Thionoesters: A Native Chemical Ligation-Inspired Approach to Cysteine-Triggered H2S Donors

Two spirobifluene-based turn-on fluorescent probes for highly selective detection of Cysteine and the applications in cells two-photon fluorescence imaging

Two spirobifluene-based fluorescent probes SPF1 and SPF2, were designed and synthesized. The probes displayed "turn-on" fluorescence response for Cysteine. One of the challenges in developing a Cysteine probe is to secure high selectivity. SPF1/SPF2 can discriminate Cysteine from GSH as well as Hcy, and showed high substrate selectivity. The detection limit of SPF1 is 36 nM, which is excellent comparing with other optical sensors for Cysteine. The sensing mechanism of SPF1/SPF2 was verified by experimental data and theoretical calculations. There was a good linear relationship between the fluorescence intensity of SPF1/SPF2 and the concentration of Cysteine. The MTT tests indicated that SPF1/SPF2 had low cytotoxicity and good biocompatibility. Theoretical calculations demonstrated that SPF1, SPF2, and their related reaction products with Cysteine exhibited good two-photon absorption properties. Finally, SPF1/SPF2 had been successfully applied to the imaging of Cysteine in living cells under two-photon excitation.

Synthesis of sinomenine derivatives with potential anti-leukemia activity

In recent years, with sinomenine hydrochloride as the main ingredient, Qingfengteng had been formulated as various dosage forms for clinical treatment. Subsequent findings confirmed a variety of biological roles for sinomenine. Here, 15 H2S-donating sinomenine derivatives were synthesized. Target hybrids a11 displayed substantial cytotoxic effects on cancer cell lines, particularly against K562 cells, with an IC50 value of 1.36 μM. In-depth studies demonstrated that a11 arrested cell cycle at G1 phase, induced apoptosis via both morphological changes in nucleus and membrane potential collapse in mitochondria. These results indicated a11 exerted an antiproliferative effect through apoptosis induction via mitochondrial pathway.

Discovery of novel β-elemene hybrids with hydrogen sulfide-releasing moiety possessing cardiovascular protective activity for the treatment of atherosclerosis

Herein, a series of novel β-elemene hybrids with different types of hydrogen sulfide (H2S) donors was designed and synthesized for the first time. In addition, all compounds were tested for H2S release in phosphate buffer solution assay, among which the derivatives with 5-p-hydroxyphenyl-3H-1,2-dithiole-3-thione (ADT-OH) as the H2S donor released the best level. The results of the isolated vasodilation assay revealed that all the compounds exhibited a degree of vasodilatory effect, and the representative compound "β-elemene-H2S gas donor" hybrid L13-2h produced more than 50% vasodilatory activity at a concentration of 20 μM. Furthermore, L13-2h possessed good concentration dependence and significantly better vasodilatory activity than the lead compound L13. In the RAW 264.7 cellular lipid inhibition against oxidized low-density lipoprotein (ox-LDL) stimulation assay, eight compounds, including L13-2g and L13-2h, produced significant cellular lipid-lowering activity. The results of the further antioxidant activity study showed that the representative compounds L13-2g and L13-2h improved H2O2-induced oxidative damage in HUVEC cells and compound L13-2h exhibited excellent antioxidant damage protection activity compared to the positive control. Moreover, none of the target compounds appeared to be significantly cytotoxic at the tested concentrations. These results suggest that the hybridization of hydrogen sulfide donors with β-elemene provides a promising approach for the discovery of novel anti-atherosclerotic drugs from natural products.

