10 



DAVID PRESSMAN- 



TABLE 1 

 Closeness of fit in various para systems 

 Hapten 



* 



CH3 



System 



NN 



/ 



NN' 



NN 



NN' 



NN 



NN 



NN 



/ 



N(CHJ., 



AsOiH^ 



1.0 



1.45 



Ko 



0.86 



1.05 



1.0 2.7 ! 1.1 1.0 



AsO^H ' 1.0 



1.9 



0.78 



C 



0.21 



2.0 



2.9 



0.52 



1.0 1.8 .21 .03 .03 



C 



O 



1.0 



3.0 



.66 



.08 



,18 



3.9 



6.0 



1.98 



10 



ring). It can be seen that the fit is much closer in the last three systems than in 

 the first two. The fit around the positive charged hapten and the azobenzene- 

 azobenzenearsonate hapten is loose enough along one side so that the extra 

 benzene ring of the naphthyl or the ortho or para-xne\\\y\ groups can be accom- 

 modated. This is taken as an indication that the antibodies formed here are 

 of the slit trench type. The antibody presumably was formed against one side 

 of the hapten leaving the other side free. 



Table 2 shows how closely anti-or///(;-azobenzoate antibody fits around the 

 4 position of the benzoate ion. The combining constants of the antibody with 

 the />ara-fluoro, -chloro, -bromo and -iodobenzoates are listed. The hapten can 

 fit into the antibody site in only one way. Fluorine, which is not much larger 

 than hydrogen, decreases the relative constant to a value of 0.6. Chlorine de- 

 creases it somewhat more, and bromine still more. The differences are signifi- 

 cant. Iodine shows a slightly increased combining constant over that of bromine 

 and is about the same as that of chlorine. This order is an example of balance 

 between steric effect and van der Waals interaction. As the size increases there 

 is a greater steric interaction, but frequently the van der Waals attraction in- 

 creases in a compensating manner, as occurs here with iodine. 



