An ROS-Responsive Donor That Self-Reports Its H2S Delivery by Forming a Benzoxazole-Based Fluorophore.

Hydrogen sulfide (H2S), an endogenous signaling molecule, is known to play a pivotal role in neuroprotection, vasodilation, and hormonal regulation. To further explore the biological effects of H2S, refined donors that facilitate its biological delivery, especially under specific (patho) physiological conditions, are needed. In the present study, we demonstrate that ortho-substituted, aryl boronate esters provide two unique and distinct pathways for H2S release from thioamide-based donors: Lewis acid-facilitated hydrolysis and reactive oxygen species (ROS)-induced oxidation/cyclization. Through a detailed structure-activity relationship study, donors that resist hydrolysis and release H2S solely via the latter mechanism were identified, which have the added benefit of providing a potentially useful heterocycle as the lone byproduct of this novel chemistry. To highlight this, we developed an ROS-activated donor (QH642) that simultaneously synthesizes a benzoxazole-based fluorophore en route to its H2S delivery. A distinct advantage of this design over earlier self-reporting donors is that fluorophore formation is possible only if H2S has been discharged from the donor. This key feature eliminates the potential for false positives and provides a more accurate depiction of reaction progress and donor delivery of H2S, including in complex cellular environments.

Enol-mediated delivery of H2Se from γ-keto selenides: mechanistic insight and evaluation.

Like hydrogen sulfide (H2S), its chalcogen congener, hydrogen selenide (H2Se), is an emerging molecule of interest given its endogenous expression and purported biological activity. However, unlike H2S, detailed investigations into the chemical biology of H2Se are limited and little is known about its innate physiological functions, cellular targets, and therapeutic potential. The obscurity surrounding these fundamental questions is largely due to a lack of small molecule donors that can effectively increase the bioavailability of H2Se through their continuous liberation of the transient biomolecule under physiologically relevant conditions. Driven by this unmet demand for H2Se-releasing moieties, we report that γ-keto selenides provide a useful platform for H2Se donation via an α-deprotonation/ß-elimination pathway that is highly dependent on both pH and alpha proton acidity. These attributes afforded a small library of donors with highly variable rates of release (higher alpha proton acidity = faster selenide liberation), which is accelerated under neutral to slightly basic conditions-a feature that is unique and complimentary to previously reported H2Se donors. We also demonstrate the impressive anticancer activity of γ-keto selenides in both HeLa and HCT116 cells in culture, which is likely to stimulate additional interest and research into the biological activity and anticancer effects of H2Se. Collectively, these results indicate that γ-keto selenides provide a highly versatile and effective framework for H2Se donation.

Intramolecular Thiol- and Selenol-Assisted Delivery of Hydrogen Sulfide.

Arylthioamides have been frequently employed to assess the chemical biology and pharmacology of hydrogen sulfide (H2 S). From this class of donors, however, extremely low H2 S releasing efficiencies have been reported and proper mechanistic studies have been omitted. Consequently, millimolar concentrations of arylthioamides are required to liberate just trace amounts of H2 S, and via an unidentified mechanistic pathway, which obfuscates the interpretation of any biological activity that stems from their use. Herein, we report that H2 S release from this valuable class of donors can be markedly enhanced through intramolecular nucleophilic assistance. Specifically, we demonstrate that both disulfide- and diselenide-linked thioamides are responsive to biologically relevant concentrations of glutathione and release two molar equivalents of H2 S via an intramolecular cyclization that significantly augments their rate and efficiency of sulfide delivery in both buffer and live human cells.

An Innovative Hydrogen Peroxide-Sensing Scaffold and Insight Towards its Potential as an ROS-Activated Persulfide Donor.

Reactive sulfur species, such as hydrogen sulfide, persulfides, and polysulfides, have recently emerged as key signaling molecules and important physiological mediators within mammalian systems. To better assess the therapeutic potential of their exogenous administration, we report on the development of a unique hydrogen peroxide (H2 O2 )-sensing motif and its capacity for providing cellular protection against oxidative stress while serving as a reactive oxygen species (ROS)-activated persulfide donor. With the strategic implementation of a gem-dimethyl group to promote both stability and cyclization, we found the initial rate of payload release from this newly derived scaffold to be directly proportional to the concentration of H2 O2 and to proceed via an unprecedented pathway that avoids the production of electrophilic byproducts, a severe limitation that has plagued the physiological application of previous designs.