30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 73 



TABLE II 



Quantum Yields for Photolysis at 253.7 mp of Some Nucleic Acid 



Constituents and Related Compounds at 



Neutral pH and ~10 -4 M 



° R. L. Sinsheimer Radiation Research 1, 505 (1954). 



6 A. M. Moore and C. H. Thomson, 4th Intern. Conf. Radiobiol., Cambridge, 1955, p. 75 (1956). 



c K. L. Wierzchowski and D. Shugar, Acta Biochim. Polon. 6, 313 (1959) ; Ann. Congr. Chem. Ges. D.D.R., 

 Leipzig, 1956 p. 85 (1957); Biochim. et Biophys. Acta 25, 355 (1957). 



d D. Shugar and K. L. Wierzchowski, Postepy Biochem. 4. Suppl., p. 243 (1958). 



e L. P. Zill, Federation Proc. 16, 276 (1957). 



' See also S. Y. Wang, J. Am. Chem. Soc. 80, 6199 (1958). 



" For 10" 5 M concentration; see K. L. Wierzchowski and D. Shugar, Acta Biochim. Polon. 6, 313 (1959), 

 for influence of concentration. 



h Moore and Thomson 6 report </> = 19 X 10"» at 10"» M and 37 X 10"» at 10" 2 M for uridine-2'(3')-phosphate; 

 see K. L. Wierzchowski and D. Shugar, Acta Biochim. Polon 6, 313 (1959), for more extensive data. 



' See A. Rorsch, R. Beukers, J. Ijlstra, and W. Berends, Rec. trav. chim. 77. 423 (1958). 



b. Reversible Photolysis of Cytosine Analogs 



Irradiation at 253.7 rrux of a variety of cytosine derivatives, in particular 

 nucleosides and nucleotides 39 ' 116 ' 120 ' J 4o-i42, 148, 149 as we u as som e other 6- 

 amino derivatives, 39142 also results in the formation of photoproducts 

 which revert to the original products to a greater or lesser degree in the 

 "dark" either by heating or acidification. 



For cytosine and 3-methylcytosine in aqueous unbuffered solution, pho- 

 todecomposition proceeds much more slowly than for uracil but with a 

 similar decrease in absorption; following irradiation, heating restores to a 

 considerable extent the original spectrum 116 (Fig. 11). If, however, irradia- 

 tion is more prolonged, the degree of reversibility is considerably reduced. 141 



148 R. L. Sinsheimer, Radiation Research 6, 121 (1957). 



149 D. Shugar and K. L. Wierzchowski, Intern. Symposium Macromolecules, Prague, 

 1957, Paper No. 123; J. Polymer Sci. 31, 269 (1958). 



