PHYSICAL PROPERTIES OF NUCLEIC ACIDS 455 



Furberg has shown in cytidine, the corresponding bond Hes in, or very 

 nearly in, the plane of the pyrimidine ring, and the considerable resonance 

 in the adenine molecule would suggest that this is true for adenosine. If so, 

 the two rings will be nearly perpendicular to each other, as in cytidine. 

 Although the existing data do not definitely exclude other configurations, 

 this structure appears to give the most satisfactory explanation of the 

 available information. The data of Hendricks'^ can be reinterpreted on the 

 basis of this model. Gulland et al.-^ have pointed out that, in the structure 

 proposed by Hendricks, the sugar was a-lyxose and not D-ribose; and, in 

 the structure proposed by Furberg, two parallel planes approximately 1.5 

 A. apart can be recognized, one containing the purine ring and Ci , C4 , and 

 Cs , the other passing through C2 , C3 , and O2 .^^ Furberg^* considers that 

 guanosine has a structure similar to adenosine. 



2, Dissociation Constants 

 a. Pyrimidines and Purines 



The ^K'a values of some pyrimidines and purines are given in Table I. 

 The acid-base properties of uracil were first investigated by Levene et 

 aL,2i'22 who concluded from electrometric titration data that it possessed 

 two acid dissociations with p/C'a values 9.28 and 13.56, which were attrib- 

 uted to the two — CO — -XH — groups. Later the same authors-^ showed thai 

 an error had been made in applying the water correction for the titration in 

 strongly alkaline solution and a recalculation of the data showed the pres- 

 ence of only a single dissociation of p/^'a 9.45. This conclusion has been 

 confirmed by Taylor.^^ The reason for the extreme weakness of the second 

 dissociation constant of uracil is still obscure. Levene et alP considered that 

 in the neutral molecule the two — CO — NH — groups were of comparable 

 strength, a view that is supported by the ^K'a values of 1- and 3-methyl- 

 uracil which are similar to that of uracil (Table I), and that the ionization 

 of the second group was inhibited by the ionization of the first to such an 

 extent that its pK'a was too high to be detected by titration in aqueous 

 solution. However, calculation of the difference between pA'^^ and p/v'ao ," 

 using the formulas of Kirkwood and Westheimer-^-^ and assuming that 



" J. M. Gulland, G. R. Barker, and D. O. Jordan, Nature 151, 109 (1943). 



^^ P. A. Levene and H. S. Simms, J. Biol. Chem. 65, 519 (1925). 



« P. A. Levene, H. S. Simms, and L. W. Bass, /. Biol. Chem. 70, 243 (1925). 



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



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



Univ., London, England, 1946. 

 ^* D. O. Jordan, Progr. Biophys. and Biophys. Chem. 2, 51 (1951). 

 " J. G. Kirkwood and F. H. Westheimer, J. Chem. Phys. 6, 506 (1938). 

 " F. H. Westheimer and J. G. Kirkwood, J. Chem. Phys. 6, 513 (1938). 



