86 D. SHUGAR 



II, Chapter 28) provides a small oligonucleotide suitable for photochemical 

 studies, although less useful from a general point of view. It has been found 

 that the RNA core is inactivated very slowly, with a quantum yield cer- 

 tainly not greater than 10~ 4 and probably lower than this. Following a 50% 

 loss in activity there is only a 10% decrease in metachromasia against 

 basic dyes, indicating relatively little chain degradation. Since we are deal- 

 ing here with a polypurine oligonucleotide containing a terminal pyrimidine 

 nucleotide residue which is very susceptible to irradiation, it may be con- 

 cluded from the above that the presence of this terminal pyrimidine nucleo- 

 tide is unnecessary for biological activity, a conclusion arrived at in- 

 dependently by direct removal of the terminal pyrimidine nucleotide. 180 

 The above result confirms biologically the physicochemical evidence for 

 the high radiation resistance of purine nucleotide residues in polynucleo- 

 tide chains. 



VII. Reversibility of Nucleic Acid Photolysis 



In view of the fact that the photolysis of uracil and cytosine nucleotides 

 is almost completely reversible in the dark, one is logically led to an ex- 

 amination of the likelihood of such reversal by these constituents when 

 they are incorporated into nucleotide chains. It should be recalled that 

 the quantum yields for these components are from 10-100 times greater 

 than for purine nucleotides, so that one would expect them to be first af- 

 fected during biological inactivation; and supporting evidence for this is 

 forthcoming from the observation that even extensive degradation of DNA 

 leaves the purine residues almost intact. Moreover, proposed schemes of 

 nucleic acid structure do not involve either positions 4 or 5 of the pyrimidine 

 rings in secondary bonds, so that the double bond involving these linkages 

 is, theoretically, still capable of reversible uptake of a water molecule, 

 albeit to a modified extent as a result of involvement of other positions of 

 the pyrimidine ring in secondary bonds as well as mutual interaction be- 

 tween adjacent rings. We shall now summarize the evidence, based on 

 physicochemical and biological properties of nucleic acids, which demon- 

 strates that at least direct partial reversal of the effects of photolysis may 

 indeed be achieved. 



1. Physicochemical Studies 



Physicochemical investigations of reversal of photochemical effects in 

 nucleic acids have hitherto been confined to observations of spectral 

 changes. Such data could be objected to on the grounds that hyperchrom- 

 icity is difficult to account for, and for this reason no experiments have 

 yet been undertaken with DNA where the uncertainty from this source 

 might be considerable. 



180 K. Tanaka, J. Biochem. (Japan) 46, 109 (1958). 



