68 D. SHUGAR 



Seraydarian 132 subsequently showed that complete inactivation of DPN 

 and TPN coenzyme activities could be achieved using only wavelengths 

 to the red of 210 rrux. Actually the influence of wavelength on the photo- 

 chemical destruction of coenzyme activity had been investigated quantita- 

 tively many years earlier by Warburg and Christian, 133 who found what 

 they considered the "apparent" quantum yield to increase with decreasing 

 wavelength so that at 186 rap, it was almost 120 times that at 253.7 rap. 

 Assuming that 4> was independent of X, and that no indirect inactivation 

 resulted from irradiated solvent molecules, they concluded that only a 

 portion of the coenzyme molecule was affected by irradiation; an inference 

 that is surprisingly close to the truth (see above), notwithstanding the ob- 

 jections that could be made to the assumptions on which it is based. 



Irradiation of DPNH in the region of its absorption band at 340 rap is 

 without effect on its coenzyme activity. 134 



4. Pyrimidines and Pyrimidine Nucleosides and Nucleotides 



Up to about 5 years ago it was generally accepted that irradiation of 

 pyrimidine derivatives led to irreversible degradation of the aromatic ring; 

 as a result of this, some of the earlier studies are now only of historical 

 interest. The intense flurry of excitement aroused by Kelner's report of 

 photoreactivation in microorganisms in 1949 somehow obscured the dis- 

 covery in the same year by Sinsheimer and Hastings 120 that the photolysis 

 of uracil, uridine, and cytidylic acid could be reversed in the "dark" by 

 heating or acidification, the criterion for reversal being the reappearance of 

 the original absorption spectra. It was not until Sinsheimer 135 took up this 

 question again five years later that its significance began to be appreciated 

 in connection with the photochemistry of nucleic acids and its bearing on 

 the phenomenon of photoreactivation. Today the first question to resolve 

 in studying the photochemistry of a pyrimidine derivative is whether the 

 reaction leads to the reversible or irreversible formation of a new derivative, 

 or to irreversible degradation. 



The first quantitative study of the photolytic degradation of a pyrimidine is that 

 of Uber and Verbrugge 136 who found that the pyrimidine component of thiamine, 

 2-methyl-5-ethoxymethyl-6-aminopyrimidine, is destroyed by irradiation at 253.7 nrux 

 with a quantum yield of 1.84 X 10~ 2 , at pH 4.8. Actually the absorption spectra shown 

 by the authors indicate that the compound possesses a pK of about 6.2 so that the 

 amino group was fully protonated. The conclusion that one-third of the amino nitro- 

 gen is split off during the reaction to form ammonia is, however, not in accord with 



132 m. W. Seraydarian, Biochim. et Biophys. Acta 19, 168 (1956). 



"3 O. Warburg and W. Christian, Biochem. Z. 282, 221 (1935). 



134 D. Shugar, Experientia 7, 1 (1951); Biochim. et Biophys. Acta 6, 548 (1951). 



136 R. L. Sinsheimer, Radiation Research 1, 505 (1954). 



136 F. M. Uber and F. Verbrugge, J. Biol. Chem. 134, 273 (1940). 



