PROTEINS 487 



the one or other kind will determine the sign of the charge of the 

 electrolyte-free protein molecule. We may imagine three possible 

 surface distributions of the NH3+ and C00~ groups, as follows 

 (the upper layer representing the outer surface of the molecule) : 



+-+++-++ --+ +- -+-+-+-+ 



RRRRRRRR RRRRRRRR RRRRRRRR 



-+ +-- ++-+++-+ +-+-+-+- 



1 2 3 



If the orientation is as in case 1, then the protein molecule is 

 predominantly positive; if as in case 2, then it is predominantly 

 negative; and if as in case 3, it is neutral. The negative charge of 

 a pure electrolyte-free protein is due to an excess of the exposed 

 C00~ groups over the NH3+ groups. 



The charged surface of a protein molecule will attract and hold 

 polar molecules such as those of water. If the orientation of the 

 Zwitterions is as in case 1 above, then the water dipoles will be 

 held by their oxygen ends, thus : 



Pauli has illustrated this for colloidal particles with a positive 

 surface charge (Fig. 177). 



If the ionic orientation is as in case 2, the water dipoles will be 

 grouped with their positive hydrogen atoms toward the surface of 

 the molecule, thus: 



- O 



+ + + + H2 



R R R R R 



+ + + + + 

 5 



If the orientation is as in case 3, both types of arrangement of the 

 water molecules will exist. Hydration will be more pronounced 

 in case 4 than in case 5, because the negative oxygen end of the 

 water dipole clings more firmly than does the hydrogen end. 



