POLYPEPTIDE STRUCTURE OF PROTEINS 



157 



enol type of union carries with it the possibility of the following types 

 of reaction : 



H 



COH=N 



Na 



OH" 



and 



COH N + H+ + 



( CONa) ++ 



( COH) 



yielding, in each case only protein ions. 



This conception of the mode of union of proteins with inorganic 

 acids and bases affords an explanation of what would otherwise con- 

 stitute a very puzzling fact, namely, that while compounds of acids 

 or bases with protein form very good conductors of electricity through 

 their solutions, and must, therefore, be electrolytically dissociated into 

 ions, yet no evidence is afforded by these solutions of the existence in 

 them of any ions derived from the combined acid or base. Thus 

 chlorides of the proteins yield only traces of chlorine ions in solution, 

 as is shown by the difficulty or sluggishness with which they react with 

 silver salts to form silver chloride. The calcium compound of Casein 

 does not dissociate any appreciable proportion of calcium ions, and 

 compound of the proteins with silver, mercury, lead, copper, etc., do 

 not yield up these ions to the solution. The conductance of electricity 

 through the solution of a protein compound cannot be due to contami- 

 nation of the protein with dissociable inorganic salts, because the con- 

 ductivities observed are too large to be accounted for in this way and, 

 moreover, the conductivity of the protein compound varies in a very 

 striking and regular manner with the proportion of acid or alkali 

 bound, increasing, as one might expect, with the number of acid or 

 alkali radicals which have entered into combination with the protein. 

 Furthermore, in many instances, e. g., Casein and Serum-globulin, it is 

 easy to show that the protein participates in the conductance of the 

 current owing to the fact that the protein is precipitated at one of 

 the electrodes, and the amount of protein so precipitated is proportional 

 to the amount of current which has traversed the solution. It is a 

 very striking fact that casein in alkaline solution, although it is only 

 precipitated at the one electrode (the anode) yet migrates toward both 

 electrodes when a current is passed through the solution, indicating 

 that certain portions of the casein are engaged in transporting positive, 

 while others are transporting negative charges. Migration of the 

 proteins toward both electrodes has also been observed in solutions of 

 Hemoglobin and in solutions of Fibrinogen. 



