120 INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



consequently negligible when two molecules are separated by any ap- 

 preciable distance. They are quite strong between molecules that 

 can bring parts of their "surfaces" into close contact. 



Hydrogen bonds (also relatively short-range j consist essentially 

 of a hydrogen atom which is attracted simultaneously to two dififerent 

 atoms. For example, the two oxygens in salicylaldehyde (Fig. 9-2) 



.0. 



-c=o 



Salicylaldehyde 

 Fig. 9-2. 



are connected by a hydrogen bond. Many of the unusual properties 

 of water are due to hydrogen bonding. It is believed that hydrogen 

 bonds play an important part in maintaining the characteristic con- 

 figurations of protein molecules. 



The role of coulomb forces in holding antibody and antigen to- 

 gether was removed from the realm of pure hypothesis by the ex- 

 periments of Singer (1957). This worker pointed out that if a nega- 

 tively charged group is involved in an antibody-antigen bond, it is 

 possible to calculate the effect of pH on antibody-antigen combination. 

 The assumption is made that, if the negative group is in the antigen, 

 the antibody contains a corresponding positively charged group, and 

 vice versa. For our present purposes it is immaterial which molecule 

 contains the negative group. Singer and Campbell (Singer, 1957) 

 suggested that, if there is one negative group characterized by an 

 intrinsic hydrogen ion association constant K^ and if we neglect the 

 nonspecific repulsion between antibody (Ab) and antigen (AG) 

 molecules, the following relation should hold in the acid region : 



log (l/K-l/Ko) =\og(Ku/Ko) - pH (1) 



where K is the apparent equilibrium constant at a given pH for the 

 reaction 



Ab+Ag;eAbAg 

 and A'o is the value of K at neutral pH where both the positive and 



