64 The Structure of Protoplas^n 



cross-linkage of a residue on one chain to a residue on an adjacent 

 chain by means of primary valence bonds. These are merely 

 chemical bonds such as in any molecule. We are more interested in 

 what may be termed cohesion bondings through electrostatic and 

 van der Waals' forces. In all cases of combinations of molecules we 

 are thinking of the protein as being especially prominent, and there- 

 fore we are concerned with the surfaces or faces of the protem 

 particles. 



It will be recalled that certain faces of the 50 A protein molecule 

 may have as many as fifty to sixty residue end groups exposed, 

 forming some sort of mosaic pattern. On this only about ten to 

 twenty are likely to be polar, and perhaps even fewer are ionized. 

 Thus the face will have patterns also based on ionized groups. Some 

 of these will be positively, some negatively, charged. The effect of 

 these charges is strongly localized at a short distance above the 

 surface, and resultants of these begin to appear as the distance from 

 the face increases, first in accordance with the arrangement of the 

 charges with respect to one another on the surface; and farther out, 

 perhaps at 50 A or more, the resultant effect is a blend of all charges, 

 internal as well as those on the surface, or is, in effect, the net 

 charge, or the algebraic sum, of all of the charges on the particle. 

 Interaction of two such particles may occur, providing their net 

 charge is opposite in sign. The resultant attraction of unlike and 

 the repulsion of like charges should produce a mutual rotation of 

 one particle with respect to another until their most attractive sur- 

 faces become adjacent. The surfaces should then turn until they 

 reach a position of best fit (99) . There is some doubt as to whether 

 the particles would be able to make direct contact because of the 

 possible presence of a 10 A layer of "electrically saturated" water 

 (16) around the ionized groups on their surface; that is, water 

 which is completely polarized and oriented. As the two oppositely 

 charged particles approached each other, they would reach a point 

 at a separation distance of about 20 A where their shells of similarly 

 oriented water molecules would touch; if their attraction for each 

 other is not large, it is possible that this might act as a barrier to 

 further approach (100) . However, in protoplasm other aggregating" 

 mechanisms may aid in squeezing out the water molecules and in 

 bringing the elementary protein particles into more intimate contact. 

 For example, van der Waals' attractive forces (19) become effective 

 at short distances of particle separation, and as the particles approach 

 still closer, hydrogen bridges between polar residues may become 





