Photochemical cross sections in cytidylyl-(3′-5′)-cytidine

Ultraviolet irradiation of cytidylyl-(3′-5′)-cytidine (CpC) results in the production of only one dimer, [Formula: see text], and for ultraviolet exposures in the range of biological interest, only the single hydrate (CpC)*. A second dimer of CpC, [Formula: see text], arises from [Formula: see text] through a dark reaction. Both these dimers deaminate in the dark to dimers of uridylyl-(3′-5′)-cytidine (UpC) and then to [Formula: see text]. The cross sections and quantum yields for the production of [Formula: see text] and (CpC)* have been determined at a number of wavelengths. In the wavelength range 240–289 mμ, the cross sections for dimer production are at least two times that for hydrate production. Approximate molar extinction coefficients have been determined for a mixture of [Formula: see text] and [Formula: see text], yielding values at ail wavelengths which are at least four times larger than for thymine dimers. The cross sections for dimer reversal of [Formula: see text] and [Formula: see text] are about the same and are larger than those of thymine dimers by at least a factor of 6.0. The quantum yield for reversal of CpC dimers is about 1.0 and independent of wavelength. These photochemical findings prove that in CpC, dimers are a much more important class of photoproduct than hydrates at low ultraviolet exposures. A similar result could be expected in polymers containing cytosine such as occur in DNA and RNA.