ANTIGENS AS BIOCATALYSTS 15 



substances other than hydrogen or hydroxy! ions are expected to act as 

 acids or bases from this viewpoint and this reahzation has clarified the 

 picture of a great number of complex reactions. 



Often the even broader concept of acids and bases proposed by 

 G. N. Lewis is called upon. According to this theory, substances which 

 accept a share in an electron pair are acids, and the donor substances 

 are bases. This picture not only includes as a special case the substances 



H:Ci: +H:d:H^[^'9'"l'^+[:Cl:l 



treated by the Bronsted-Lowry concept, but also extends to neutraliza- 

 tions involving no proton transfer at all : 



CI H CI H 



C1:'b + :N:H >► C1:B:N:H 



ci H ci H 



acid base 



These viewpoints of the nature of acids, bases, and solvents will 

 be incorporated into the discussion of examples of processes where 

 either specific acids or bases act as catalysts or where any acid or base 

 would be effective to an extent dependent upon its strength. In the 

 latter case, i.e., general acid-base catalysis, the symbols HA and A~ will 

 serve for acids and bases in general. 



Furthermore, the course of some of the reactions will employ the 

 concept of quantum-mechanical resonance to justify the existence of 

 intermediate substances. Where this resonance stabilizes the inter- 

 mediate, it will be shown in customary style by a double-headed arrow 

 (<^) connecting the several canonical resonating structures which 

 serve as our designation of the resonance hybrid.* 



In representing the electron pairs in certain of the diagrams, dashes 

 are used. Thus 



H— O— H instead of H:0:H 



The hydrogen ion will often be written as a base proton, H"^, 

 although it is realized that the proton is solvated. This procedure is 

 followed largely as a matter of convenience, but it should be pointed 

 out that it is justified on other counts. First, the exact extent of 



*The reader is referred to such works as L. Pauling's The Nature of the Chemical 

 Bond (1944) for discussion of the fundamental basis of the resonance phenomenon. 



