178 



THE CELL AND PROTOPLASM 



bridges has been demonstrated by infra-red 

 absorption methods (Ellis and Bath 1938). 

 A conception of the hydrogen bridge may 

 be formed by the help of the diagram in 

 Fig. 6 where the approach of the plus and 



Fig. 5. A diagram to show the perspective ar- 

 rangement of coordinated water molecules. Each 

 molecule contains two residual plus and two minus 

 charges which are located at the corners of a 

 tetrahedron (Bernal and Fowler 1933). 



good agreement, as indicated in Table VII 

 where the principal hydrophilic groups of 



TABLE VII 



Coordination of Water by Various 

 Polar Groups 



the proteins are shown with the number of 

 water molecules which may be bridged to 

 them. 



That these coordinated water molecules 

 are bound to the i)rotein by hydrogen 



Fig. 6. A diagram to illustrate the hydrogen 

 bridge. Two water molecules are represented by 

 the large circles. The central © circles indicate 

 the positions of the oxygen nuclei which are con- 

 nected by solid lines to the hydrogen protons, the 

 smaller ffi circles. The mechanism proposed as the 

 ' ' hydrogen bridge ' ' is indicated by the proton 

 and the negative charge © on the line between the 

 two oxygen nuclei. 



minus points bring the resi^ective water 

 molecules closer together, 2.75 A, than the 

 van der Waals' cohesion forces, which 

 usually produce a zone approximating 3.5 

 A between molecules (Robertson 1935). 



As a consequence of the presence of these 

 oxygen and nitrogen atoms, hydration cen- 

 ters occur at various places on the protein 

 chain. There is a potential possibility of 

 two chains becoming fastened together, in 

 an extreme case, by means of 150 or more 

 water molecules forming bridges between 

 the two back-bones where these atoms oc- 

 cur uniformly and are spaced only short 

 distances apart. The layer of water mole- 

 cules attached in this manner between the 

 two back-bones would form a strong seam 

 of water molecules between the two chains, 

 separating them by about the thickness of a 

 water molecule. 



The hydration centers at the ends of the 

 side chains, on the other hand, are rela- 

 tively few and even if spaced uniformly 

 are rather far apart. Attachment through 

 these, therefore, should be weaker than 

 thi-oiigli the l)a('k-h(tii('s. These predictions 



