36 



pentose molecule to give a nucleoside. They may add phosphoric acid residues, 

 thus increasing the complexity of the molecule. Finally, they may combine with 

 each other to produce the nucleic acids. We have found that as the complexity 

 of the molecule increases, the sensitivity to the action of X-rays decreases. For 

 example, irradiation of cytosine with 50,000 r produced a large decrease in the 

 absorption spectrum. Addition of pentose to cytosine to form cytidine decreased 

 considerably the action of X-rays (Figure 1). 



ALLEN: Could you tell us what these molecules are'' 



BARRON: Cytosine is 2 hydroxy-6-aminopyrimidine 



Introduction of a pentose residue to the N in position 3 gives cytidine, 3 p-ribo- 

 furanosido -cytosine 



NHz 



N CH 



OC CH O ^^Z^^ 



^N-^1 C^ ^C 



' ^C — C^ ' 



H ' ' H 



OH OH 



Addition of ribose protects the cytosine molecule against the effects of X irradia- 

 tion. 



ALLEN: What is the ion pair yield for this loss of ultraviolet absorp- 

 tion? 



BARRON: I will come to it later. 



ALLEN: May I ask what group is responsible for the ultraviolet ab- 

 sorption? 



COHN: It is the pyrimidine ring and primarily the double bonds in this 

 ring. There are 3 double bonds in the cytosine ring. 



ALLEN: Are the double bonds reduced? 



! ^ARRON: AccQrdipg t9 Cavalieri (9), the absorption is due mainly to 

 the -C = C - <i = N or -C = C - C = O chromophore of the ring. Reduction of the 

 double bonds is rather difficult, although it may be produced by hydrogen in the 

 presence of colloidal palladium. 



COHN: Destruction of the double bonds does not necessarily mean re- 

 duction. It could be by oxidation. 



