INTERMOLECULAR FORCES 



85 



most of the long range forces between biological molecules are of electrostatic 

 origin rather than being of the London type which is most common in gases. 



The short range forces between two biological molecules frequently require 

 the consideration of two or more valence structures corresponding to the 

 possibility of chemical reactions taking place during the collision. 



And linally, in biological reactions (such as in photosynthesis) electronically 

 excited intermediates are frec}uently formed which have unusually large long 

 range forces with other molecules. 



Thus the problems of intermolecular forces in biological systems are orders 

 of magnitude more difficult than those for the interaction of two noble gas 

 atoms with which we are reasonably familiar. However, we can examine the 

 qualitative nature of the biological forces. 



In the large biological molecules some groups have a large electron affinity 

 and suck electrons away from other groups, which become positively charged. 

 The zwitter-ions provide the classical example. The separations between these 

 positive and negative charge centers often are large compared to the separation 

 between the points of closest approach of two interacting molecules. In such 

 cases the dipole-dipole interactions are cjuite different than would be supposed 

 on the basis of "ideal dipole" forces. Consider the interaction of two zwitter-ions 

 A and B whose centers are separated by the distance Rab • Let us suppose that 

 the charges on A have the absolute value Ca and those on B have the absolute 

 value ee . Then the energy of dipole-dipole interaction of these molecules is 



E = CaC 



A^B 



lRa+, b+ 



+ 



1 



R, 



R 



A+, B- 



Ra-,b+_ 



(1) 



Here Ra+, b+ is the separation between the center of positive charge on A and 

 the center of positive charge on B; Ra-, b- is the separation between the cen- 

 ter of negative charge on A and the center of negative charge on B; etc. 



If as shown in Fig. 1, the positive charge on A comes close to the negative 



Fig. 1 



