30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 



65 



5 



10 15 



Irradiation time (hrs.) 



20 



Fig. 9. Relative rates of photodeeomposition of 0.8 X 10~*M aqueous, unbuffered 



solutions of adenine (a) under oxygen, (6) under nitrogen; and of hypoxanthine (c) 

 under oxygen and (d) under nitrogen [M. J. Kland and L. A. Johnson, J. Am. Chem. 

 Soc. 79, 6187 (1957)]. 



and hypoxanthine was actually isolated chromatographically from the reac- 

 tion mixture. On the other hand hypoxanthine was also obtained from 

 adenine irradiated under nitrogen; furthermore the quantities of hypox- 

 anthine actually isolated were extremely small, although this could have 

 been due to the much greater radiation sensitivity of this compound. 



It should also be noted that all the above derivatives were irradiated in 

 unbuffered solutions, the initial pH values of which varied from 5.35 for 

 uric acid to 5.94 for adenine. Since these two compounds possess pK values 

 at 5.4 and 4.15, respectively, a mixture of ionic species with different ab- 

 sorption spectra was present in each case (see Volume I, Chapters 3, 13, 14). 

 Furthermore, the liberation of ammonia during decomposition 36 (see below) 

 undoubtedly led to important pH changes during the course of the reac- 

 tion. 



The decomposition products of irradiated purine derivatives include both 

 urea and ammonia and this question has been extensively investigated for 

 purines and pyrimidines, as well as their nucleosides and nucleotides, by 

 Canzanelli et a/. 36 who found that liberated urea is derived almost exclu- 

 sively from the pyrimidine ureide group, and is quantitatively greatest for 

 those compounds with a keto group in position 2, as might be expected. The 

 pyrimidine ureide group is also the source of most of the ammonia. 



A significant observation is that the greatest formation of urea is asso- 

 ciated with the least yield of ammonia, and conversely, from which it may 



