486 PROTOPLASM 



negative (Na+ + G~) ; and when in acid solution, it is positive 

 (G+ + Cl~) ; consequently, in an electric field, the positive protein 

 ion will travel to the cathode, and the negative one to the anode. 

 Midway between the state of complete combination with acid and 

 complete combination with base will be a point where no migra- 

 tion takes place. This is the isoelectric point. At this point, 

 there is no (or a minimum) migration, minimum combination of 

 the protein with acid or with base, and minimum solubility. 

 The isoelectric point may be in an acid, a neutral, or an alkaline 

 region, depending on the relative number and strength of the 

 acidic and basic groups. As the protein ion is either positive or 

 negative depending on the pH of the solution, it is evident that 

 to state the cationic or anionic nature of a protein salt is meaning- 

 less unless the pH of the solution is given. This applies not only 

 to proteins but to living cells which may be coated with protein. 



So much for protein salts which ionize into a protein anion in an 

 alkaline solution and into a protein cation in an acid solution. 

 With pure or electrolyte-free protein, the story is slightly different. 

 We still have to do with ionization, not of a protein salt but of 

 amino and carboxyl groups within the protein molecule. 

 Albumin, after weeks of electrodialysis, is almost, if not wholly, 

 free of acid and base, yet the albumin is still slightly negatively 

 charged and exhibits feeble cataphoretic properties. 



An important concept, expressed by the term Zwitterion (liter- 

 ally, hermaphroditic), has been introduced into protein chemis- 

 try; it signifies that ionized amino acids are compound ions; thus: 

 +NH3 — R — C00~, which for glycine, as we have seen, becomes 



/NH3+ 

 CH2< 



^coo- 



Amino acids as Zwitterionen, or double ions, acquire and there- 

 fore give to proteins properties peculiar to themselves. Among 

 these is the possibility of precipitation with either the cation or 

 the anion of a salt. Our present interest in double ions is their 

 orientation within the protein molecule. 



If the numerous NH2 and COOH groups within a protein mole- 

 cule are ionized, then whatever the configuration of the protein 

 molecule as a whole may be, some of the ionized NH3+ and C00~ 

 radicals will be exposed at the surface, and the excess in number of 



