64 D. SHUGAR 



in absorption at wavelengths above 250 m/z during irradiation of adenine 

 is shown in Table I of a paper by Canzanelli et a/., 36 but no significance was 

 apparently attached to this. It is claimed by Garay and Guba 119 that irra- 

 diated ATP exhibits an enhanced absorption and that, with time, the spec- 

 trum reverts to that of ATP; but no data are presented to substantiate this. 



One fact of some importance in relation to the photochemistry of poly- 

 nucleotides, and upon which all observers are in agreement, is that purine 

 derivatives (particularly of adenine and guanine) are considerably more 

 resistant to irradiation than pyrimidines. 108 ' U8 ' 12 ° Irradiation of a DNA 

 solution to the point where 30 % of the pyrimidine bases are destroyed ap- 

 parently leaves the guanine residues intact while more than 90% of the 

 adenine can be recovered from a hydrolyzate of the photoproduct. 121 



For purposes of comparison with pyrimidine derivatives in studies on 

 the photochemistry of nucleic acids, rough measurements have been made 

 of the quantum yields for adenine and guanine at 253.7 mju, based on the 

 loss in absorption of the characteristic maxima. The values obtained were 

 0.6 X 10 4 for adenine and 2 X 10 -4 for guanine. 39 



The conclusion of Loofbourow and Stimson, 122 that the lability of purines 

 and pyrimidines to irradiation parallels the number of carbonyl groups in 

 the ring, is perhaps too broad a generalization although it does appear to 

 apply roughly to purines since the stability of these is (in decreasing order) 

 adenine, hypoxanthine, guanine, xanthine, and uric acid. The presence of a 

 carbonyl group in position 2 is probably of greater significance. 36 



The only attempt to examine the kinetics of photodecomposition of 

 purines is that of Kland and Johnson. 123 Irradiation was under nitrogen or 

 oxygen, using a bank of germicidal lamps, and loss of absorption was the 

 criterion for destruction of the purine ring. With the exception of adenine 

 under nitrogen, there is an initial "induction" period; while at high irra- 

 diation intensities the rate is proportional to the square of the exposure 

 time, adenine excluded. The kinetic data made possible a division of the five 

 purines examined into two classes: those substituted in position 6, adenine 

 and hypoxanthine, which decompose more slowly under nitrogen; and di- 

 substituted purines (guanine and xanthine) and uric acid, which are less 

 sensitive under oxygen. 



The considerably greater sensitivity of adenine (Fig. 9) under aerobic 

 conditions was regarded as supporting evidence for oxidative breakdown 

 at the amino groups 



RNH 2 ^ > [RNH:OH] ^ > ROH 



119 K. Garay and F. Guba, Acta Physiol. Acad. Set. Hung. 5, 393 (1954). 



120 R. L. Sinsheimer and R. Hastings, Science 110, 525 (1949). 



121 M. Errera, Biochim. et Biophys. Acta 8, 30, 115 (1952). 



122 J. R. Loofbourow and M. M. Stimson, J. Chem. Soc. p. 844 (1940). 



123 M. J. Kland and L. A. Johnson, J. Am. Chem. Soc. 79, 6187 (1957). 



