INTERACTION OF ORGANIC MOLECULES WITH PROTEINS 93 



of the medium on this interaction. For this purpose I have indicated schemati- 

 cally (Fig. 1) theonly oneor two theoretical ecjuations which I shall have here. 



The free energy of the binding can be thought of as consisting of two terms. 

 One is an intrinsic afhnity of the protein for the organic molecule — you can 

 think of that factor as being essentially the affinity present if you just blot out 

 the n G in Fig. 1 from your vision and imagine that there is no charge on the 

 protein. Under these circumstances there is merely the intrinsic affinity of some 

 particular site on the protein surface for the organic molecule. 



Then, in addition, there is the repulsive factor. If there is a negative charge, 

 there must be an electrostatic repulsion factor. So I put a minus sign before the 

 term AF elect, in Fig. 1. 1 do not mean that this is actually a negative free energy 

 term; that would be an incorrect indication. I just want to indicate that this 

 factor, the electrostatic factor (due to anion and a negative protein), is opposite 

 to the intrinsic factor and tends to decrease the extent of the interaction. 



Now, if you examine the nature of this electrostatic factor in more detail, 

 without going into all the quantities involved, you find that it depends, among 

 other things, upon the shape and the charge on the protein, and on the size of 

 the organic ion, but I have lumped all of those terms in one factor and empha- 

 sized the dielectric constant D as the one thing to which we want to pay par- 

 ticular emphasis. It occurs in the denominator of the bottom equation in Fig. 

 1. This would be the macroscopic dielectric constant of the medium as such. 

 So you can see that if you could, let us say, decrease the dielectric constant, 

 this negative term c^/Z) would become greater and you would expect a greater 

 repulsion factor and a decrease in binding. 



This has been tried (Fredericq, 1954) but I think the results cannot be con- 

 sidered unequivocal because the kinds of substance which you might add, let 

 us say dioxane, will decrease the dielectric constant, but simultaneously these 

 molecules can compete with the negative ion for attachment to the protein, 

 so that two factors occur simultaneously to decrease the binding. 



Conseciuently, we thought we ought to try to operate in the other direction, 

 to try to increase the dielectric constant in an aqueous solution. Essentially, 

 I think the only effective way of doing that is to add an amino acid in its iso- 

 ionic form and to operate at a pH, of course, at which the amino acid would 

 remain in its isoionic form. So, as I will show you in a moment, if you increase 

 the concentration of glycine from zero to two molar in an aqueous solution so 

 that the dielectric constant rises, roughly, from 80 to 120, the denominator in 

 <i)/D (Fig. 1) will become larger. Consequently this factor is decreased and you 

 would expect less of a contribution of the electrostatic factor and thus an in- 

 crease in the attraction of protein and organic molecules, since you have de- 

 creased the effect of the repulsion force. 



The next illustration (Fig. 2) shows these results (Klotz and Ayers) summa- 

 rized in terms of the free energies. The increase in — AFi° means increased bind- 