Smart Lipid Nanoparticle that Remodels Tumor Microenvironment for Activatable H2S Gas and Photodynamic Immunotherapy

Hydrogen sulfide (H2S) has shown promise for gas therapy. However, it is still controversial whether H2S can remodel the tumor microenvironment (TME) and induce robust antitumor immunity. Here, a tumor-targeting and TME-responsive "smart" lipid nanoparticle (1-JK-PS-FA) is presented, which is capable of delivering and releasing H2S specifically in tumor tissues for on-demand H2S gas and photodynamic immunotherapy. 1-JK-PS-FA enables a burst release of H2S in the acidic TME, which promptly reduces the embedded organic electrochromic materials and consequently switches on near-infrared fluorescence and photodynamic activity. Furthermore, we found that high levels of H2S can reprogram the TME by reducing tumor interstitial fluid pressure, promoting angiogenesis, increasing vascular permeability, ameliorating hypoxia, and reducing immunosuppressive conditions. This leads to increased tumor uptake of 1-JK-PS-FA, thereby enhancing PDT efficacy and eliciting strong immunogenic cell death during 808 nm laser irradiation. Therefore, 1-JK-PS-FA permits synergistic H2S gas and photodynamic immunotherapy, effectively eradicating orthotopic breast tumors and preventing tumor metastasis and recurrence. This work showcases the capacity of H2S to reprogram the TME to enhance H2S gas and immunotherapy.

A new dual functional H2S donor for fluorescence imaging and anti-inflammatory application

This study explored FL-H2S, a novel fluorescein-based H2S donor, as an anti-inflammatory agent. The results demonstrated the efficient release of H2S by FL-H2S, along with its biocompatibility, real-time intracellular H2S release and imaging capability. In vivo experiments using a rat model confirmed the anti-inflammatory effects of FL-H2S, evidenced by reduced foot swelling. We also successfully elucidated the anti-inflammatory mechanism through ELISA and WB analysis.

Silver Nanoparticle Sensor Array-Based Meat Freshness Inspection System

The series of biochemical reactions, metabolic pathways, and regulatory interactions that occur during the storage of meat are the main causes of meat loss and waste. The volatile compounds produced by these reactions, such as hydrogen sulfide, acids, and amines, can directly indicate changes in the freshness of meat during storage and sales. In this study, a one-pot hydrothermal method based on a surface control strategy was used to develop nanoparticles of silver with different reactivities, which were further immobilized in agar powder to develop a colorimetric sensor array. Due to the different chemical interactions with various volatile compounds, the colorimetric sensor array exhibited distinct color changes. The study demonstrates significant differences between 12 different volatile compounds and provides a quantitative and visual method to reveal rich detection indicators. The colorimetric sensor array is an economical and practical multi-analyte identification method. It has many potential applications such as food packaging, anti-counterfeiting, health monitoring, environmental monitoring, and optical filters.

A unique reaction of diphenylcyclopropenone and 1,2-aminothiol with the release of thiol for multiple bioconjugation

Selective reaction of diphenylcyclopropenone (DPCP) and 1,2-aminothiol in water at pH 7.4 produces an amide conjugate with the release of thiol. In addition, structural modifications of DPCP enable the coupling rate to be tuned with a reaction constant of +3.68. Based on this chemistry, triple labelling was demonstrated by treating an N-terminal cysteine peptide with DPCP-Cl followed by thiol-maleimide and tyrosine-diazonium couplings in one pot. We anticipate that the DPCP motif will be a useful toolkit for multiple bioconjugation.

Nitric Oxide Scavenging and Hydrogen Sulfide Production Synergistically Treat Rheumatoid Arthritis

To restore the disordered endogenous gas levels is an efficient alternative for the treatment of rheumatoid arthritis (RA). Both insufficient hydrogen sulfide (H₂ S) and excessive nitric oxide (NO) contribute to synovial inflammation. Herein, a new block polymer PEG₁₀ -b-PNAPA₃₀ -b-PEG₁₀ composed of an NO-responsive monomer and a cysteine-triggered H₂ S donor, which can simultaneously scavenge NO and release therapeutic H₂ S for RA treatment, is reported. In vitro experiments demonstrate that the polymer exhibits a synergistic effect on suppressing reactive oxygen species levels and pro-inflammatory cytokine production via NF-κB signaling pathway. It leads to the polarization of macrophages from M1 to M2 phenotype. Moreover, the released H₂ S further restrains NO production by suppressing the expression of iNOS. In vivo experiments with an RA rat model show that the system markedly mitigates the synovial inflammation, osteoporosis, and clinical symptoms of RA rats, which is attributed to the combination therapy of H₂ S release and NO depletion. This work provides new insight into the synergistic treatment of RA and endogenous gas-related diseases.

