Thionoesters: A Native Chemical Ligation-Inspired Approach to Cysteine-Triggered H₂S 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 (H 2 S), we hypothesized that thionoesters, which are constitutional isomers of thioesters, would undergo a similar NCL reaction in the presence of cysteine to release H 2 S under physiological conditions. Herein, we report mechanistic and kinetic investigations into cysteine-mediated H 2 S release from thionoesters. We found that this reaction proceeds with high H 2 S-releasing efficiency (∼80%) and with a rate constant (9.1 ± 0.3 M –1 s –1 ) 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.