MOLECULES AND STRUCTURE FORMATION 7 



to an ion-dipole interaction with water molecules and so become 

 hydrated, the interaction energy amounting to about 5 kcal per 

 mole of bound water (Dole and McLaren, 1947). 



The last of the forces operating at close approach are the London- 

 van der Waals attractive forces, which result from the polarization 

 of each group of atoms in the fluctuating electrical field arising from 

 the instantaneous configurations of the electrons and nuclei of ad- 

 jacent groups. For small groups, the energy varies with the inverse 

 6th power of the distance between centers and directly with the 

 molar refractions of the groups involved (Pauling and Pressman, 

 1945). 



Specificity Effected by Short-Range Interactions 



It is of importance to note that both the hydrogen-bond and 

 van der Waals interactions vary, respectively, inversely with the 

 3d and 6th powers of the distance of separation of the groups. If 

 the maximum interaction energy is released when, for example, two 

 CHl' groups are 4 A apart, separating these groups by an additional 

 2 A will reduce the interaction energy by a factor of about 11. Like- 

 wise for the hydrogen bond, stretching the bond by 1 A essentially 

 terminates the interaction. We see at once an extraordinary speci- 

 ficity can be accounted for by the requirement that an adequate 

 interaction energy be obtained only when a variety of groups hav- 

 ing different interaction characteristics are brought into their proper 

 positions. These groups would include unlike charges, those form- 

 ing hydrogen bonds, and the bulky groups, interacting through van 

 der Waals forces. The latter, which will essentially define the sur- 

 face of a large molecule, must fit so that the protuberances on one 

 surface find appropriate cavities on the other surface. Remembering 

 that a stable interaction is produced when the total interaction 

 energy is about 10 to 15 X kT, and that all local or group interaction 

 energies are summed to obtain the total interaction energy, sug- 

 gests that the smaller the area of interaction, the more perfect the 

 fit between interacting surfaces or the more numerous the groups 

 giving rise to stronger interactions. 



Even the most specific of interactions can now be rationalized 

 on the basis given; for, as is evident, minute alterations in struc- 

 ture may lead to imperfect steric fit, and since energies of inter- 



