452 



SCIENCE 



[N. S. Vol. LII. No. 1350 



tin solutions are red whose pH is above 4.Y, 

 while the others are colorless. 



On the acid side of the isoelectric point, i. e., 

 at pH < 4.7, the gelatin is in combination 

 with the anion of the salt used. This can be 

 demonstrated in the same way by bringing 

 different doses of powdered gelatin to differ- 

 ent pH and treating them for one hour with 

 a weak solution of a salt whose anion easily 

 betrays itself, e. g., M/128 K^FeCClSr)^. If 

 after this treatment the powdered gelatin is 

 washed six times with cold water to remove 

 the FeCCN')^ not in chemical combination 

 with gelatin and if 1 per cent, solutions of 

 these different samples of gelatin are made, 

 it is found that when the pH is < 4.Y the 

 gelatin solution turns blue after a few days 

 (due to the formation of ferric salt), while 

 solutions of gelatin with a pH of 4.7 or above 

 remain permanently colorless. Hence gelatin 

 enters into chemical combination with the 

 anion re(CN)j only when pH is < 4.7. The 

 same can be demonstrated through the addi- 

 tion of ferric salt when gelatin has been 

 treated with JSTaCNS, the anion CNS being 

 in combination with gelatin only where the 

 pH is < 4.7. Acid dyes, like acid fuchsin, 

 combine with gelatin only when pH is < 4.7. 



In this way it can be shown that when the 

 pH is > 4.7 gelatin can combine only with 

 cations; when pH is < 4.7 it can combine 

 only with anions, while at pH 4.7 (the iso- 

 electric point) it can combine with neither 

 anion nor cation. The idea that both ions 

 influence a protein simultaneously is no 

 longer tenable. 



It follows also that a protein solution is not 

 adequately defined by its concentration of 

 protein but that the hydrogen ion concentra- 

 tion must also be known since each protein 

 occurs in three different forms — possibly 

 isomers — according to its hydrogen ion con- 

 centration. 



In the experiments just discussed it was 

 necessary to wash the powdered gelatin to 

 find out at which pH an ion was in combina- 

 tion with the gelatin. This has led some 

 authors to the belief that in all my experi- 

 ments the washing was a necessary uart of 



the procedure. I therefore will call especial 

 attention to the fact that the experiments to 

 be described in the rest of the paper were 

 carried out with isoelectric gelatin to which 

 just enough acid or alkali was added to bring 

 it to the hydrogen ion concentration required 

 for the purpose of the experiment. 



When a protein is in a salt solution, e. g., 

 NaOl, it will combine with Na forming 

 sodium proteinate as soon as the pH is higher 

 than the isoelectric point of the protein; 

 when, however, the pH falls below that of the 

 isoelectric point of the protein the Na is 

 given off and protein chloride is formed. 



Moreover, the writer has been able to show 

 by volumetric analysis that the quantity of 

 anion or cation in combination with the pro- 

 tein is an imequivocal function of the pH, 

 When we add HCl to isoelectric gelatin and 

 determine the pH we always find the same 

 amoimt of CI in combination with a given 

 mass of originally isoelectric gelatin for the 

 same pH ; so that if we know the pH and the 

 concentration of originally isoelectric gelatin 

 present we can also tell how much CI is in 

 combination with the protein for this pH. 

 The same is true when we add an alkali to 

 the isoelectric gelatin. For the same pH the 

 amoimt of cation in combination is always 

 the same. These facts have led the writer 

 to propose the following theory. When we 

 add an acid, e. g., HCl, to isoelectric gelatin 

 (or any other isoelectric protein) an equili- 

 brium is established between free HCl, pro- 

 tein chloride, and non-ionogenic or isoelectric 

 protein; when we add alkali an equilibriimi 

 is established between metal proteinate, non- 

 ionized protein, and the hydrogen ions. 

 Sorensen was led to a similar view on the 

 basis of entirely different experiments. 



This fact that the hydrogen ion concentra- 

 tion of a protein solution determines the 

 quantity of protein salt formed is the basis on 

 which the following proof for the purely 

 chemical character of the combination be- 



