The Photochemistry of Thymidylyl‐(3′‐5′)‐5‐methyl‐2′‐deoxycytidine in Aqueous Solution¶

Abstract

ABSTRACTThe photochemistry of the dinucleoside monophosphate thymidylyl‐(3′‐5′)‐5‐methyl‐2′‐deoxycytidine (Tpm5dC) has been studied in aqueous solution using both 254 nm and UV‐B radiation. A variety of dinucleotide photoproducts containing 5‐methyl‐cytosine (m5C) have been isolated and characterized. These include two cyclobutane dimers (CBD) (the cis‐syn [c,s] and transsyn forms), a (6–4) adduct and its realted Dewar isomer, and two isomers of a product in which the m5C moiety was converted into an acrylamidine. Small amounts of thymidylyl‐(3′‐5′)‐thymidine (TpT) were also formed, presumably as a secondary photoreaction product. In addition, a photoproduct was characterized in which the m5C moiety was lost, thus generating 3′‐thymidylic acid esterified with 2′‐deoxyribose at the 5‐hydroxyl on the sugar mioety. The c,s CBD of Tpm5dC readily undergoes deamination to form the corresponding CBD of TpT. The kinetics of this deamination process has been studied; the corresponding enthalpy and entropy of activation for the reaction have been evaluated at pH 7.4 as being, respectively, 73.4 kJ/mol and −103.5J/K mol. Deamination was not observed for the other characterized photoproducts of Tpm5dC.