256 
CHEMISTRY: K J. CORN 
Proc. N. a. S. 
globule of water having a radius of equilibrium which is a function of the 
humidity of the air. 
1 McClelland, J. A. and Nolan, P. J., Proc. Roy. Irish Acad., A 33, 1916 (29); Nolan, 
P. J., Ibid., A33, 1916 (10); McClelland, J. A. and Nolan, P. J., Ibid., AZ5y 1919 (1). 
THE RELATION BETWEEN THE ISOELECTRIC POINT OF A 
GLOBULIN AND ITS SOLUBILITY AND ACID 
COMBINING CAPACITY IN SALT SOLUTION 
By Edwin J. Cohn 
Harriman Research Laboratory, RooseveivT Hospital, New York 
Communicated by L. J. Henderson, March 11, 1920 
Globulins are, by definition, soluble in solutions of the salts of strong 
acids and strong bases ^ and tuberin, the protein of the potato, has ac- 
cordingly been classified as a globulin. ^ In the juice that can be squeezed 
from the potato, tuberin is dissolved on account of the high concentra- 
tion of electrolytes.^ It is precipitated by dialysis of the juice, and readily 
redissolves in solutions of neutral salts. The effect of neutral salts in 
dissolving tuberin is, however, variable with the hydrogen ion concentra- 
tion. The nature of this variation, and to some extent its causes and its 
meaning for the classification of proteins, is considered in this paper. 
The tuberin that was employed in these experiments was precipitated 
by dialyzing potato juice that had been freed from starch by filtration 
through pulp, in a collodion bag against tap water. After a period of days 
the tuberin had largely precipitated and settled. The contents of the 
collodion bag were then evenly suspended, transferred in 100 cc. samples 
to flasks, and brought to different hydrogen ion concentrations by the addi- 
tion of the amounts of sodium hydroxide and hydrochloric acid recorded 
in table I. The systems containing tuberin and acid, or tuberin and base 
in identical concentration were next divided, and added to vessels contain- 
ing different amounts of sodium chloride. The systems, therefore, varied 
in two respects; in the first place in the amount of free, and, therefore, 
of combined protein, acid or base; in the second place in the amount of 
salt that they contained. After they had stood at constant temperature 
for about 6 hours, and it was believed that equilibrium had been established, 
the hydrogen ion concentrations of the systems were electrometrically 
determined, and the solubility of the tuberin in them was estimated by 
determining nitrogen in an aliquot part after the precipitated tuberin had 
been removed by centrifuging. In certain of the systems it will be noted 
that all of the protein dissolved. In these cases, therefore, the data do 
not represent solubility; that is solutions saturated with respect to protein. 
The results obtained in this preliminary investigation were not, it is 
true, altogether regular. Abnormal values of nitrogen and also of hydro- 
