30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 



77 



TABLE III 



Juantum Yields (Moles/einstein X 10 3 ) for Photolysis of Cytosine 



Derivatives, Concentration ~10 -4 M, at 



253.7 ni/i and at Various pH Values' 1 



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

 Wierschowski, Intern. Symposium Macromolecules, Prague, 1957, Paper No. 123 (1957); J. Polymer Sci. 31. 269 

 (1958). Values are "effective" quantum yields uncorrected for reverse reaction which is rapid at pH 11-12. 



b Unbuffered solution; for influence of buffer see a, first ref. 



c Reaction reversible in the dark; for cytosine and 1-methylcytosine only partial reversibility 



d Quantum yield increases appreciably during course of reaction. 



e R. L. Sinsheimer [Radiation Research 6, 121 (1957)] reports a quantum yield of 17 X 10~ 3 for cytidine-2'- 

 phosphate and cytidine-3'-phosphate. 



1 A. M. Moore and C. H. Thomson {Can. J. Chem. 35. 163 (1957)] report a quantum yield of 7 X 10 _ » at a 

 concentration of 10"" 2 M . 



heating. 39 Work in progress in our laboratory shows that the number of 

 pyrimidine derivatives giving rise to stable photoproducts containing the 

 pyrimidine ring is fairly extensive and should be of interest not only to the 

 physical chemist and biologist, but also to the synthetic organic chemist. 



Of special interest are those derivatives substituted in positions 4 or 5 

 such as thymine, 5-methylcytosine, etc. A number of observers have as- 

 sumed that such compounds do not give rise to reversible photoproducts 

 although Moore and Thomson 142 reported "slight" reversibility for 4-meth- 

 yluracil. This question is of paramount importance in relation to the photo- 

 chemistry of nucleic acids and, particularly, photoreactivation (Section X). 



From an investigation of a wide variety of substituted pyrimidines it 

 was concluded that the ability to take up a water molecule reversibly at 

 the 4,5 double bond is dependent on the electron distribution in the pyrimi- 

 dine ring and is not necessarily attributable to steric hindrance resulting 

 from substitution of one of the positions at 4 or 5. 39 Investigations in prog- 

 ress on this and related problems have now shown that, although no re- 



