CHEMISTRY OF PURINES AND PYRIMIDINES 



115 



Fig. 1. Ultraviolet absorption spectra of 5-»i€thylcytosine at pH values (1.0 to 14) 

 indicated. The curve for pH 2, not shown, is identical with that for pH 1.0; those (not 

 shown) for pH 7 to 10 are identical. Isoshestic point a is that for pA'ai (= 4.6); b, c, 

 and dare for pA'a2 (= 12.4). (Adapted from D. Shugar and J. J. Fox, Biochim. ctBtophys. 

 Acta 9, 199 (1952).) 



form. Such a derivative would appear to exhibit the phenomenon of "meso- 

 meric tautomerism" which "results in a manifestation of dual character by 

 a compound essentially homogeneous."^-^ The existence of individual tauto- 

 mers (hence a slow tautomeric change) has been inferred^''* from titrimetric 

 data for the related 6-hydroxypteridine and xanthopterin (2-amino-4,6- 

 dihydroxpteridine), but thus far not for pyrimidines and purines. 



The principles mentioned above are illustrated for 5-methylcytosine, the 

 spectral changes of which (cone. ca. 10~'* molar) are shown in Figure P^' 

 over the pH range 1 to 14. The curve for pH 1.0 is the same as that for 

 pH 2.0 and shows e = 9,790 at wavelength 283.5 m/x. As the pH is increased, 

 this absorption decreases (hypochromic effect) and the position of this 

 maximum shifts towards shorter wavelengths (hypsochromic effect). The 

 values remain constant from pH 7 through 10 (e = 6,230 at 273.5 mju) and 

 hyperchromic and bathochromic effects are observed with further increase 

 in pH until 14 is reached. (The basicity of ring nitrogens, which would be 



3" L. Hunter, /. Chem. Soc. 1945, 806. 



