title: Rose Bengal-photosensitized oxidation of 4-thiothymidine in aqueous medium: evidence for the reaction of the nucleoside with singlet state oxygen.
abstract: The photoreactivity of 4-thiothymidine (S(4)TdR) under visible light in the presence of Rose Bengal (RB), acting as a photosensitizer, was investigated in aqueous solutions at pH 7 and 12, using UV-vis, FTIR-ATR and (1)H-NMR spectroscopic techniques, time resolved absorption spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Evidence for the generation of thymidine (TdR) as the main product, after one hour of irradiation, was obtained from UV-Vis data, that suggested 4-thiothymidine photodegradation to be faster at basic pH, and confirmed by FTIR-ATR and (1)H-NMR data. Clues for the presence of a further product, likely corresponding to a dimeric form of S(4)TdR, were obtained from the latter techniques. Besides indicating the presence of thymidine, the ESI-MS and MS/MS spectra of the reaction mixtures enabled the identification of the additional product as a S-S bridged covalent dimer of 4-thiothymidine. The concentration of the dimeric species could be estimated with the aid of (1)H-NMR data and was found to be lower than that of thymidine in pH 7 reaction mixtures and almost negligible in the pH 12 ones. From a mechanistic point of view, time-resolved absorption spectroscopy measurements provided direct evidence that the formation of the two products cannot be ascribed to a photoinduced electron transfer involving S(4)TdR and the excited triplet state of RB. Rather, their generation can be interpreted as the result of a bimolecular reaction occurring between singlet state oxygen ((1)O2), photogenerated by RB, and S(4)TdR, as demonstrated by the direct detection of (1)O2 through IR luminescence spectroscopy. More specifically, a sequential reaction pathway, consisting in the generation of an electrophilic hydroxylated form of S(4)TdR and its subsequent, rapid reaction with S(4)TdR, was hypothesized to explain the presence of the S-S bridged covalent dimer of 4-thiothymidine in the reaction mixtures. The described processes make S(4)TdR an interesting candidate in the role of molecular probe for the detection of (1)O2 under different pH conditions.
authors.affiliation: Università degli Studi "Aldo Moro" di Bari, Dip. Chimica, Via Orabona, 4-70126 Bari, Italy. pinalysa.cosma@uniba.it.
authors.forename: Vito
authors.initials: V
authors.lastname: Rizzi
authors.forename: Ilario
authors.initials: I
authors.lastname: Losito
authors.forename: Andrea
authors.initials: A
authors.lastname: Ventrella
authors.forename: Paola
authors.initials: P
authors.lastname: Fini
authors.forename: Aurore
authors.initials: A
authors.lastname: Fraix
authors.forename: Salvatore
authors.initials: S
authors.lastname: Sortino
authors.forename: Angela
authors.initials: A
authors.lastname: Agostiano
authors.forename: Francesco
authors.initials: F
authors.lastname: Longobardi
authors.forename: Pinalysa
authors.initials: P
authors.lastname: Cosma
mesh_terms: D008027:Light; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010782:Photolysis; D017319:Photosensitizing Agents / Q000737:chemistry*; D012395:Rose Bengal / Q000737:chemistry*; D026082:Singlet Oxygen / Q000737:chemistry*; D013872:Thionucleosides / Q000737:chemistry; D013936:Thymidine / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: C414668:4-thiothymidine; D017319:Photosensitizing Agents; D013872:Thionucleosides; C070456:4'-thiothymidine; D026082:Singlet Oxygen; D012395:Rose Bengal; D013936:Thymidine