30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 101 



ribouridylic acid (poly-U) exhibits only 5-6% hyperchromicity. The syn- 

 thetic small linear and cyclic thymine oligonucleotides of Tener et al. nh 

 would be ideal compounds for clarification of this problem. Aside from the 

 photochemical aspects, the source of the above difference in hyperchromic- 

 ity merits investigation from the point of view of nucleotide chain struc- 

 ture. 



The enormous increase in sensitivity (on a survival basis) of E. coli 

 cells containing 5-bromouracil, in place of thymine, in their DNA suggested 

 that such DNA is itself less stable to irradiation. 226 An interesting sequel 

 to this observation .is the finding that the sensitivity of phage 0X174, 

 which contains a single-stranded DNA molecule, is increased when it in- 

 corporates 5-bromouracil in place of thymine, while that of phage T2 

 is unaffected. Increased radiation sensitivity following bromouracil in- 

 corporation is suggested as a feasible test for the single-stranded character 

 of DNA in multiplying cells, and has been utilized to follow the fate of 

 DNA during its transfer from parental to progeny phage. 227 



It would obviously be of interest to examine the action spectrum for 

 brominated $X174. A preliminary investigation has shown that the action 

 spectrum for ordinary <£X174 resembles the spectrum of cytosine plus 

 thymine, and not that of ordinary DNA. 228 In view of this result, the action 

 spectrum of TMV (Fig. 18) merits re-examination. 



Procedures have now been worked out for studying the photochemistry 

 of model heterologous oligonucleotides which exhibit high hypochromicity 

 that must be corrected for in quantitative work 222 (see Section VII, 1,6). 

 The extensive data of Michelson 79 ' 229 on hyperchromicity in model oligo- 

 nucleotides is very useful in such investigations. The results substantially 

 clarify the role of base sequences in reversible photolysis in nucleotide 

 chains, suggest a different interpretation of the kinetics of photolysis of 

 UpUp, 69 and make available a new tool for investigations of hypochromicity 

 and structure in oligonucleotides. A related study 230 on absorption spectra 

 and structure of dihydropyrimidines (Section IV,3) and hydrogenated 

 uracil-containing oligonucleotides provided useful supplementary data in 

 the foregoing investigations. 



The relatively stable hydroperoxides formed by the X-irradiation of 

 pyrimidines (but not purines) and nucleic acids 231 ■ 232 and which, in the 



225 G. M. Tener, H. G. Khorana, R. Markham, and E. H. Pol, /. Am. Chem. Soc. 80, 

 6223 (1958). 



226 S. Zamenhof, Ann. N. Y. Acad. Sci. 81, 784 (1959). 



227 A. W. Kozinski and W. Szybalski, Virology 9, 260 (1959). 



228 R. Setlow, Biochim. et Biophys. Acta 39, 180 (1960). 



229 A. M. Michelson, Acta Biochim. Polon. 6, 335 (1959); J. Chem. Soc, p. 3655 (1959). 



230 C. Janion and D. Shugar, Acta Biochim. Polon., 7, No. 2-3, in press (1960). 



231 G. Scholes, J. Weiss, and C. M. Wheeler, Nature 178, 157 (1956); 185, 305 (1960). 



232 B. Ekert and R. Monier, Ann. insl. Past. 92, 556 (1957). 



