30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 



53 



220 



240 260 

 Afnyi) 



Fig. 5. 



280 



240 260 



A (mjd) 



Fig. 6. 



Fig. 5. Absorption spectra (a) of uracil at neutral pH; (6) of dihydrouracil at 

 neutral pH; (c) of dihydrouracil at pH 13, corrected for alkaline decomposition [curve 

 c from R. D. Batt, J. K. Martin, J. McT. Ploeser, and J. Murray, J\ Am. Chem. Soc. 

 76, 3663 (1954)]. 



Fig. 6. Absorption spectra at neutral pH of (a) deoxycytidine, (6) dihydrodeoxy- 

 cytidine; extinction values for the latter curve are only approximate, see text. Spec- 

 trum of dihydrocvtosine is similar to curve b [Wierzchowski and Shugar, unpub- 

 lished]. 



barbital and barbituric acid derivatives at acid pH. 61 The spectra of the 

 dihydropyrimidines and their glycosides are of interest in relationship to 

 the photochemistry of the parent compounds, but the optical properties 

 of these biologically important substances have received very little atten- 

 tion, although some of them are now readily available through hydrogena- 

 tion with a rhodium catalyst. 62 ' 64 This is due in part to the fact that the 

 absorption spectra are located largely in the somewhat less accessible region 

 below 230 m/x, although newer instruments (Section IV, 1) have now sur- 

 mounted this obstacle. 



Figure 5 shows the spectrum of uracil at neutral pH, as well as that of 

 dihydrouracil at neutral and alkaline pH; the latter curve has been extrapo- 

 lated to zero time to correct for the drop in extinction due to the alkaline 



61 J. J. Fox and D. Shugar, Bull. soc. chim. Beiges 61, 44 (1952). 



62 W. E. Cohn and D. G. Doherty, J. Am. Chem. Soc. 78, 2863 (1956). 

 m M. Green and S. S. Cohen, J. Biol. Chem. 225, 397 (1957). 



64 M. Green and S. S. Cohen, J. Biol. Chem. 228, 601 (1958). 



