ANTIGENS AS BIOCATALYSTS 13 



O— NO OH 



/ / 



2. 2SO2 +Ho0^2S0. +N0O3 



\ " \ " 



OH OH 



The gaseous mixture is brown; with the formation of nitrosylsulphuric 

 acid the brown color disappears. With its decomposition into sulphuric 

 acid and N2O3 the color reappears. These two equations represent 

 distinct consecutive actions and not partial equations of an interaction, 

 for one can observe the deposition of nitrosylsulphuric acid crystals 

 when a glass flask is used and the supply of water is deficient. The 

 N0O3 liberated in the second equation is immediately available to take 

 part in the successive cycles a large number of times. In this process 

 NoOa functions as a catalyst and the nitrosylsulphuric acid is the inter- 

 mediate labile complex between a catalyst and the reacting substances. 

 It corresponds to the enzyme substrate complexes generally believed to 

 form in enzyme reactions. 



1. Catalysis of Organic Reactions 



Some of the outstanding general catalyses of organic reactions in 

 homogeneous systems are: (a) general acid and base catalysis; (b) 

 catalysis in aqueous concentrated acid systems; and, (c) acid catalysis 

 in non-aqueous systems. 



Before dealing with these catalytic processes, it will be helpful to 

 review the modern concepts of acids and bases as well as the role of 

 the solvent in processes involving acids and bases. 



To extend the considerations of acids and bases beyond treatment 

 of aqueous systems, Bronsted, Lowry, and others have defined acids as 

 substances which yield protons (H+) and bases as substances which 

 accept protons. The relative ease with which the protons are given up 

 (or accepted) is the measure of the acid (or base) strength. The 

 solvent itself may act as an acid or base, and, indeed, the acidity or 

 basicity of the solvent is an important factor in the ionization processes 

 of solutes, as shown by the following generalized reaction scheme: 



HA + S ^ H+.S 4- A- 

 acid solvent solvated base 



(base) proton 



(acid) 



