822 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



of the effect of the ions on the solvent water and not directly on the solute 

 molecules. It has been generally found that the activity coefficents of or- 

 ganic nonelectrolytes increase with the salt concentration, and that the 

 relationship is adequately described by the equation: 



log y = k,,s (15-80) 



where A^,,, is the salting coefficient. However, the magnitude of the effect 

 is dependent on the salt used and is not related only to the ionic strength; 

 that is, the value of k„, varies with the salt used and the nonelectrolyte 

 studied (Harned and Owen, 1958, p. 532). The values of k„, for addition of 

 KCl vary from 0.06 to 0.12 in most cases and the mean value for ten non- 

 electrolytes is about 0.10. Thus the addition of KCl to bring about a change 

 of + 0.1 in the ionic strength would increase the activity coefficient by a 

 factor of 1.023, and a change of + 0.5, which is about maximal for most 

 enzyme work, by a factor of 1.122. Thus the effects on interactions of the 

 nonelectrostatic type are small and probably do not contribute greatly 

 to changes in the dissociation constants of EI complexes. What effect 

 there is produces a change in K^ in the opposite direction to that occurring 

 in ionic interactions and so if nonelectrostatic forces form a significant 

 fraction of the total interaction energy of an inhibitor with an enzyme, 

 the expected increase in Kj based on a purely ionic interaction may be re- 

 duced somewhat. 



The effects of the ionic strength on dipolar interactions and hydrogen 

 bonding are much more difficult to treat. The activity coefficients of the 

 strongly dijDolar amino acids may be either increased or decreased with 

 increasing salt concentration, depending for one thing on the ratio of polar 

 to nonpolar groups in the molecules. Arguments on the j)ossible effects of 

 ionic strength on hydrogen bonding have been presented in a discussion of 

 a paper by Kirkwood (1954, p. 23). No experimental data are available. 

 Boyer and Hammett both feel that there should be very little if any ef- 

 fect of ionic strength while Kirkwood maintains that since the interaction 

 is electrostatic, there should be a small effect. It would seem that one 

 might expect an effect similar to that observed in ion-ion interactions but 

 much smaller, inasmuch as the ionic atmosphere surrounding dipoles is 

 certainly of less electrical density than around ions. 



The interactions of many inhibitors with enzymes involve a number of 

 different types of forces and the energy terms corresponding to each of these 

 forces will vary in characteristic ways with the ionic strength. Thus the over- 

 all effect of a change in the ionic strength on K^ may be complex. In certain 

 protein interactions an increase in the binding with an increase in the ionic 

 strength has been observed and Kirkwood has termed this an inverse ionic 

 strength effect (Kirkwood, 1954). A possible explanation was suggested by 

 Gutfreund in the discussion (p. 21): if the binding depends on a large van 



