DIELECTRIC CONSTANT 245 



many ionic groups distributed over the molecule and thus such conside- 

 rations are not as irrelevant as might appear at first glance. The induction 

 of the adsorbed molecule is shown to be an important contribution in in- 

 teractions with an ionic surface and the same probably holds for many 

 molecules bound to enzymes. 



The importance of dispersion energies in ion-ion interactions is evident 

 in the binding of the — N(CH3)3"^ to the active site on acetylcholinesterase. 

 Although an important contribution is made by the interaction of this cat- 

 ionic group with an anionic group on the protein, the contribution of the 

 methyl groups is indicated by the decrease in binding energy as the methyl 

 groups are removed from the nitrogen atom. Cations such as K"^, Na"^, 

 or NH4"^ also do not interact with the enzyme anionic site as readily as 

 the —N( 0113)3^ group, whereas on the basis of purely coulombic interaction 

 one might expect the smaller cations to be bound more tightly due to 

 their smaller radius. Hydration of these ionic groups introduces another 

 factor and a more detailed discussion of such interactions will be left until 

 the role of the solvent is considered. 



DIELECTRIC CONSTANT 



The ratio of the force between two charged particles in a vacuum to the 

 force between them when they are immersed in some medium is the die- 

 lectric constant of that medium. It is also the ratio of the potential energies 

 in a vacuum and the medium (Eqs. 6-2 and 6-4). Heretofore interactions 

 in a vacuum have been assumed and it is now necessary to consider the sit- 

 uation when the interacting molecules are dissolved in water. The decrease 

 in force or potential energy between two charges when placed in a medium 

 is due to the polarization of the molecules of the medium. In solvents com- 

 posed of dipolar molecules the most important factor is the orientation or 

 alignment of the solvent molecules in the electrical field of the charges. 

 An ion or charged group will orient the water molecules around it and this 

 alters the electrical field due to the charge. The bulk dielectric constant 

 D of water at 37.5'' is 74.1. Charges separated by large distances (above 

 at east 20 A) will have their potential energy reduced by this factor. How- 

 ever, as the charges approach one another, the effective dielectric constant 

 decreases until it reaches a low value when the charged species are in contact. 



The dielectric constant for water varies with temperature according to 

 the equation: 



D = D,e-Tl0 (6-73) 



where D^, = 305.7 and 6 = 219 (Gurney, 1953). Thus D = 87.9 at Oo, 

 78.2 at 250, and 55.7 at lOO". In this book, body temperature will be assumed 

 in the formulation but when experiments are performed at other tempera- 

 tures the appropriate correction should be made. 



