14 



DAVID PRESSMAN 



methylammonium group with two haptens of identical configuration, except 

 that one has the trimethylammonium group and the other a tertiary butyl 

 group as shown below (Pressman and Siegel, 1953a; Pressman, Grossberg and 

 Pauling, 1943). 



B.2 H 



N O NN 



-O3S 



N(CH3)3 



SO, 



-03S' 



C(CH3)3 



The compounds were prepared by coupling ^ara-azobenzene-trimethylam- 

 monium ions and /Jara-azo-^er/iar^'-butylbenzene to "H acid." The teriiary- 

 butyl group has the same configuration and size as the trimethylammonium 

 group but lacks the charge. Experimentally we observed a factor of 8 between 

 the relative combining constants of these two haptens with the antibody. 

 This factor of 8 represents a free energy of interaction due to the positive charge 

 of about 1100 cal., assuming that the van der Waals interactions for the two 

 groups are the same. On calculation of the distance between the positive charge 

 on the hapten and the corresponding negative charge on the antibody which 

 would give an energy interaction of this magnitude, we find that the two 

 charges approach within about 3 A. of the distance to the closest possible 

 approach. The calculation is made on the basis of Schwartzenbach's evalua- 

 tion of the dielectric constant of water for relatively small distances between 

 charges. This is indicated in Fig. 7. 



Table 6 shows the effect of increasing the size of the positive ion by replacing 

 the trimethylammonium group with either the trimethylarsonium group or 

 the triethylammonium group. We find that the combining constant decreases 

 directly with the increasing radius, and we conclude that the decreased com- 

 bination may be attributed to a greater distance between the charge of the 

 central atom and the negative ion of the antibody in the complementary posi- 

 tion. 



Since antibodies in general seem to fit closely about certain particular sub- 

 stances and the closeness of fit can be determined by detecting substances of 

 known configuration, information can be obtained about the steric configura- 

 tion of systems in aqueous solution. An interesting example of this is the case 



