Potential of 4-thiothymidine as a molecular probe for H2O2 in systems related to PhotoDynamic therapy: A structuristic and mechanistic insight by UV-visible and FTIR-ATR spectroscopies and by ElectroSpray ionization mass spectrometry

An investigation, based on the synergy between UV-Vis and FTIR-ATR spectroscopies and ElectroSpray Ionization Mass Spectrometry (ESI-MS), on the reactivity of 4-thiothymidine (S-4-TdR) with H2O2 , selected as a Reactive Oxygen Species (ROS) potentially related to Photo Dynamic Therapy (PDT), was performed. The nucleoside main absorption band in the UV-Vis range, centered at 337 nm, was monitored at several reaction times, confirming the S-4-TdR reactivity with a process leading to thymidine (TdR), as the main reaction product, detected through its absorption at 270 nm. Actually, the generation of at least another by-product, absorbing at slightly higher wavelengths compared to S-4-TdR, was inferred from UV-Vis spectra. FTIR-ATR and ESI-MS, MS/MS and MS3 measurements were then performed to unveil the nature of these reaction products, evidencing the presence of a hydroxylated S-4-TdR as important intermediate reaction product. In particular, ESI-MS analyses suggested that the latter was generated as a couple of isomeric compounds, with the OH linked either to the SH group arising from C-4=S tautomerization or to the methyl carbon (C-7) of S-4-TdR. Interestingly, the generation of a covalent dimer of S-4-TdR, characterized by the presence of a disulfide (S-S) bond, was also clearly inferred. The evolution of the cited products with reaction time was also be monitored using ESI-MS, confirming the role of hydroxylated S-4-TdR as an intermediate towards the generation of the ultimate product, TdR. The described processes make S-4-TdR a promising candidate as a molecular probe for the detection of H2O2 in PDT-related systems. (C) 2018 Elsevier B.V. All rights reserved.