30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 79 



chemist, is the behavior of a number of derivatives in nonaqueous solvents, leading 

 to the formation of new, occasionally analogous, photoproducts. 39 ' 142 



Beukers et al. lbS have investigated the reversible photolysis of some uracil and 

 cytosine derivatives in frozen solutions and found marked differences in sensitivity 

 for some compounds as compared to that in aqueous solution. Their conclusion that 

 the behavior in frozen solution more adequately represents that to be expected for 

 the same compounds in nucleotide chains is open to question, although in no way de- 

 tracting from the interest of their findings which are undoubtedly worth following up. 



VI. Photochemistry of Nucleic Acids 



1. Physicochemical Studies 



Ultraviolet irradiation of DNA is accompanied by a decrease in vis- 

 cosity, 156 which drops almost to zero 121 before changes in absorption spec- 

 trum take place involving a decrease in the height of the principal absorp- 

 tion band at 260 rruz. The decrease in viscosity proceeds with a quantum 

 yield of the order of 10 -6 while the initial decrease in absorption is due 

 largely to the destruction of pyrimidine rings since irradiation to the point 

 where 30 % of the pyrimidines are destroyed leaves the purine rings prac- 

 tically intact. 121 No attempt appears to have been made to determine 

 whether one of the pyrimidine components is more susceptible, although 

 deoxycytidylic acid would be expected to disappear more rapidly than 

 thymidylic acid because of the large difference in quantum yields for 

 photolysis of the free nucleotides (Section V, 4). 



Under the influence of high doses of radiation a small percentage of 

 dialyzable products appears, 121 while under more extreme conditions rup- 

 ture of internucleotide linkages may take place with the appearance of 

 appreciable quantities of inorganic phosphate. 157 No attempts appear to 

 have been made to isolate some of the products of degradation under these 

 conditions. 



Analogous high doses of radiation have been used to determine the ex- 

 tent to which DNA is susceptible to degradation under conditions resem- 

 bling those in vivo, by irradiating rat thymocytes and then extracting the 

 DNA. 158 The yield of polymerized DNA from irradiated thymocytes was 

 only one-half that from a nonirradiated control homogenate, additional 

 evidence for fragmentation being based on qualitative determinations of 

 large amounts of thymine in the supernatant fluids from the ultraviolet 

 treated thymocytes during the extraction procedure. It follows from this 



156 R. Beukers, J. Ijlstra, and W. Berends, Rec. trav. chim. 77, 729 (1958). 



166 A. Hollaender, J. P. Greenstein, and W. V. Jenrette, J . Natl. Cancer Inst. 2, 23 

 (1941). 



157 M. W. Seraydarian, A. Canzanelli, and D. Rapport, Am. J. Physiol. 172, 42 (1953). 

 168 S. J. deCourcy, J. O. Ely, and M. H. Ross, Nature 172, 119 (1953). 



