PROTEINS 485 



would become chemically inactive glycine: 



/CH2\ 

 0C< >NH3 



\ / 



These older views are still possible even though augmented by 

 newer ideas. Bjerrum and others regard glycine (99 per cent of it) 

 as existing as a Zwitter or double ion (the German term Zwitterion 

 is generally used for these double ions). Such an ion is cation and 

 anion in one, i.e., possesses both a positive and a negative charge, 

 and may be written +NH3 — R — C00~, which for glycine 

 becomes, +NH3-CH2-COO-. 



Electric Charge. — While organic colloidal particles have been 

 regarded as mammoth ions (page 371), and they may be so in 

 certain aspects of their behavior, there is a fundamental difference 

 between a typical metal colloidal particle and an ion; the electric 

 charge of the latter is due to the loss or gain of an electron, while 

 that of the former is due to the adsorption of ions on to the surface 

 of the particle. Proteins possess electric charges which they owe 

 apparently to both ionic and colloidal properties. 



While we are, for the moment, interested in the electric charge 

 on proteins, it must be remembered that the stability (and solubil- 

 ity, etc.) of proteins rests usually in part, perhaps at times solely, 

 upon hydration. Pauli first suggested that hydration is a factor 

 in protein stability. Kruyt and deJong extended the idea to 

 include other substances (agar, etc.) (page 147). Protein parti- 

 cles in aqueous suspension when decharged by electrolytes or 

 dehydrated by alcohol may remain in suspension, but they fall 

 when both are added. 



Let us first consider proteins as ions. When in the presence of 

 acids or bases, proteins form salts which ionize. They are, there- 

 fore, electrolytes, with acidic or basic properties, and for this 

 reason are known as ampholytes, thus indicating both their 

 amphoteric and their ionization properties when in solution. 

 Ions migrate in an electric field. Proteins should, therefore, show 

 cataphoretic flow, and this they do (page 375). The sign of the 

 charge and therefore the direction of migration will depend upon 

 the degree of acidity of the solution. As we have seen, a protein 

 salt, e.g., gelatin chloride or sodium gelatinate, ionizes and in such 

 a way that when in alkaline solution, the protein (gelatin) ion is 



