydrogen ion concentration than 10"^ N. 
These results are reported in table I. If represented graphically, the 
solubility of tuberin plotted as ordinates, and hydrogen ion concentration 
as abscissae, the curves representing the solubility of tuberin in the pres- 
ence of different amounts of sodium chloride converge, and intersect each 
other within a narrow range of hydrogen ion concentrations, slightly lower 
than 10-4 N, probably nearest to 0.5 X 10"* {pH 4.3). 
The Isoelectric Point of Tuberin. — In a previous investigation the method 
of cataphoresis was employed in determining "the Isoelectric Points of 
the Proteins in Certain Vegetable Juices."^ Potato juice was brought to 
different hydrogen ion concentrations and the migration of tuberin in 
an electric field determined. The direction of migration was found to 
change at a hydrogen ion concentration slightly lower than 10 ~^ N. 
At greater acidities tuberin migrated to the cathode, at more nearly neutral 
reactions to the anode. At hydrogen ion concentrations slightly lower 
than 10-4 N, tuberin migrated in neither direction. At this reaction, 
therefore, it was isoelectric. 
The Ionization of Tuberin. — The ionization of a protein compound can 
be inferred from the direction of its migration during cataphoresis. In 
neutral potato juice and potato juice less acid than 10 A^, tuberin bore 
a negative charge. Presumably therefore tuberin as it existed in the 
juice of the potato was ionized as an acid. This compound decomposed 
on the addition of the stronger acid HCl, and tuberin was freed at its iso- 
electric point. In the neighborhood of its isoelectric point tuberin was, 
moreover, less ionized than at greater or at lesser acidities and was also 
* This requires further study. 
