Identification of the Two cis-syn [2 + 2] Cycloadducts Resulting from the Photoreaction of 3-carbethoxypsoralen with 2′-deoxycytidine and 2′-deoxyuridine

Summary

Near-ultraviolet photolysis of 2′-deoxycytidine (dCyd) and 3-carbethoxypsoralen (3-CPs) in the dry state was found to generate two main stable photoadducts which were separated by thin-layer and high-performance liquid chromatography. Fast atom bombardment and plasma desorption mass spectrometry analyses suggested that the bound molecule to 3-CPs is dCyd. These two compounds were found to produce the corresponding 2′-deoxyuridine (dUrd) derivatives through a deamination process when left in aqueous solutions with a lifetime close to 24h at 20°C. The chemical structure of the deaminated photoadducts was confirmed by photochemical synthesis using dUrd as the substrate. UV and fluorescent measurements indicated that the furan moiety of 3-CPs is involved in the photobinding reaction. The cyclobutane type structure of the modified dUrd derivatives was established on the basis of its photoreversibility and detailed ¹H NMR analysis. The cis–syn stereoconfiguration of the two photocycloadducts was inferred from coupling constant considerations and on the basis of the complete assignment of the cyclobutyl protons, requiring the synthesis of deuterated nucleosides at pyrimidine carbon C(6). Further confirmation of the diastereoisomeric relationship between the two cis–syn dUrd3-CPs was provided by circular dichroism measurements.