30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 69 



the reported changes in absorption spectrum; furthermore, ammonia frequently ap- 

 pears as a photolysis product of the pyrimidine ring (see below) as in the case of the 

 purines 36 (Section V, 2). 



From a comparison of the relative resistance to irradiation of 2-chloro-6-amino- 

 pyrimidine, as compared to uracil 137 and barbituric acid, 122 Stimson and Loofbourow 138 

 concluded that susceptibility to photolysis parallels the number of carbonyl groups 

 in the ring. However, as in the case of purines, it is rather the presence of a carbonyl 

 group in position 2 which is of greater importance. 36 Even this latter generalization 

 is limited in its application if we compare the quantum yield of 1.84 X 10 -2 for 2- 

 methvl-5-ethoxymethyl-6-aminopyrimidine, mentioned above, with that for some 

 uracil derivatives (see Table II). 



Urea and ammonia are among the products of extensive degradation of pyrimidine 

 derivatives at 253.7 nnx, both originating in the ureido group; e.g., cytosine does not 

 give a greater yield of ammonia than uracil. 36 These results, however, give no infor- 

 mation as to the degradation mechanism involved. 



Prolonged irradiation of uracil at 253.7 nuz, followed by concentration 

 of the irradiated solution led to the isolation of four unidentifiable products 



H 2 N— C=0 HN— C=0 



0=C 



H 2 N— C=0 HN— C=0 



(V) (VI) 



Oxamide Parabanic acid 



in relatively pure form, as well as crystalline oxamide (V) and parabanic 

 acid (VI). 125 It is clear that neither of these two latter products could result 

 from the rupture of a single bond in the uracil ring, while the existence of 

 four additional products (plus others not isolated) indicates the existence 

 of a number of reactions. We shall return to this question below. 



a. Reversible Photolysis of Uracil Analogs 



A turning point in our understanding of the photochemistry of pyrimidine 

 nucleotides was the publication of Sinsheimer's 135 investigation on uridylic 

 acid in 1954. Using as source a resonance lamp, from which wavelengths 

 below 210 m/j were filtered out, it was shown that irradiation of uridylic 

 acid results in the disappearance of the characteristic absorption spectrum; 

 at this point removal of the light source followed by acidification of the 

 solution results in the reappearance of the original spectrum with an effi- 

 ciency of 90-100%. In place of acidification, the same result could be 

 achieved by heating at neutral pH. The photolytic reaction was reported 

 to be first -order with a quantum yield of 0.0216 mole/einstein for both uri- 

 dine-2'-phosphate and uridine-3'-phosphate; while the reverse reaction was 



137 F. F. Heyroth and J. R. Loofbourow, J. Am. Chem. Soc. 53, 3441 (1931). 



138 M. M. Stimson and J. R. Loofbourow, J. Am. Chem. Soc. 63, 1827 (1941). 



