IONIC ATMOSPHERE 



261 



usually expressed as Ijx, would be approximately 7.5 A. If the distance of 

 closest approach /"q is taken as 3.0 A (radius of enzyme or inhibitor ionic 

 group = 1.7 A and radius of atmosphere ion = 1.3 A) the potential energy 

 at equilibrium, correcting for repulsion, would be: 



(Pe = 183 ^ eo.i33(3-d^) kcal/mole 



(6-89) 



If the ions retain their primary hydration, ?•(, would be approximately 

 10.2 A (diameter of two water molecules = 7.2 A) and the potential energy 

 would be: 



go.o834(io.2-d^) kcal/mole (6-90) 





When it is reasonable to assume that the interacting ions reach an equili- 

 brium distance equivalent to the closest approach of the atmosphere ions, 

 the exponential factor disappears; when this is not the case, the variation of 

 the exponential factor with distance may be estimated from Fig. 6-14. It 



Fig. 6-14. Variation of the exponential factor with distance. Curve A: 

 y _- g0.i33(4-de); curve B: / = e0.o834(io.2-(ie). curve C: /= e^-^iO-de). 



