MESOMERIC CONCEPTS IN BIOLOGY 

 R:A- :A:R or R:A: -ArR 



IV 



possesses an unpaired electron (odd electron) and the right A, an elec- 

 tron pair; in structure b, the situation is reversed. The shift can be 

 represented as an oscillation of one electron between the two symmetrical 

 molecules: R : A. : A : R. The existence of an unpaired electron gives 

 rise to paramagnetism because the neutralized magnetic moments of 

 paired electrons is abolished when the electron lacks its partner. 

 The type of intermolecular mesomerism illustrated in formula IV will 

 be discussed later in this essay. 



The frequency of the oscillations which the electrons in meso- 

 meric structures undergo is very high; and it is therefore impossible 

 to consider a mesomeric group as possessing for an appreciable interval 

 of time either structure a or b. There is, however, a type of meso- 

 merism in which it is possible to distinguish between the two structures. 

 Tautomerism is a classical illustration of this type of mesomerism. 



In tautomerism, both an electron and a hydrogen atom parti- 

 cipate in the oscillation. Since the hydrogen possesses a significant 

 mass, the oscillation is considerably slower than in the purely electronic 

 mesomerism; and it is therefore possible to distinguish between the 

 two symmetrical states. Two examples of tautomerism are: (1) the 

 enol-keto tautomerism of carbonyl compounds (V); and (2) the lac- 

 tam-lactim shift of the hydroxy purines (VI). 



-H 



The so-called hydrogen bond results from the attraction of a 

 hydrogen atom attached to one electronegative atom (e. g., fluorine, 

 oxygen, nitrogen) for an unshared electron pair of another electronega- 

 tive atom (see formula VII): 



R— O— H . . . O— R 

 VII 



231 



