30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 



87 



220 



240 



260 



280 



Fig. 



16. Irradiation and subsequent partial reversibility of photolysis of RNA 



at neutral pH: (a) prior to irradiation, (b) after 65 minutes irradiation, (c) irradiated 

 solution after 5 minutes at 92° [from D. Shugar and K. L. Wierzchowski, Intern. 

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

 269 (1958)]. 



a. Natural Nucleic Acids 



Initial experiments, 39 ' 149 which were confined to RNA, were subsequently 

 extended to apurinic acid (APA) since this provides a chain containing 

 only two components, thymidylic and deoxycytidylic acids, for which only 

 the latter is subject to reversibility. A typical experiment is illustrated in 

 Fig. 16 for RNA, from which it will be observed that somewhat more than 

 50 % of the drop in absorption due to irradiation may subsequently be re- 

 stored by heating. The results for APA are analogous; but, assuming that 

 the drop in absorption is due mainly to the deoxycytidylic acid component, 

 the quantum yield is found to be about 3 times that to be expected. It will 

 be observed from Fig. 16 that the "regenerated" spectrum exhibits a small 

 permanent increase in absorption at all wavelengths to the violet of 230 

 fci/jl; this phenomenon is fairly general and suggests that some permanent 

 damage has resulted which may explain the failure to observe 100% re- 

 versibility. 



Deaminated RNA and APA, which at first failed to reveal any signs of 

 reversibility, have since been found to exhibit reversal under appropriate 

 conditions. 



b. Synthetic Oligo- and Polynucleotides 



An extensive series of experiments, along the lines outlined in the pre- 

 ceding section, has now been completed with homopolymers of uridylic 



