OPTICAL PROPERTIES OF NUCLEIC ACIDS 



501 



200 



220 



240 260 



Wavelength, m/i 



Fig. 6. Xanthine (Johnson, unpublished). 



320 



of absorbance ratios and values for e at 260 m/x determined by Cohn'^ 

 (with subsequent revisions and additions), whose complete spectra were 

 unfortunately not available. In view of the limitations to the accuracy of 

 absorbance ratios mentioned above, in Table I the values of Cohn are 

 quoted for the sake of consistency whenever they are available, supple- 

 mented from other sources when necessary. Some variations from values 

 obtained by inspection of the absorption curves will therefore be found. 

 Table I is intended to give data for individual ionic species, and the pK 

 values in the second column are placed between the two species to whose 

 intercon version they apply. Wherever possible the determinations for each 

 species were carried out at a pH removed at least 1 .5 and preferably 2 units 

 from the nearest pK, and in solutions of low ionic strength. 



The determination of absorption spectra of purines and pyrimidines at 

 low temperatures has been carried out on sublimed films at 90° K. by Brown 

 and Randall,^^ and on sublimed films and also solutions in mixed organic 

 solvents which form glasses at 77° K. and 23° K. by Sinsheimer ct al.^* 

 The vibrational fine-structure is enhanced, particularly in pyrimidines such 



" G. L. Brown and J. T. Randall, Nature 163, 209 (1949). 



" R. L. Sinsheimer, J. F. Scott, and J. R. Loofbourow, J. Biol. Chem. 187, 299, 313 

 (1950); J. Am. Chem. Soc. 74, 275 (1952). 



