460 D. O. JORDAN 



TABLE III 

 Values of Kj for the Ampholytic Nucleotides 



(nucleotide) (nucleoside) Ki 



Guanylic acid 0.7 



Adenylic acid 0.9 



Cytidylic acid 0.8 



have been calculated^^ assuming (1) that only the primary phosphoric acid group 

 and the amino group are important in determining the position of the isoelectric 

 region and the value of Ki , (2) that the constant for the dissociation of the nucleotide 



H3NROPO(OH)2 + H2O ^ H2NROPO(OH)2 + H3O+ 

 is identical with that for the nucleoside 



H3NR + H2O ^ H2NR + H3O+ 

 Then^s 



A'l (nucleotide) 



K.t = ~ ' ~ r — 1 

 Ai(nucleoside) 



This assumption is analogous to that generally made for calculating K^ for the amino 

 acids, when it is assumed that the dissociation of the amino group in the positively 

 charged acid is identical with that of the corresponding ester.'* It is justified here 

 by the fact that the undissociated phosphoric acid group is unlikely to have any 

 inductive effect on the amino group, from which it is separated by six atoms, and 

 the furanose ring is a nonresonating system. The values of K^ so calculated are given 

 in Table III. In guanylic acid, because of the low value of the pK'a of the amino group 

 in guanosine, the concentration of the zwitterionic form is only seven times that of 

 the uncharged form. In adenylic and cytidylic acids, however, the ratio is very much 

 greater and the concentration of the uncharged form in the isoelectric region is 

 negligible. 



It is evident from Table II that the introduction of the negatively charged 

 phosphate ion into the nucleoside produces a weakening of the acid strength 

 of the amino and the — NH — CO — dissociations. This observation is in 

 agreement with the field effect, first recognized by Bjerrum^^ and treated 

 c^uantitatively by Kirkwood and Westheimer.-^" The degree of the shift 

 of the pK'a will be a function of the distance l)etween the charged groups, 

 the smaller this distance the greater the p/v'a shift. This relationship has 

 been used by Cavalieri'"' in an attempt to establish the configuration of the 



37 W. D. Kumler and J. J. Eiler, J. Am. Chem. Soc. 65, 2355 (1943). 



38 E. J. Cohn and J. T. Edsall, "Proteins, Amino Acids and Peptides." Chemical 

 Catalog Co., New York, 1943. 



39 N. Bjerrum, Z. phtjsik. Chem. 106, 219 (1923). 



'0 L. F. Cavalieri, /. Am. Chem.. Soc. 74, 5804 (1952). 



