Nitric oxide (NO) has a range of biological activities in living systems. Because NO is a gaseous and short-lived molecule, the development of methods to enable a temporally and spatially resolved NO supply has been a challenging task. In this report, 3-aryl 4-alkoxy furoxan is revealed as a potent photo-induced NO-donor. While negligible levels of NO release were observed under ambient fluorescent light, a significant amount of NO was released under UV irradiation ( = 300-400 nm). The thiol mediator bearing a pendant amino group proved indispensable for the NO release from aryl alkoxy furoxans. Analysis of the NO-releasing reaction of aryl alkoxy furoxan revealed that the main coproduct is the corresponding aryl nitrile derived from furoxan ring-fragmentation. From these results, a plausible photo-induced NO-releasing mechanism was proposed, in which the key steps are nucleophilic attack by the thiol at the 3position of furoxan leading to the furoxan ring-fragmentation, and the formation of tetrahydrothiazole bearing a C-nitroso group. Comparison between fluorofuroxan and alkoxy furoxan is also discussed.
Nitric oxide (NO) is an endogenous signaling molecule used in multiple biochemical processes. The development of switchable NO donors that deliver an NO payload under spatiotemporal control harbors many medicinal benefits. Previously, 4-fluorofuroxans were found to function as a UV light-induced NO donor under physiological conditions based on the photoinduced isomerization mechanism; however, the isomerization of fluorofuroxans with longer wavelength light is desired for further application into living systems. Herein, we report the use of photosensitizers in the photochemical isomerization of fluorofuroxan, enabling the use of visible light to induce isomerization. Among the tried photosensitizers, anthraquinone derivatives showed a good sensitizing ability to isomerize 4-fluorofuroxan to 3-fluorofuroxan using visible light. This new phenomenon was applied to the synthesis of a water-soluble anthraquinone-fluorofuroxan all-in-one molecule, which demonstrated promising NO-releasing ability using 400-500 nm irradiation. A high level of control is displayed with "on" and "off" NO-release functionality suggesting that photosensitizer-furoxan hybrids would make valuable donors. Furthermore, unlike most furoxans, NO is released in the absence of thiol cofactor.
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