Polythionoester Vesicle: An Efficient Polymeric Platform for Tuning H2S Release

Hydrogen sulfide (H₂S) serves as a key gaseous regulator that not only directs many physiological activities, but also manifests therapeutic benefits to many diseases. Developing H₂S vehicle platforms for its local delivery and long-acting release is important to achieve target gas therapy. Most of the known H₂S-donating polymers contain labile thioester scaffolds within their structures that suffer from the issue of low gas releasing efficiency. Here we present the use of thionoester, a constitutional isomer of thioester, as the functional unit to build a structural platform of cysteine-triggered H₂S donor polymer, polythionoester. Simple exchange of the sulfur and oxygen positions in the carbonyl sulfide scaffold makes the polythionoesters undergo a distinct mechanism of H₂S production, which can largely improve the gas-releasing efficiency (>80%). Moreover, the thionoester-containing block copolymers can self-assemble into vesicles in an aqueous media. We discover that control over the size effect can adjust the vesicle disassembly rate and gas-releasing kinetics. A tunable half-life of H₂S generation (2.6-9.8 h) can be accessed by tailoring the vesicle dimension. This allows such polymersomes to be potential as a gas nanodelivery system for long-lasting gas therapeutics.

Progress and perspective on hydrogen sulfide donors and their biomedical applications

Following the discovery of nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H₂ S) has been identified as the third gasotransmitter in humans. Increasing evidence have shown that H₂ S is of preventive or therapeutic effects on diverse pathological complications. As a consequence, it is of great significance to develop suitable approaches of H₂ S-based therapeutics for biomedical applications. H₂ S-releasing agents (H₂ S donors) play important roles in exploring and understanding the physiological functions of H₂ S. More importantly, accumulating studies have validated the theranostic potential of H₂ S donors in extensive repertoires of in vitro and in vivo disease models. Thus, it is imperative to summarize and update the literatures in this field. In this review, first, the background of H₂ S on its chemical and biological aspects is concisely introduced. Second, the studies regarding the H₂ S-releasing compounds are categorized and described, and accordingly, their H₂ S-donating mechanisms, biological applications, and therapeutic values are also comprehensively delineated and discussed. Necessary comparisons between related H₂ S donors are presented, and the drawbacks of many typical H₂ S donors are analyzed and revealed. Finally, several critical challenges encountered in the development of multifunctional H₂ S donors are discussed, and the direction of their future development as well as their biomedical applications is proposed. We expect that this review will reach extensive audiences across multiple disciplines and promote the innovation of H₂ S biomedicine.

Cysteine-Activated Small-Molecule H2Se Donors Inspired by Synthetic H2S Donors

The importance of selenium (Se) in biology and health has become increasingly clear. Hydrogen selenide (H₂Se), the biologically available and active form of Se, is suggested to be an emerging nitric oxide (NO)-like signaling molecule. Nevertheless, the research on H₂Se chemical biology has technique difficulties due to the lack of well-characterized and controllable H₂Se donors under physiological conditions, as well as a robust assay for direct H₂Se quantification. Motivated by these needs, here, we demonstrate that selenocyclopropenones and selenoamides are tunable donor motifs that release H₂Se upon reaction with cysteine (Cys) at pH 7.4 and that structural modifications enable the rate of Cys-mediated H₂Se release to be tuned. We monitored the reaction pathways for the H₂Se release and confirmed H₂Se generation qualitatively using different methods. We further developed a quantitative assay for direct H₂Se trapping and quantitation in an aqueous solution, which should also be operative for investigating future H₂Se donor motifs. In addition, we demonstrate that arylselenoamide has the capability of Cys-mediated H₂Se release in cellular environments. Importantly, mechanistic investigations and density functional theory (DFT) calculations illustrate the plausible pathways of Cys-activated H₂Se release from arylselenoamides in detail, which may help understand the mechanistic issues of the H₂S release from pharmacologically important arylthioamides. We anticipate that the well-defined chemistries of Cys-activated H₂Se donor motifs will be useful for studying Se biology and for development of new H₂Se donors and bioconjugate techniques.

