PHYSICAL PROPERTIES OF NUCLEIC ACIDS 459 



TABLE II 



pK'o Values of Nucleosides and Nucleotides 



tides with those of the respective nucleosides, shows that the dissociating 

 groups of the latter are supplemented by two additional dissociations hav- 

 ing pK'a values 0.7-1.0 and 5.9-6.0. These are of the correct order for the 

 first and second dissociations of a sugar phosphate; ribose phosphates have 

 not been studied, the analogous glucose-3-phosphate has pK'a values of 

 0.84 and 5.67,^® and similar values have been obtained for other sugar 

 phosphates.^^ 



The assignment of the pK'a values to the various groups is best carried 

 out by reference to the data for the corresponding nucleosides and sugar 

 phosphates. The following are the relevant data for adenylic acid: 



Adenosine pK'a 3.5 



Sugar phosphates p^'oi 0.8-1.1 pK'a2 6.0-6.5 



Adenylic acid pK'ai 0.9 pK'a2 3.7 pK'a^ 6.0 



The pK'ai value of adenylic acid is clearly that of a primary phosphoric 

 acid dissociation, pK'a2 an amino dissociation, and pK'as a secondary phos- 

 phoric acid dissociation. In the isoelectric region, therefore, it is evident 

 that adenylic acid, and hence also guanylic and cytidylic acids, will exist 

 largely in the zwitterionic form. It is therefore to be expected that the 

 nucleic acids will behave similarly. 



The values of K^ for the nucleotides, where 



[H3NROPO(OH)o] 



K, = 



[H.NR0P0(0H)2] 



3^ W. E. Fletcher, On the Structure of Nucleic Acids, Doctoral thesis, London Univ., 



London, England, 1948. 

 " O. Meyerhof and K. Lohmann, Biochem. Z. 185, 113 (1927). 



