OPTICAL PROPERTIES OF NUCLEIC ACIDS 495 



to plot absorptivities on a logarithmic basis for relatively simple spectra 

 such as those of purines and pyrimidines. The details of shape are thereby 

 blunted and the tedious process of obtaining quantitative estimates from 

 small-scale curves is made even more tiresome and inaccurate. Even if the 

 above conditions are fulfilled it is always desirable to provide in addition a 

 table giving molar absorptivities and wavelengths of principal features, 

 including minima as well as maxima. 



II. Bases, Nucleosides, and Mononucleotides 



The structural significance of the ultraviolet absorption spectra of purines 

 and pyrimidines has been well reviewed by Jordan,^ who, following Mar- 

 shall and Walker,' has emphasized the role played by ionization in the 

 spectral changes observed on varying the pH of solutions. Marshall and 

 Walker have pointed out that the more accessible pyrimidines, including 

 those concerned here, have a number of functional groups as substituents 

 which make detailed interpretation of their spectra very difficult. These 

 workers, and also Boarland and McOmie^ and Brown and Short, ^ give 

 spectra for many simple pyrimidines, including pyrimidine itself. The 

 ultraviolet absorption spectra of the simple diazines have been discussed 

 by Halverson and Hirt,'" who, however, employ the older spectrum of 

 pyrimidine determined by Heyroth and Loofbourow." Less attention has 

 been devoted to a general examination of the purines. Here again, however, 

 Stimson and Reuther^- and Cavalieri et al.^^ have attributed the spectral 

 changes observed with change in pH to keto-enol tautomerism, although 

 the close coincidence of pH values determined by titration with the pH 

 regions where such changes occur, points unmistakably to their ionic charac- 

 ter. The importance of pH control when measuring purine absorption 

 spectra was emphasised as early as 1930 by one of us (E. R. H.^^), but 

 measurements are still frequently made at an arbitrary pH value which 

 lies so close to a pK that a mixture of ionic species is present. Spectroscopic 

 measurements have in fact proved a useful method for the determination of 

 many additional pK values, some of which lie outside the range of con- 

 venient measurement by conventional means. The work of Shugar and 



* D. O. Jordan, Progr. Biophys. and Biophys. Chem. 2, 51 (1951) ; Ann. Rev. Biochem. 

 21, 209 (1952). 



7 J. R. Marshall and J. Walker, J. Chem. Soc. 1951, 1004. 



8 M. P. V. Boarland and J. F. W. McOmie, J. Chem. Soc. 1952, 3716, 3722, 4942. 



9 D. J. Brown and L. N. Short, J. Chem. Soc. 1953, 331 . 



1" F. Halverson and R. C. Hirt, J. Chem. Phys. 19, 711 (1951). 



'1 F. F. Heyroth and J. R. Loofbourow, J. Am. Chem.. Soc. 56, 1728- (19.34). 



12 M. M. Stimson and M. A. Reuther, J. Am. Chem. Soc. 65, 153 (1943). 



" L. F. Cavalieri, A. Bendich, J. F. Tinker, and G. B. Brown, J. Am.. Chem. Soc. 70, 



3875 (1948). 

 " E. R. Holiday, Biochem. J. 24, 619 (1930). 



