MESOMERIC CONCEPTS IN BIOLOGY 



sented the first oxidation step, i. e., the product which lost only one 

 electron. In that case, the chance of existence of B, possessing as it 

 does an odd electron, would be infinitely small and the potential bar- 

 rier, therefore, very high. However, if B were able to form an inter- 

 molecular mesomeric bond with A of the type illustrated in formulas 

 IVfl and b, and in IX (that is, a three-electron bond), the chances of 

 getting the reaction started would be much better. 



This could be illustrated in the following way: Let A be the 



r 



product to be oxidized, A the first oxidation product (^ signifies the un- 

 neutralized electromagnetic moment of an unpaired electron), and A°^ 

 the final product. The broken line signifies the potential barrier 

 (or activation energy) of the noncatalyzed reaction, and the unbroken 

 line, that of the catalyzed reaction in which A forms a mesomeric com- 



plex (signified by A f^^ A or by A: ?=^ .A) with A, both being in close 

 proximity to each other on the specific catalyst. In other words, the 

 state of activation is not confined to one molecule but is "spread" 

 over two or more molecules. Yet only one of the molecules, perhaps 

 that closest to the electron acceptor, donates the second electron and is 

 thus converted to A°^. 



It might be useful, in order to make the idea more intelligible, 

 to select as a concrete example the enzymic oxidation of an alcohol to a 

 carbonyl group. The specific catalyst of this reaction could be classi- 

 fied as an alcohol dehydrogenase or as an acetaldehyde reductase 

 (hydrogenase) with equal right, inasmuch as the enzyme is equally 

 specific toward the alcohol and the aldehyde. The first step in the 

 oxidation is the formation of the free radical (see also Chapter 14) 

 which, however, has very little chance of formation if it cannot in some 

 way or other undergo mesomerism with another molecule. Inasmuch 

 as the protein-substrate combination brings the various oxidation 

 products of the substrate in close proximity, one is justified in assuming 

 that a good opportunity for the formation of mesomeric complexes is 

 at hand. The sequence of phenomena in such a dehydrogenation might 

 be described as follows. It is well known that, in oxidations of metab- 

 olites such as hydrocarbons, alcohols, and aldehydes, not only elec- 

 trons but hydrogen too is removed. Most metabolites like hydrocarbons 

 and alcohol are infinitely weak acids and have very little tendency to 

 form hydrogen ions except at extremely alkaline reactions. 



