H. M. KALCKAR 



Biological Significance of Mesomerism 



What is the significance and what are the implications of meso- 

 meric phenomena in biological reactions? The great significance of 

 mesomerisin lies in the fact that it invariably endows the molecular 

 group with a considerable amount of additional stability. The word 

 stability in this connection is used in its broadest sense. It has been a 

 custom to distinguish between thermodynamic stability and the kind 

 of stability implied in terms such as "willingness" of a group to react 



Scheme i 

 Sequence of Reaction 



"ACTIVATED" MOLECULE 

 I 



ACTIVATION ENERGY 



OF REACTION: 



A:=:±:(B):;^C 



Scheme ii 

 Sequence of Oxidation 



■OF THE REACTION: 

 A=;=i:C 



A^ 



r- 



Ia^aI 



a 



MOLECULAR 

 ACTIVATION ENERGY 



1) OF 

 — NONCATALYTIC 

 OXIDATION 



2) OF CATALYTIC 

 OXIDATION 



spontaneously. The distinction must be considered artificial. The 

 work of Polanyi, Eyring and Stearn — cf. Eyring (3) — indicates strongly 

 that the activation of the substrate, i. e., the problem of how reactive 

 complexes are formed from stable molecular groups, is essentially a 

 thermodynamic one. Scheme I illustrates some of these relationships. 

 "A" signifies the starting product and "C," the end product 

 of the reaction A — > C. AFis the change in free energy of the reaction, 

 I. e., the amount of potential energy which is lost when A is converted to 

 C. The free energy or potential energy of the system drops when A is 

 converted to C. However, in order to start the reaction, A must be 

 activated in some way, i. e., the potential barrier which represents the 

 so-called activation energy must be overcome. Both the activation 

 energy and the free energy change (AF) are influenced by mesomerism. 

 Since the significance of mesomerism for our understanding of energy 

 coupling in biological systems has been discussed elsewhere (6), this 



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