500 



G. H. BEAVEN, E. R. HOLIDAY, AND E. A. JOHNSON 



in6NHCI 



inO.lA'HCI 



in pH 7 buffer 



inNaOH, pH 10.9 



inA'NaOH 



260 

 Wavelengtn, m/j 



320 



Fig. 5. Guanine (Johnson, unpublished). 



Ploeser and Loring,^^ and Andrisano and Modena^^ discuss the spectra of 

 numerous pyrimidines including uracil, at various pH values, without 

 mention of a pK. Wyatt'*^ and Cohn^^ first gave data for 5-methylcytosine, 

 and Wyatt and Cohen^^ have reported 5-hydroxymethylcytosine isolated 

 from bacteriophage nucleic acids, with maxima at 279 m/x, e = 9700, in 

 0.1 N hydrochloric acid, at 269.5 mn in pH 7.4 buffer, and at 283.5 m^ in 

 0.1 A^ sodium hydroxide. For all except this last compound very complete 

 data have been given by Shu gar and Fox,^^ and the curves published here 

 (Figs. 7 to 10) have been re-drawn from their paper. It has been pointed 

 out by Cohn (private communication) that uracil has a pK in the region of 

 pH 0.5; the spectrum in 6 A" HCl was therefore determined in this labora- 

 tory and added to those of the other ionic species. 



Good general agreement was found between the data for the pyrimidines 

 by Shugar and Fox, those given here for the purines, and an extensive series 



" J. M. Ploeser and H. S. Loring, J. Biol. Chem. 178, 431 (1949). 



29 R. Andrisano and G. Modena, Gazz. chim. ital. 81, 405 (1951). 



30 G. R. Wyatt, Biocheyn. J. 48, 581 (1951). 



31 W. E. Cohn, /. Am. Chem. Soc. 73, 1539 (1951). 



32 G. R. Wyatt and S. S. Cohen, Nature 170, 1072 (1952). 