Hydrogen Sulfide: An Emerging Precision Strategy for Gas Therapy

Advances in nanotechnology have enabled the rapid development of stimuli-responsive therapeutic nanomaterials for precision gas therapy. Hydrogen sulfide (H₂ S) is a significant gaseous signaling molecule with intrinsic biochemical properties, which exerts its various physiological effects under both normal and pathological conditions. Various nanomaterials with H₂ S-responsive properties, as new-generation therapeutic agents, are explored to guide therapeutic behaviors in biological milieu. The cross disciplinary of H₂ S is an emerging scientific hotspot that studies the chemical properties, biological mechanisms, and therapeutic effects of H₂ S. This review summarizes the state-of-art research on H₂ S-related nanomedicines. In particular, recent advances in H₂ S therapeutics for cancer, such as H₂ S-mediated gas therapy and H₂ S-related synergistic therapies (combined with chemotherapy, photodynamic therapy, photothermal therapy, and chemodynamic therapy) are highlighted. Versatile imaging techniques for real-time monitoring H₂ S during biological diagnosis are reviewed. Finally, the biosafety issues, current challenges, and potential possibilities in the evolution of H₂ S-based therapy that facilitate clinical translation to patients are discussed.

Ni-Catalyzed Synthesis of Thiocarboxylic Acid Derivatives

A Ni-catalyzed cross-coupling of readily accessible O-alkyl xanthate esters or thiocarbonyl imidazolides and organozinc reagents for the synthesis of thiocarboxylic acid derivatives has been developed. This method benefits from a fast reaction time, mild reaction conditions, and ease of starting material synthesis. The use of transition-metal catalysis to access a diverse range of thiocarbonyl-containing compounds provides a useful complementary approach when compared with previously established methodologies.

Cell-Trappable BODIPY-NBD Dyad for Imaging of Basal and Stress-Induced H2S in Live Biosystems

H₂S is a gaseous signaling molecule that is involved in many physiological and pathological processes. In general, the level of intracellular H₂S (<1 μM) is much lower than that of GSH (∼1-10 mM), leading to the remaining challenge of selective detection and differentiation of endogenous H₂S in live biosystems. To this end, we quantitatively demonstrate that the thiolysis of NBD amine has much higher selectivity for H₂S over GSH than that of the reduction of aryl azide. Subsequently, we developed the first NBD-based cell-trappable probe 1 (AM-BODIPY-NBD) for highly selective and ultrasensitive imaging of intracellular H₂S. Probe 1 demonstrates a 207-fold fluorescence enhancement at 520 nm after reaction with H₂S/esterase to produce a bright BODIPY (quantum yield 0.42) and a detection limit of 15.7 nM. Probe 1 is water-soluble, cell-trappable, and not cytotoxic. Based on this excellent chemical tool, relative levels of basal H₂S in different cell lines were measured to reveal a positive correlation between endogenous H₂S and the metastatic potential of colon and breast cancer cells. In addition, H₂S biogenesis in vivo was also validated by probe 1 both in tobacco leaves under viral infection and in zebrafish after tail amputation. It is anticipated that probe 1 will have widespread applications in imaging and for investigating different H₂S-related biological processes and diseases.

An Efficient and Straightforward Approach for Accessing Thioesters via Palladium-Catalyzed C-N Cleavage of Thioamides

We first report the coupling of activated thioamides with alcohols to efficiently form thioesters via palladium-catalyzed C-N cleavage strategy. The new approach employs the thioamides as thioacylating reagent to give...

