30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 63 



exhibiting characteristic absorption in the range 240-290 mju. 107, 109 How- 

 ever, since analogous changes may be observed in solutions of monosac- 

 charides and polysaccharides under the influence of alkali alone, 110 ' m the 

 enhanced effect resulting from irradiation at this pH is most likely due to 

 excited hydroxyl ions. It is of some interest that analogous products arc 

 produced by X-rays 112, U3 and a-irradiation 114 at both neutral and alkaline 

 pH (for detailed discussion and literature on this subject see Phillips et 

 a/. 115 ). 



Purine and pyrimidine nucleoside-bound carbohydrates, however, do not 

 exhibit instability in alkali and their resistance to ultraviolet even under 

 these conditions is testified to by the fact that irradiation of a 10 -4 M solu- 

 tion of adenosine in O.Of M NaOH with a resonance lamp, filtered to remove 

 radiation below 240 mp., at an intensity of f0 17 quanta/cm . 2 /min. for 1 hour 

 is without effect on the orcinol reaction for ribose. 116 



One may therefore conclude that, under the pH conditions normally 

 used in photobiological studies and at wavelengths at least above 220 m/x, 

 the carbohydrate components of nucleic acids are not directly affected by 

 radiation; and that, in those instances where destruction of sugar residues 

 does occur this is a consequence of secondary reactions, such as energy 

 transfer, following absorption of light by the aromatic rings. 



2. Purine Derivatives 



With one exception the photochemistry of purines has been investigated 

 largely from a qualitative viewpoint, thus frequently rendering difficult a 

 comparison of results from different laboratories. The commonly applied 

 criterion has been the loss of characteristic absorption between 220 and 

 300 iruz; e.g., overnight irradiation with a germicidal lamp of 0.5 X 10" 4 M 

 solutions of various purines suffices in all cases completely to disrupt the 

 absorption spectrum. 117 



It has been reported by Christensen and Giese 118 that prolonged irradia- 

 tion of adenine (but not guanine or guanylic acid) leads to an increase in 

 absorption over the range 230-290 mp. An analogous increase (about 7%) 



09 T. C. Laurent, J. Am. Chem. Soc. 78, 1875 (1956). 



10 F. Petuely and N. Meixner, Chem. Ber. 86, 1255 (1953). 



11 W. G. Berl and C. E. Feazel, J. Agr. Food Chem. 2, 37 (1954). 



12 P. Holtz and J. P. Becker, Arch, exptl. Pathol, u. Pharmakol. 182, 160 (1936). 



13 C. T. Bothner-By and E. A. Balazs, Radiation Research 6, 302 (1957). 



14 M. A. Khenokh, Doklady Akad. Nauk S.S.S.R. 104, 746 (1955). 



15 G. O. Phillips, G. J. Moody, and G. L. Mattock, J. Chem. Soc. pp. 3522, 3534 

 (1958); G. O. Phillips and G. J. Moody, ibid. pp. 754, 762 (1960). 



16 D. Shugar and K. L. Wierzchowski, Biochim. et Biophys. Acta 23, 657 (1957). 



17 D. Rapport and A. Canzanelli, Science 112, 469 (1950). 



18 E. Christensen and A. C. Giese, Arch. Biochem. Biophys. 51, 208 (1954). 



