MESOMERIC CONCEPTS IN BIOLOGY 



aspect will be treated rather briefly in the present essay. The influence 

 of mesomerism on AFcan be summarized as follows: If C has an addi- 

 tional amount of mesomeric stability which A does not possess, the drop 

 in AF of the reaction A — > C is greater than it would have been if C 

 did not possess that extra stability. We have already mentioned the 

 carboxylate ion as a typical representative of a molecular group with 

 extra mesomeric stability. If another molecule is introduced into such 

 a mesomeric group, as by esterification, the symmetry of the group is 

 disturbed and the mesomerism decreases or vanishes, which again 

 implies that the potential energy of the complex is raised. Thus, acetic 

 acid anhydride, CH3 — CO— O— CO — CH3, in which both carboxyl 

 groups have lost their state of mesomerism, possesses a much higher 

 potential energy than that of the two acetic acids formed by hydrolysis. 

 In terms of scheme I, acetic acid anhydride would correspond to A 

 and the two acetic acid molecules to C. 



The living cell contains at least three types of substances in 

 which the mesomerism of two groups is mutually blocked. The first 

 type includes the carboxyl phosphates (acyl phosphates), the second 

 group, the amidine phosphates (phosphocreatine, phosphoarginine), 

 and the third group, the pyrophosphates. The carboxyl phosphates are 

 the primary oxidation products of a reaction in which a carbonyl 

 phosphate complex undergoes enzymic oxidation. It is important to 

 point out that the oxidation is catalyzed by an enzyme specific only 

 for the phosphate complex. Thermodynamically speaking, the 

 oxidation of a carbonyl-water complex to free carboxylate would be 

 greatly favored, and the chances of forming a carboxyl phosphate would 

 be vanishingly small, if the latter reaction were not specifically catalyzed 

 by an enzyme. 



This brings up the question of the nature of enzyme catalysis. 



An unusually promising approach toward an understanding of 

 oxidation-reduction catalysis on the basis of mesomeric concepts has 

 been made by Michaelis and his group. Since this topic is discussed in 

 Chapter 14, only certain aspects of the problem will be treated 



here (8). 



It is now generally recognized that oxidation of organic com- 

 pounds, involving the removal of a pair of electrons, takes place step- 

 wise. The removal of one electron prior to the other gives rise to the 

 formation of a free radical displaying paramagnetism because of the 