Thiobenzophenones: tunable hydrolysis-based donors for intracellular H2S delivery

H2S, the third gasotransmitter, is involved in many physiological and pathological processes. Compounds that can release H2S slowly under physiological conditions are useful chemical tools for studying H2S biology as...

A cysteine-triggered fluorogenic donor base on native chemical ligation for tracking H2S delivery in vivo

A hydrogen sulfide (H₂S) donor is a fundamental molecular tool used as an exogenous source in biological studies and therapies. However, finding a controllable and visual fluorescent H₂S donor is difficult. We report a new H₂S donor, HSD560, the H₂S release of which is triggered by cysteine. Importantly, the H₂S generation is accompanied with enhanced green fluorescence, which could be utilized to track H₂S release in cells using microscopy. H₂S release from HSD560 undergoes a non-enzymatic native chemical ligation (NCL) process, which provides an accurate match with activated fluorescence and localization of H₂S in zebrafish.

Efficacious One-pot Synthesis of 2-thiazolines and 2-oxazolines Under Solvent and Metal-Free Conditions

Background:

2-oxazolines and 2-thiazolines are important heterocycles due to their extensive applications in chemistry, biochemistry, and pharmacology. Most of the precedent methods for preparing these heterocycles involve one or more drawbacks, including harsh reaction conditions, long reaction times, low yields of products, high temperature, and toxic solvents.

Objective:

The aim of this study was to develop a new and eco-efficient method for the preparation of 2-oxazolines and 2-thiazolines.

Methods:

Amino alcohols were condensed with nitriles in a sealed tube under solvent-free and metal- free conditions.

Results:

Our procedure appears to be highly eco-efficient and promotes quantitative access to 2-oxazolines and 2-thiazolines using simple and minimum manipulation.

Conclusion:

This simple approach allows high conversion for different nitriles yielded from 78 to 99% and easy isolation of the targeted products without further purification. To the best of our knowledge, our procedure is the most efficient and fast method reported to date in terms of chemical yields, number of steps, and atom economy.

Design and Development of a Bioorthogonal, Visualizable and Mitochondria-Targeted Hydrogen Sulfide (H2 S) Delivery System

Hydrogen sulfide (H₂ S) is an important endogenous gasotransmitter, but the targeted delivery and real-time feedback of exogenous H₂ S are still challenging. With the aid of density functional theory (DFT) calculations, we designed a new 1,3-dithiolium-4-olate (DTO) compound, which can react with a strained alkyne via the 1,3-dipolar cycloaddition and the retro-Diels-Alder reaction to generate carbonyl sulfide (COS) as the precursor of H₂ S, and a thiophene derivative with turn-on fluorescence. Moreover, the diphenylamino substituent in DTO greatly increases the mitochondrial targeting of this H₂ S delivery system. Such a bioorthogonal click-and-release reaction has integrated three functions in one system for the first time: (1) in situ controllable H₂ S release, (2) concomitant fluorescence response, and (3) mitochondria-targeted delivery. In addition, we investigated the mitochondrial membrane potential loss alleviation by using this system in H9c2 cells under oxidative stress.

A Dinuclear Persulfide-Bridged Ruthenium Compound is a Hypoxia-Selective Hydrogen Sulfide (H2 S) Donor

Hydrogen sulfide (H2 S) is a gaseous molecule that has received attention for its role in biological processes and therapeutic potential in diseases, such as ischemic reperfusion injury. Despite its clinical relevance, delivery of H2 S to biological systems is hampered by its toxicity at high concentrations. Herein, we report the first metal-based H2 S donor that delivers this gas selectively to hypoxic cells. We further show that H2 S release from this compound protects H9c2 rat cardiomyoblasts from an in vitro model of ischemic reperfusion injury. These results validate the utility of redox-activated metal complexes as hypoxia-selective H2 S-releasing agents for use as tools to study the role of this gaseous molecule in complex biological systems.

