74 



D. SHUGAR 



220 



240 260 



260 



220 



280 



240 260 



Fig. 11. Fig. 12. 



Fig. 11. Absorption spectrum of cytosine at neutral pH, unbuffered: (a) before 

 irradiation, (6) after 85 minutes irradiation, (r) irradiated solution heated 10 minutes 

 at 80° [from D. Shugar and K. L. Wierzchowski, Biochim. et Biophys. Acta 23, 657 

 (1957)]. 



Fig. 12. Reversible photolysis of cytidine or cytidylic acid at neutral pH: (a) 

 before irradiation, (6) after irradiation, (c) irradiated solution after 16 hours at room 

 temperature or 5 minutes at 80° [from R. L. Sinsheimer, Radiation Research 6, 121 

 (1957), and K. L. Wierzchowski and D. Shugar, Biochim. et Biophys. Acta 25, 355 

 (1957)]. 



In acid medium the effects of irradiation are not reversible, while in alkaline 

 medium a new unknown pyrimidine derivative is formed which is relatively 

 unstable. 



For nucleosides (and nucleotides) the disappearance of the maximum at 

 about 270 m^t is accompanied by the simultaneous appearance of a new 

 maximum at about 236 m/x. 116 ' 140 ' 141, 148 If irradiation is not unduly pro- 

 longed following completion of the reaction, acidification or heating results 

 in 100% reversion to the original compound (Fig. 12). 



Isolation of the photoproducts of cytosine nucleosides and nucleotides 

 is more difficult than for the corresponding uracil analogs because of the 

 lower stability of the former, some of which revert spontaneously to the 

 parent compound at room temperature. 



The reverse reaction is first-order for Cp 148 as well as for all cytidylic acid 

 isomers, 149 and is more rapid than for uridine nucleotides. 39 ' n6 At neutral 

 pH and 30°C the half-times for the reverse reaction for the 2' and 3' isomers 

 of Cp are, respectively, 22 and 96 minutes; at pH 4 these values are de- 

 creased to about one-eighth and at pH 13 they are too short to measure. 148 



By analogy with the uracil analogs the mechanism may be assumed to 

 involve uptake of a water molecule at the 4,5 double bond. If such were 

 the case the quantum yield for photodecomposition should be lower in 



