Thionoesters: A Native Chemical Ligation-Inspired Approach to Cysteine-Triggered HS Donors.

Native chemical ligation (NCL) is a simple, widely used, and powerful synthetic tool to ligate N-terminal cysteine residues and C-terminal α-thioesters via a thermodynamically stable amide bond. Building on this well-established reactivity, as well as advancing our interests in the chemical biology of reactive sulfur species including hydrogen sulfide (HS), we hypothesized that thionoesters, which are constitutional isomers of thioesters, would undergo a similar NCL reaction in the presence of cysteine to release HS under physiological conditions. Herein, we report mechanistic and kinetic investigations into cysteine-mediated HS release from thionoesters. We found that this reaction proceeds with high HS-releasing efficiency (∼80%) and with a rate constant (9.1 ± 0.3 M s) comparable to that for copper-catalyzed azide-alkyne cycloadditions (CuAAC). Additionally, we found that the final product of the reaction of cysteine with thionoesters results in the formation of a stable dihydrothiazole, which is an iron-binding motif commonly found in siderophores produced by bacteria during periods of nutrient deprivation.