MESOMERIC CONCEPTS IN BIOLOGY 



groups (serine, threonine) of the protein catalyst. This suggestion 

 may be in better accordance with kinetic data. Moreover, the sug- 

 gestion may illustrate the nature of the forces responsible not only for 

 the catalytic action of enzymes but also for their specificity. 



The electron acceptor goes through most of the steps illustrated 

 here, only in the opposite order. The mesomerism between the alde- 

 hyde and the free radical is best illustrated as a complex between the 

 free radical and the aldehyde in the biradical (triplet) state in which 

 two electrons in the double bond are separated (formula XIII). In 



H H 



R:C:0: , .6:C:R 



Free radical Biradical 



aldehyde aldehyde 



XIII 



ihe biradical state, the group contains two unpaired electrons which 

 will give rise to the occurrence of paramagnetism. Most double bonds 

 are considered to exist only to a very small degree in the biradical state; 

 yet the occurrence of paramagnetism in molecules containing double 

 bonds is a reminder of the existence of unpaired electrons. In most 

 cases, the biradical state is considered a state of excitation. The energy 

 of this excitation has recently been made accessible to measurement, 

 thanks to the studies of phosphorescence spectra by Lewis and his 

 group. Readers interested in this new and interesting development are 

 referred to the papers of Lewis and his co-workers (7). 



There are, however, a few facts concerning biradical double 

 bonds which might be of direct interest for biologists. One of them is 

 that molecules like oxygen and nitrate exist mainly in the biradical 

 state and that the double bond configuration represents the labile 

 state. Correspondingly, these molecules display a high degree of 

 paramagnetism. Pauling (9) describes the stable biradical (or triplet) 

 state of the oxygen molecule as a single bond with two three-electron 

 bonds (formula XIV). It would be interesting to know whether this 



:0 : O; 



• • -4 > • 



XIV 



is the form of oxygen which reacts with the coenzymes of the respira- 

 tion enzymes. 



