ELECTRICAL FORCES 



41 



Iniermolecular Forces 



At the molecular level electrostatic interactions occur of such a profound 

 nature that they are reflected all the way up to the physiology of the system. 

 In this group we discuss not only charge-charge (ion-ion) forces, but also 

 those arising from interactions involving dipoles, and even induced dipoles. 

 With these concepts, along with that of electron dispersion in an atom-atom 

 bond, we can then describe not only the "Coulombic forces" but also the so- 

 called "London-van der Waals forces" operating between big molecules 

 such as lipoproteins; and finally, with the concept of proton (H + ) exchange 

 between neighboring groups (two oxygens, for example), we can describe 

 the extremely important "hydrogen bond." 



For reasons which are reviewed in Chapter 4, in a molecule which is not 

 symmetric, such as CO, one end accumulates more of the electronic charge 

 than the other. In CO, the oxygen atom has the extra bit of negative charge, 

 and the carbon is left slightly positive, by difference. The molecule has 

 within it a permanent charge separation, and is called a permanent dipole. 

 This and its weaker sister, the induced dipole, are shown in Figure 2-5. 



8* 



OS- 



S' 



permanent dipole 



i p v i 



0^ ^Osi.*NH 3 \ 



8* 



-^B- 



induced dipole 



Figure 2-5. Electrostatic Charges in 

 Molecules. 



Water is a permanent dipole, its hydrogen ends being positive to the nega- 

 tive oxygen. The — CONH — linkage between amino acids in proteins is 

 also a permanent dipole, as are the — COOH groups of organic acids, and 

 many others. 



Although these are small charges, Coulomb's law applies to them, and 

 fairly strong electrostatic forces can exist, firstly between permanent charges 

 and permanent dipoles, and secondly between one permanent dipole and 



