FORMATION OF CYCLOBUTANE DIMERS AND (6-4)-PHOTOPRODUCTS UPON FAR-UV PHOTOLYSIS OF 5-METHYLCYTOSINE-CONTAINING DINUCLEOSIDE MONOPHOSPHATES

The far-UV photochemistry of 5-methylcytosine, a minor DNA base, was studied in three dinucleoside monophosphates, including m(5)dCpT, Tpm(5)dC, and m(5)dCpdC. The model compounds were exposed to 254-nm radiation, and the resulting photoproducts were isolated by reverse-phase HPLC and characterized as cyclobutane dimers, (6-4) adducts, and the related Dewar valence isomers by UV, mass, and H-1 NMR spectroscopies. The rate of formation of the different photoproducts was compared with those obtained by photolysis of TpT and the corresponding cytosine dinucleoside monophosphates, including dCpT, TpdC, and dCpdC. The formation of deaminated m(5)dC-containing photoproducts was observed in each of the far-UV irradiated solution of m(5)dCpT, Tpm(5)dC, and m(5)dCpdC. They were shown to be generated mainly through a photochemical process since methylation of the C5 atom of the cytosine ring appeared to dramatically decrease the deamination rate of the C5-C6 saturated photoproducts.