28 f CHEMICAL STATICS 



monacid bases to a solution of an organic polybasic acid of the 

 type R(COOH)to, and the formation of salts by the replacement 

 of H atoms in the — COOH groups, the osmotic pressure of the 

 solution would increase (provided the salts were highly dissociated) 

 in the same proportion, namely 2, 3, 4, etc. The experimental 

 fact, for casein (58), is that the osmotic pressure (= depression 

 of the freezing-point) increases in the proportion 2, 4, 6, etc., 

 i.e., each successive equivalent of neutralized base gives rise to 

 an equal number of ions. This obviously corresponds with what 

 would be expected were the union and its mode of dissociation 

 of the type outlined above. 



In addition to these, a host of minor details in the behavior 

 of the protein salts, which would be very hard to explain upon 

 any other basis, admit, as we shall see, of a simple explanation 

 on the basis of the hypothesis outlined above. 



The poly-amino-acid structure of the proteins, however, carries 

 with it other possibilities which are of importance in interpreting 

 their behavior. Some measure of the dissociations 



/NH2 



n( +H+ 



^ COO' 



and 



/NH2 /NH3+ 



r; +H20 = r; +oh' 

 ^ cooh ^ cooh 



must undoubtedly occur, although, in the majority of cases (a 

 notable exception being afforded by the protamins) these modes 

 of dissociation must play a very subordinate part. Then the 

 terminal — NH2 and —COOH groups (and at least one of these 

 must exist at either end of a chain of amino-acids) may neutralize 

 themselves internally, thus: 



NH3 



/ \ 

 R-COO 



forming what Winkelblech terms an "internal salt" (73). Such a 

 molecule, whatever the mode of union, would of course form 

 compounds with acids or bases with much more difficulty than 

 the protein which had not undergone such internal neutralization, 

 since, before dissociation could occur the ring-formation would 



