MESOMERIC CONCEPTS IN BIOLOGY 



The condition for stabilization of the semiquinone by meso- 

 merism is satisfied only when the two structures are equivalent. This 

 requirement is satisfied when the molecule is dissociated as an anion. 

 The undissociated semiquinone in which the presence of the hydrogen 

 atom eliminates the symmetry of the two structures does not fulfill the 

 required condition. Correspondingly, Michaelis and his group showed 

 that the semiquinone of phenanthrene-3-sulfonate is relatively very 

 stable in alkaline solution in which it exists as the symmetrical anion. 

 The semiquinone of /^-phenylenediamine, on the other hand, has a fair 

 chance of existence in strongly acid solutions because only then does 

 the symmetrical phenylenediaminium cation exist. In complete 

 accordance with the ideas just developed is the observation that the 

 free radical of paraquinones accumulates only in the alkaline pH range, 

 whereas those of the paradiamine compounds accumulate in measur- 

 able amounts only at strongly acid reactions. The "catalyzing" effect 

 of hydroxyl ions or hydrogen ions on these two types of oxidation- 

 reductions has actually been explained in terms of mesomerisms. 



The intriguing question is whether it is possible to explain en- 

 zymic catalysis in terms of the same principles. There are observa- 

 tions which may provide confirmation for such an explanation. Some 

 years ago, Haas (4) found that riboflavin phosphate, when linked to a 

 specific protein, forms a semiquinone when undergoing reduction. 

 This semiquinone is not observed during the reduction of free riboflavin 

 phosphate in neutral solution, but accumulates when the reaction is 

 acid enough to insure complete ionization. In other words, the 

 enzyme is able to stabilize a product at neutral reaction which other- 

 wise would exist only at strongly acid reaction. This observation may 

 be taken as a clear indication that oxidation-reduction enzymes in 

 some way or other are concerned with the formation of mesomeric 

 free radicals. 



Before going deeper into the discussion of the nature of enzyme 

 catalysis, it is worth while to introduce a very ingenious theory proposed 

 by Delbriick, dealing with the nature of reproduction phenomena. 

 Delbriick advanced the idea that, in processes like gene reproduction, 

 mesomeric phenomena may play an outstanding role. According to 

 classical concepts, the probability of bringing the same components 

 together in the same sequence as that of the original molecule is in- 

 finitely small, Delbriick is probably the first who has expressed the 



