OPTICAL PROPERTIES OF NUCLEIC ACIDS 



497 



Fig. 2. Relation between absorbance ratios and pH for the isomeric cytidylic 

 acids (Cohn, unpublished). 



5'- and deoxy- 



3'- 



2'- 



where cha and €a are the absorptivities of the two ionic species, and « is 

 the absorptivity determined at the given pH. The thermodynamic vakie 

 of pKa may be obtained by appropriate correction for the ionic strengths. 

 The tables given here also include absorbance ratios based largely on 

 the very comprehensive work of Cohn^^ (with subsequent additions and 

 revisions). These have considerable utility for the identification of com- 

 pounds in solutions of unknown concentration, but their precise values 

 will be critically dependent on the wavelength calibration of the spectro- 

 photometer, and variations must be expected when different instruments 

 are used. The relationships of these ratios to pH values and to the absorp- 

 tion spectra are illustrated for the isomeric cytidylic acids by Figs. 1 and 2 

 (Cohn, unpublished). It may be noted that the phosphate ionizations ap- 

 pear to be without significant effect on the spectra, but this is not neces- 

 sarily always true (see Table III). 



'« E. Volkin and W. E. Cohn, Methods of Biochemical Analysis, Vol. 1, p. 287, Inter- 

 science, New York, 1954. 



