SALT COXCEXTRATION AXD IONIC STRENGTH 823 



der Waals' component (due to a structurally favorable complementariness 

 over an extensive area) but is reduced by the presence of oppositely charg- 

 ed groups, an increase in the ionic strength would reduce the electrostatic 

 repulsion and hence increase the interaction energy. Actually, in any case 

 in which the binding is impeded by repulsion between ionic groups, even 

 though these groups are not part of the specifically interacting regions, the 

 expected decrease in binding with an increase in the ionic strength will be 

 either reduced or inverted. The intimate nature of the interaction should 

 also be considered because in some instances the opposed groups may be 

 shielded from the influence of ions in the medium and hence the interaction 

 energy between them would not vary in the same way as if they were ex- 

 posed to the medium. 



Changes in the Ionic Strength Produced by Additions of Substrate or 



Inhibitor 



Ionic substrates or inhibitors increase the ionic strength of the medium 

 and, when they are added in increasing concentrations, some of the effects 

 on the enzyme rate may be related to changes in ionic strength. This is 

 particularly true, of course, when the affinities for the enzyme are low and 

 high concentrations must be used. This secondary factor could so modify 

 the kinetics that deviations from linearity would be observed in the ana- 

 lytical plots described in Chapter 5. For example, in the plot of 1/f ^ against 

 (I) (type D), the slope is ^,„/7„,^,(S); if K^ becomes larger with increasing 

 inhibitor concentration (due to the increasing ionic strength), the slope will 

 decrease and the curve will become concave downward. Some of the slight 

 activations and inhibitions reported for various ions, either inorganic or 

 organic, may be due. at least in part, to a nonspecific effect of the ionic 

 strength. One way of minimizing such effects is to use a medium with a 

 relatively high ionic strength, either from buffers or neutral salts, so that ionic 

 strength changes due to the substrate or the inhibitor will be less effective. 



Effects of Changes in the Ionic Strength on Rate Constants 



Variations in ionic reaction rates with salt concentration were first 

 put on a firm theoretical basis by Bronsted (1922, 1928) who introduced 

 the activity coefficients for the reactants into the rate equation. Thus for 

 the bimolecular reaction: 



A + B^X^C + D (15-81) 



where X is the activated complex, the rate is given by: 



V = A:o(A)(B) ^"^ (15-82) 



