30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 



to 



heavy water because of the higher activation energy required, while the 

 reverse reaction should be more rapid since D 2 is less basic than H 2 0, 

 as a result of which the photoproduct will more effectively compete for the 

 deuteron in D 2 than for the proton in H>0; hence the concentration of 

 the conjugate acid, and the rate of the elimination reaction, will be 

 greater. 150 



For 3-methyluracil, cytosine, and cytidine the quantum yields in D 2 

 were indeed found to be half those in H 2 and the reverse reactions 2-3 

 times faster in D 2 in support of the above argument 116 ; so that the reaction 

 mechanism may be represented as follows: 



8 CH 



4 CH 



hv 



< 



. (-) 

 CH 



(+) 

 CH 



\ (-) 

 CH 



H2O 



H 

 /OH 



\H 



H 3 



(+) 

 CH(H 2 0) 



a 



^H 



H 

 .OH 



^H 



.H 



-HsO 



CH 



CH 



An essentially similar mechanism has been proposed for the reverse reaction 

 by Moore and Thomson. 143 Wang 145 suggests that the primary process may 

 involve an ionic intermediate 



O 



hy 



o- 



It would undoubtedly be profitable to examine a number of uracil and 

 cytosine derivatives by flash photolysis (Section II, 3). Finally, it should 

 be noted that there is no direct chemical evidence to indicate, as in the case 

 of uracil derivatives, whether the water hydroxyl is taken up at position 

 4 or 5. 



If we compare the absorption spectrum of the photoproduct of cytidine 

 (Fig. 12) with that of dihydrodeoxy cytidine (Fig. 6), it will be seen that 

 good correspondence prevails between the two. A similar comparison of 

 the photoproduct of cytosine (Fig. 11) with that of dihydrocytosine (which 

 is similar to that of dihydrodeoxycytidine, Fig. 6) shows that the spectrum 

 of the former cannot be explained on the assumption of simple uptake of a 

 water molecule, and this question remains an open one, which is rendered 



150 K. B. Wiberg, ('hem. Revs. 55, 713 (1955). 



