TABLE II 

 Apparent Dissociation Constants* of Pyrimidines and Purines 



Compound 



Pj^rimidine 



Cytosine 



5-Meth}'lcytosine 



Uracil 



Thymine 



l-MethyluraciH 



3-Methyluracil* 



1 ,3-Dimethylurat'il 



Orotic acid 



Barbituric acid 



Barbital 



Purine 



Adenine 



Guanine 



Hypoxanthine 



Xanthine 



Uric acid 



Method of measurement 



Spectrophotometric 



pA'a, 



2.52 



7.53 

 5.4 



pA\ 



8.90 



11.63 

 10.6 



Ref. 



* Expre.ssed as pA'a , where \>Ka = 1/log Ka , and A'q = (A~)(H+)/(AH) for "acidic" and = 

 (B)(H + )/(BH'^) for "i)asic" dissociations. Thf designations pifai and pifo2 refer to the first and second dis- 

 sociations actually measured. Only for pyrimidine (unsubstituted) and purine do the listed p/Cai values 

 refer to the basicity of ring nitrogen. 



" A. Albert, R. Goldacre, and J. Phillips, J. Chem. Soc. 1948, 2240. 



'' D. Shugar and J. J. Fox, Btochim. el Biophys. Acta 9, 199 (1952). 



<" P. A. Levene, L. W. Bass, and H. S. Simms, J. Biol. Chem. 70, 229 (1926) 



'^ Referred to as "3-methyluracil" by an alternative nomenclature: D. Shugar and J. J. Fox, Biochim. et 

 Biophys. Acta 9, 199 (1952). 



' Referred to as "1-methyluracil": see previous reference. 



/ Orotic acid shows pKaz > 13. The carboxyl dissociation (pKai) appears to have a negligible effect on 

 the second and third dissociations since they are very similar to the corresponding values for uracil and thy- 

 mine. 



» M. Bachstez, Ber.63, 1000 (1930). 



'' J. J. Fox and D. Shugar, Bull. soc. chim. Beiges 61, 44 (1952). 



' .1. K. Wood, J. Chem. Soc. 89, 1831 (1906); cone, probably .W/64. 



' M. E. Krahl, J. Phys. Chem. 44, 449 (1940). 



* A. Bendich, P. J. Russell, Jr., and J. J. Fox, J. Am. Chem. Soc. 76, (1954), in press. 

 ' A. Albert and D. J. Brown, J. Chem. Soc. 1954, 2060. 



■" H. F. W. Taylor, J. Chem. Sec. 1948, 765; cone, from 0.0012 to 0.007 M. 



" pKas for guanine = 12.3. 

 H. F. W. Taylor, "Acid Base Properties of Nucleic Acids," Doctoral Thesis, London Univ., London, 

 1946; taken from D. O. .Jordan, Progr. Biophys. and Biophys. Chem. 2, 51 (1951); Ann. Rev. Biochem. 21, 209 

 (1952). 



^ A. G. Ogston, J. Chem. Soc. 1935, 1376. 



' W. His, Jr., and T. Paul, Z. physiol. Chem. 31, 1 (1900). 



"^ A. L. Bernoulli and A. Loebeastein, Helv. Chim. Acta 23, 245 (1940); apparently these two .separate 

 dissociations (pKai = 5.78 and pKai = 5.85), involving a total of 2 equivalents of alkali per mole of uric acid, 

 escape detection in the spectrophotometric analysis and appear as one {pKai = 5.4). 



' L. F. Cavalieri, J. J. Fox, A. Stone and N. Chang, J. Am. Chem. Soc. 76. 1119 (1954). 



' E. A. Johnson, Biochem. J. 51, 133 (1952). 



113 