Insights into the Mechanism of Thiol-Triggered COS/H2S Release from N-Dithiasuccinoyl Amines

The hydrolysis of carbonyl sulfide (COS) to form H₂S by carbonic anhydrase has been demonstrated to be a viable strategy to deliver H₂S in a biological system. Herein, we describe N-dithiasuccinoyl amines as thiol-triggered COS/H₂S donors. Notably, thiol species especially GSH and homocysteine can trigger the release of both COS and H₂S directly from several specific analogues via an unexpected mechanism. Importantly, two representative analogues Dts-1 and Dts-5 show intracellular H₂S release, and Dts-1 imparts potent anti-inflammatory effects in LPS-challenged microglia cells. In conclusion, N-dithiasuccinoyl amine could serve as promising COS/H₂S donors for either H₂S biological studies or H₂S-based therapeutics development.

Polymers with Dithiobenzoate End Groups Constitutively Release Hydrogen Sulfide upon Exposure to Cysteine and Homocysteine

Dithioesters are well-established as efficient reversible addition-fragmentation chain transfer (RAFT) agents. More recently, certain small molecule dithioesters have been reported to release the biological signaling molecule hydrogen sulfide (H₂S) upon exposure to cysteine. Herein, we examine the propensity of polymers synthesized using RAFT with a dithioester chain transfer agent to release H₂S via reaction of cysteine with constitutive dithioester end-groups. Homocysteine-triggered release of H₂S from these materials is also observed, with evidence suggesting that the mechanism is analogous to the reaction with cysteine.

A Reactive Oxygen Species (ROS) Activated Hydrogen Sulfide (H2S) Donor with Self-Reporting Fluorescence

Hydrogen sulfide (H₂S) is an important cellular signaling molecule, and its physiological and pathophysiological properties have been under intensive investigation. In this study, a novel ratiometric fluorescent H₂S donor (HSD-B) has been developed, which exhibited the following advantages: (i) scavenging ROS and producing H₂S simultaneously; (ii) providing ratiometric fluorescence for visualization and quantification of H₂S releasing; and (iii) targeting mitochondrion specifically. Moreover, it demonstrated protective effects on myocardial ischemia reperfusion injury in a cellular model. These attractive features promise this HSD-B as a fluorescent H₂S donor for future research studies.

Activatable Small-Molecule Hydrogen Sulfide Donors

Significance: Hydrogen sulfide (H2S) is an important biological signaling molecule involved in many physiological processes. These diverse roles have led researchers to develop contemporary methods to deliver H2S under physiologically relevant conditions and in response to various stimuli. Recent Advances: Different small-molecule donors have been developed that release H2S under various conditions. Key examples include donors activated in response to hydrolysis, to endogenous species, such as thiols, reactive oxygen species, and enzymes, and to external stimuli, such as photoactivation and bio-orthogonal chemistry. In addition, an alternative approach to release H2S has utilized the catalyzed hydrolysis of carbonyl sulfide (COS) by carbonic anhydrase to generate libraries of activatable COS-based H2S donors. Critical Issues: Small-molecule H2S donors provide important research and pharmacological tools to perturb H2S levels. Key needs, both in the development and in the use of such donors, include access to new donors that respond to specific stimuli as well as donors with well-defined control compounds that allow for clear delineation of the impact of H2S delivery from other donor byproducts. Future Directions: The abundance of reported small-molecule H2S donors provides biologists and physiologists with a chemical toolbox to ask key biological questions and to develop H2S-related therapeutic interventions. Further investigation into different releasing efficiencies in biological contexts and a clear understanding of biological responses to donors that release H2S gradually (e.g., hours to days) versus donors that generate H2S quickly (e.g., seconds to minutes) is needed.

