Dimer Photoproducts in Cytidylyl‐(3′‐5′) Cytidine

When CpC is exposed to ultraviolet (u.v.) radiation, dimers are produced both photochemically and by dark reactions. A reaction scheme for the production of these dimers is presented. We have shown that a single dimer, Cp̂C 1 , is initially produced photochemically. This is converted by a spontaneous reaction to a second dimer, Cp̂C 2 . The dimer Cp̂C 2 may also spontaneously revert back to Cp̂C 1 and if no side reactions were involved a thermal equilibrium between these txo dimers would be established with about two to three times the concentration of Cp̂C 2 , as of Cp̂C 1 . The rate constants for the interconversion of these two dimers vary from about 0.05 to 0.4 hr ‐1 over the pH range 2.7 to 8.4. The dimers of CpC have maximum stability near pH 7. The two dimers Cp̂C 1 and Cp̂C 2 can also be reversed to CpC by u.v. irradiation. Both these dimers in turn deaminate to dimers of UpC and then to Up̂U. The rate constants for these deamination reactions are presented. These lie in the range 0.01 to 0.5 hr ‐1 for the pH values studied. Possible structures for the two dimers are suggested.