Abstract
In this study, we report the synthesis and the photochemical behavior of a series of new "click-on" fluorescent probes designed to detect singlet oxygen. They include a highly fluorescent chemical structure, an aryloxazole ring, linked to a furan moiety operating as singlet oxygen trap. Their activity depends on both the structure of the aryloxazole fluorophore and the electron-donating and electron-accepting properties of the substituents attached to the C-5 of the furan ring. All probes are selectively oxidized by singlet oxygen to give a single fluorescent product in methanol and produce negligible amounts of singlet oxygen themselves by self-sensitization. The most promising dyad, (E)-2-(2-(5-methylfuran-2-yl)vinyl)naphtho[1,2-d]oxazole, FN-6, shows outstanding reactivity and sensitivity: it traps singlet oxygen with a rate constant (5,8 ± 0.1) x 107 M-1 s-1 and its fluorescence increases by a factor of 500 upon reaction. Analysis of the dyads reactivity in terms of linear free energy relationships using the modified Swain and Lupton parameter F and the Fukui condensed function for the electrophilic attack, suggests that cycloaddition of singlet oxygen to the furan ring is partially concerted and possibly involves an exciplex with a "more open" structure than could be expected for a concerted cycloaddition.
Highlights
Singlet oxygen, (O2(1Δg), hereafter 1O2), the first electronic excited state of molecular oxygen, is a well-known reactive oxygen species (ROS), that can diffuse and oxidize several types of biomolecules such as proteins, nucleic acids and lipids constituent of cell membranes [1,2,3,4,5,6,7,8]
Compounds FN-1 to FN-5 are dyads composed by an unsubstituted furan ring linked to an aryloxazole moiety through either a single C-C bond or a vinyl bridge
The results described in previous sections suggest that dyads which include a polycyclic aromatic naphthoxazole [1,2-d] system with a vinyl bridge at the 2-position of the heterocycle linked to a substituted furan ring with an alkyl group at carbon-5 are the ones with the highest fluorescent response
Summary
Singlet oxygen, (O2(1Δg), hereafter 1O2), the first electronic excited state of molecular oxygen, is a well-known reactive oxygen species (ROS), that can diffuse and oxidize several types of biomolecules such as proteins, nucleic acids and lipids constituent of cell membranes [1,2,3,4,5,6,7,8]. Due to this reactivity, 1O2 is recognized as an important ROS capable of promoting a large variety of cell responses [9,10,11]. 1O2 is one of the most central cytotoxic ROS produced by exposure of sensitizers to light in photodynamic therapy, PDT [12,13,14,15].
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