Reactivity of 4-thiothymidine with Fenton reagent investigated by UV-visible spectroscopy and electrospray ionization mass spectrometry

The reactivity of the sulfur-containing nucleoside 4-thio-(2 '-deoxy)-thymidine usually abbreviated as 4-thio-thymidine, (S-4-TdR) under Fenton conditions, ie, in the presence of H2O2 and catalytic amounts of Fe(II), was investigated by UV-vis spectroscopy and electrospray ionization single and tandem mass spectrometry (ESI-MS and MS/MS). S-4-TdR hydroxylated on the S atom was found to be a key reaction intermediate, ultimately leading to (2 '-deoxy)-thymidine usually abbreviated as thymidine, (TdR) as the main reaction product. This finding was in accordance with the outcome of the reaction between S-4-TdR and H2O2, previously investigated in our laboratory. On the other hand, the additional presence of center dot OH radicals, induced by the Fe(II)/H2O2 combination, led to the increased generation of another interesting S-4-TdR product, already observed after its reaction with H2O2 alone, ie, the covalent dimer including a S-S bridge between two S-4-TdR molecules. More importantly, multihydroxylated derivatives of S-4-TdR and TdR were detected as peculiar products obtained under Fenton conditions. Among them, a product bearing an OH group both on the methyl group linked to the thymine ring and on the C5 atom of the ring was found to prevail. The results obtained during this study, integrated by those found previously in our laboratory, indicate 4-thiothymidine as a promising molecular probe for the recognition, through a careful characterization of its reaction products, of the prevailing species among reactive oxygen species (ROS) corresponding to singlet-state oxygen, hydrogen peroxide, and hydroxylic radical.