JACQUES LOEB 581 



treated with a protein, the water will diffuse into the solution at a 

 rate increasing with increasing valency of the cation and diminishing 

 with increasing valency of the anion. It is not difficult to find out 

 by this method the critical hydrogen ion concentration at which the 

 reversal in the sign of charge of the membrane occurs. The experi- 

 ments to be described in this paper have led to the result that this 

 critical hydrogen ion concentration is connected but not identical 

 with the isoelectric point of the protein with which the collodion 

 membrane has been treated. 



Proteins are amphoteric electrolytes which can form salts with 

 metals (metal proteinates, e.g. Na proteinate) as well as with the 

 anions of acids (protein-acid salts, e.g. protein chloride). Whether 

 they do the one or the other depends upon the hydrogen ion concen- 

 tration of the solution. Below a certain hydrogen ion concentration 

 the proteins form salts of the type of metal proteinates, e.g. Na 

 gelatinate; above this critical hydrogen ion concentration they form 

 salts of the type of protein-acid salts; e.g., gelatin chloride.^ Between 

 the two concentrations proteins form salts with neither cation nor 

 anion, and this is the so called isoelectric point. For a number of 

 proteins (gelatin included) the isoelectric point lies in the neighborhood 

 of a hydrogen ion concentration of 2 X 10~^ n; for oxyhemoglobin it 

 lies at a very different hydrogen ion concentration; namely, 1.8 X 10~' 

 N. We will show in this paper that the hydrogen ion concentration 

 required to produce a positive charge in a collodion membrane pre- 

 viously treated with a protein varies in the same sense as the iso- 

 electric point of the protein used. 



//, Membranes Treated with Gelatin. 



The isoelectric point of gelatin is, according to Michaelis, at a 

 hydrogen ion concentration of about 2 X 10~^ n (or in Sorensen's 

 logarithmic symbol, pH = 4.7). A 1 per cent solution of gelatin was 

 put into a collodion bag over night and was removed the next morn- 

 ing. The bag was rinsed a number of times (ten or twenty times 

 or more) with water to remove all the gelatin except that film 

 which remained apparently attached to the inner side of the col- 



7 Loeb, J., J. Gsn. Physiol., 1918-19, i, 39, 237, 363, 483^ 559. 



