70 



D. SHUGAR 



240 



260 



280 



Fig. 10. Reversible photolysis of uridine-3'-phosphate at 253.7 m/x: (a) before 

 irradiation, (b) after 40 minutes irradiation, (c) irradiated solution acidified to pH 

 0.8 and left 42 hours at room temperature [from R. L. Sinsheimer, Radiation Research 

 1, 505 (1954)]. 



likewise first-order but more rapid for the 3' isomer. The behavior of uridine 

 was qualitatively similar to that of Up. Additional evidence for reversion 

 of the photoproduct to the original compound is furnished by the observa- 

 tion 139 that irradiated uridine could not satisfy the growth requirements of 

 a pyrimidine-requiring mutant of Neurospora crassa, but could after heating 

 at neutral pH, or acidification or slight alkalization at room temperature. 



Uracil was found to undergo photodecomposition with a quantum yield 

 of 0.052, with the formation of several products, only 50 % of which reverts 

 to uracil on acidification or heating. Rorsch et al. bl have provided additional 

 spectral evidence for the existence of several reactions on irradiation of 

 uracil, of which only one is reversible. 



A qualitatively similar behavior is exhibited by 3-methyluracil 140 ■ 141 and 

 1 ,3-dimethyluracil. 59 ' 142145 Moore and Thomson's 59 choice of dimethyl- 

 uracil for elucidation of the mechanism of the photochemical reaction was 

 a fortunate one in that its photoproduct exhibits 100% reversibility while 

 its physicochemical properties made it particularly suitable for the isola- 

 ion of the photoproduct by fractionation and chromatography. 



139 D. Rapport, A. Canzanelli, and R. Sossen, Federation Proc. 14, 386 (1955). 



140 K. Wierzchowski and D. Shugar, Ann. Congr. Chem. Ges. D.D.R., Leipzig, 1956, 

 p. 85 (1957). 



141 K. L. Wierzchowski and D. Shugar, Biochim. et Biophys. Acta 25, 355 (1957). 



142 A. M. Moore and C. H. Thomson, 4th Intern. Conf. Radiobiol., Cambridge, 1955 

 p. 75, (1956). 



143 A. M. Moore and C. H. Thomson, Can. J. Chem. 35, 163 (1957). 



144 S. Y. Wang, M. Apicella, and B. R. Stone, J. Am. Chem. Soc. 78, 4180 (1956). 



145 S. Y. Wang, J. Am. Chem. Soc. 80, 6196 (1958). 



