PHYSICAL PROPERTIES OF NUCLEIC ACIDS 



457 



TABLE I 

 pK'o Values of Pyrimidines and Purines 



P^'a. 



pK'a 



pK'a 



Reference 



12.3 



21-24 



23 



23 



23 



23,24 



23 



29 



29 



28 



28,29 



28,29 



28,29 



28,29 



24 



xanthine show no dissociation in the pH range 11.0-12.5. In adenine, the 

 pK'a2 value, 9.80, is surprisingly low when compared with the correspond- 

 ing dissociation in guanine and has been explained by Taylor'^ on the basis 

 of the crystallographic data of Broomhead.'-'" Taylor explains the unusual 

 acid strength of this group in adenine as being due to the considerable con- 

 tribution to the resonance of the structure, not only of the uncharged 

 structures, but also of those in which there is a negative charge on the 

 nitrogen atoms in the pyrimidine ring and a positive charge on those in 

 the imidazole ring. This distribution of charge would lower the p/C'aj value 



from that characteristic of an uncharged — NH — group towards one 



+ 

 characteristic of a charged — NH= group. The respective pK'a values for 

 these groups in benzimidazole are 12.3 and 5.3 and the observed value of 

 9.8 for adenine may thus be explained. In guanine, the dissociation of the 

 imidazole — NH — group takes place in a molecule which already bears a 

 negative charge in the region of the pyrimidine nitrogen atoms as a result 

 of the dissociation of the — NH — CO — group. It is therefore to be expected 

 that the resonance structures bearing a negative charge on the negative 

 atoms of the pyrimidine ring will make a smaller contribution in the gua- 

 nine than in the adenine anion, and, since resonance structures of this type 

 necessarily contain a single bond in the C4 — C5 position, Taylor^^ concludes 

 that the greater length of this bond in adenine is explained. This argument 

 has been criticized by Cochran'"* on the grounds that the resonance struc- 



81 H. F. W. Taylor, Nature 164, 750 (1949) 



