INTERACTIONS OF HAPTENS WITH ANTIBODIES 



281 



these groups cannot be estimated (since Kq for sulfonate and carboxylate is 

 given as 0), but it is certainly greater than 2.5 kcal/mole. Since steric 

 factors reducing the interaction of sulfonate and carboxylate cannot be 

 important, there must be forces other than ion-ion attraction to stabilize 

 the arsonate complex. Hydrogen bonds between the arsonate 



(— As^O") 



or phosphonate 



(-P^O ) 



groups with an adjacent :X group on the antibody surface might account 

 for part of this difference but not all. In any event, it is a very interesting 

 example of ionic specificity and similar situations will be found in enzyme- 

 inhibitor interactions. 



Effect of the Ionic Group Size 



The antibody to j5-azophenyltrimethylammonium comljines more weakly 

 with haptens as the size of the charged group is increased (Pressman et 

 al., 1946; Pres.sman, 1957). The experimental combining constants, Kq, 

 are given in Table 6-20 and calculations based on head-on interaction with 

 a carboxylate ion are given, using Eq. 6-90 to account for the effect of the 

 ion atmosphere. It is seen that the predicted Kq"s are quite in accord with 



Table 6-20 

 Effect of Ionic Group Size ox Hapten-Antibody Interaction " 



" The ionic group radii were calculated on the basis of head-on approach. The 

 combining constant Kg' for the trimethylammonium ion is assumed to be unity in 

 both cases. 



