Photodegradation of the cytosine nucleosides family by water-soluble iron(III) porphyrins.

Aqueous solutions of cytosine nucleosides such as cytidine (C),2'-deoxycytidine (dC),2',3'-dideoxycytidine (ddC), and 1-beta-D-arabinofuranosylcytosine (ara-C) were irradiated with visible light in the presence of a water-soluble iron(III) porphyrin photocatalyst (FeIIITMPyP or FeIIITSPP) in air or under an argon atmosphere. HPLC analyses revealed release of cytosine as a dominant reaction. In the case of the irradiations of isomeric C and ara-C, their interconversion was also observed as a minor reaction. Under an argon atmosphere, a stoichiometric reduction of the catalyst into FeIITMPyP or FeIITSPP was observable by UV-vis spectroscopy. Quantum yields for the cytosine release and the catalyst reduction (phi cyt and phi FeII) were very much dependent both on the presence and the stereochemistry of the 2'-OH group. For the FeIIITMPyP case, for example, (a) phi cyt and phi FeII decreased in the order C > ara-C > dC > ddC and (b) oxygen lowered the phi cyt of dC and ddC but not phi cyt of C and ara-C. These results strongly support the previously proposed mechanism, i.e., C and ara-C undergo the reaction via a C2' carbon radical (e.g., eq 4) and dC and ddC undergo the reaction via a C1' or C4' carbon radical (e.g., eq 3). The observed effects of 2-propanol and solution pH suggest that a main reactive species photogenerated from FeTSPP is a free hydroxyl radical (*OH), while that photogenerated from FeTMPyP is probably an iron-coordinated active oxygen species (crypto-OH or Fe = O).