H. M. KALCKAR 



presence of an unpaired electron, possessing unneutralized magnetic 

 moment. This free radical has, generally, a very brief existence, since it 

 either accepts an electron again or expels the remaining odd electron. 

 Since the free radical has so little chance of existence, the removal of 

 the first electron is barred, so to speak, by a high potential barrier. 

 In scheme I, the group to be oxidized would be represented by A, the 

 free radical by B, and the final oxidation products by C. The height 

 of the potential barrier would represent the activation energy. The 

 activation energy is the factor which particularly interests us in con- 

 nection with the concept of catalysis. If the potential barrier is too 

 high, the chance of forming the free radical is practically nil and the 

 rate of the reaction is zero. When the temperature is raised sufficiently, 

 the thermal movements of the molecule become so vigorous that a 

 certain percentage of molecules will slip over the potential barrier. 

 Although thermal movements, of course, are of importance for events 

 in the living cell, physiological temperatures are usually too low to 

 allow most reactions to proceed at measurable rates. In order to bring 

 about a reasonable rate, the living cell has succeeded in lowering the 

 potential barrier by a special device which we call catalysis. It is in 

 this connection that mesomerism may turn out to be of paramount 

 importance, as the model experiments of Michaelis and his associates 

 have so strikingly demonstrated. 



The reduction of /)-benzoquinone to the corresponding hydro- 

 quinone goes through a radical (semiquinone). The free radical has 

 very little chance of existence because of its asymmetry (formula VIII). 

 In basic solution, however, the semiquinone will exist as the symmetrical 

 ion oscillating between two symmetrical electronic states. The two 

 equivalent structures interchange the odd electron, similar to what 

 Pauling (9) calls a three-electron bond, either through the benzene 

 ring or by intermolecular bonds (formula IX). 



.6:H :6: < — ^ .6: 



I I I 



A fA A 



Y V Y 



:0: :0. < — > :0: 



VIII IX 



