Vol. 6, 1920 
CHEMISTRY: E. J. COHN 
261 
system coincides with the isoelectric point of the protein. Sorensen's 
studies, therefore, add another property — acid combining capacity — in 
which the singular point in protein behavior coincides with the isoelectric 
point, and suggests another method which, in the absence of complicating 
phenomena, may be used to determine the isoelectric point. 
The only apparent effects of sodium chloride upon the hydrogen ion 
concentration of systems containing tuberin and sodium hydroxide, or 
tuberin and hydrochloric acid, follow from this theory.* When sodium 
chloride was added to such systems the acid binding capacity was increased, 
and the hydrogen ion concentration decreased, if tuberin was ionized as 
base at acidities greater than the isoelectric point. The hydrogen ion con- 
centration was increased at more nearly neutral reactions. The hydrogen 
ion concentration of the system whose acid binding capacity was least 
affected by the addition of sodium chloride was slightly lower than 10 ~^ A/^, 
and, therefore, in the neighborhood of the isoelectric point of tuberin. 
Dissociation of Tuberin in Salt Solution. — ^The increased hydrogen ion 
concentration of tuberin systems less acid than pH 4.3 to which sodium 
chloride was added shows that the dissociation of the tuberin compound 
was depressed, and that as a result tuberin combined with surplus base (the 
amount of base bound by tuberin, and by depression of the dissociation 
of all electrolytes being measured by the increase in free acid). We have 
seen that under these circumstances the solubility of tuberin increased.** 
Tuberin also combined with base in the systems to which sodium hy- 
droxide was added. An increase in solubility also followed the formation 
of this tuberin compoound, which may have been similar to that formed by 
sodium chloride. Tuberin never dissolved as completely, however, in neu- 
tral solutions to which no sodium chloride was added. Apparently this 
compound of tuberin is more soluble when little dissociated. That its 
dissociation products — like those of many other globulins which appear 
upon dialysis or dilution of their solfxtions — are very insoluble, is certain. 
Small amounts of hydrochloric acid decomposed the compounds that 
were formed by the more weakly ionized acid, tuberin, at reactions less 
acid than its isoelectric point. We have seen that sodium chloride and 
sodium hydroxide favored the formation of this tuberin compound, and, 
therefore, increased its solubility. By decomposing it hydrochloric acid 
decreased the solubility of tuberin in salt solutions. 
The observations of Osborne, that have already been referred to, indicate 
that the decreased solubility of many other globulins in salt solutions, 
to which small amounts of acid had been added, resulted from their acid 
* It is immaterial whether the change of hydrogen ion concentration caused or was 
caused by the change in state of the protein. 
** The tuberin preparation was probably not a simple sodium compound. It con- 
tained phosphorus, and may have contained calcium. The increase in solubility may, 
therefore, in part, have been associated with change in the protein compound. 