Cyclic Sulfenyl Thiocarbamates Release Carbonyl Sulfide and Hydrogen Sulfide Independently in Thiol-Promoted Pathways

Hydrogen sulfide (H2S) is an important signaling molecule that provides protective activities in a variety of physiological and pathological processes. Among the different types of H2S donor compounds, thioamides have attracted attention due to prior conjugation to nonsteroidal anti-inflammatory drugs (NSAIDs) to access H2S-NSAID hybrids with significantly reduced toxicity, but the mechanism of H2S release from thioamides remains unclear. Herein, we reported the synthesis and evaluation of a class of thioamide-derived sulfenyl thiocarbamates (SulfenylTCMs) that function as a new class of H2S donors. These compounds are efficiently activated by cellular thiols to release carbonyl sulfide (COS), which is quickly converted to H2S by carbonic anhydrase (CA). In addition, through mechanistic investigations, we establish that COS-independent H2S release pathways are also operative. In contrast to the parent thioamide-based donors, the SulfenylTCMs exhibit excellent H2S releasing efficiencies of up to 90% and operate through mechanistically well-defined pathways. In addition, we demonstrate that the sulfenyl thiocarbamate group is readily attached to common NSAIDs, such as naproxen, to generate YZ-597 as an efficient H2S-NSAID hybrid, which we demonstrate releases H2S in cellular environments. Taken together, this new class of H2S donor motifs provides an important platform for new donor development.

Data-Driven Identification of Hydrogen Sulfide Scavengers

Hydrogen sulfide (H₂ S) is an important signaling molecule whose up- and down-regulation have specific biological consequences. Although significant advances in H₂ S up-regulation, by the development of H₂ S donors, have been achieved in recent years, precise H₂ S down-regulation is still challenging. The lack of potent/specific inhibitors for H₂ S-producing enzymes contributes to this problem. We expect the development of H₂ S scavengers is an alternative approach to address this problem. Since chemical sensors and scavengers of H₂ S share the same criteria, we constructed a H₂ S sensor database, which summarizes key parameters of reported sensors. Data-driven analysis led to the selection of 30 potential compounds. Further evaluation of these compounds identified a group of promising scavengers, based on the sulfonyl azide template. The efficiency of these scavengers in in vitro and in vivo experiments was demonstrated.

A computational study on H2S release and amide formation from thionoesters and cysteine

The recognition of the biological activity of H2S has drawn much attention to the development of biocompatible H2S release reactions. Thiol-, particularly cysteine-triggered systems which mimic the enzymatic conversion of cysteine or homocysteine to H2S have been intensively reported recently. Herein, a density functional theory (DFT) study was performed to address the reaction mechanism of H2S release and potential amide bond formation from thionoesters and cysteine to gain deeper mechanistic insights. Three possible mechanisms were considered and we found that the one starting from the nucleophilic addition of the ionized mercapto of cysteine on thionoester to generate a dithioester intermediate (Path A) is kinetically favored over the others starting from the nucleophilic addition of the amine of cysteine to generate thionoamide intermediates (Paths B and C). Dithioester then undergoes intramolecular nucleophilic addition of an amine group and the rate-limiting water-assisted proton transfer to generate a cyclic thiol intermediate, and finally affords H2S and dihydrothiazole via water-assisted elimination. The hydrolysis of thionoamide or dihydrothiazole to produce amide is highly difficult under neutral conditions but is operative under strong basic conditions, which explains the experimental observation that dihydrothiazole rather than amide is the major product. Meanwhile, the ring opening reaction of the cyclic thiol intermediate to form the more stable thionoamide is detrimental to H2S release and becomes competitive under basic conditions.

Dithioesters: simple, tunable, cysteine-selective H2S donors

Dithioesters have a rich history in polymer chemistry for RAFT polymerizations and are readily accessible through different synthetic methods. Here we demonstrate that the dithioester functional group is a tunable motif that releases H2S upon reaction with cysteine and that structural and electronic modifications enable the rate of cysteine-mediated H2S release to be modified. In addition, we use (bis)phenyl dithioester to carry out kinetic and mechanistic investigations, which demonstrate that the initial attack by cysteine is the rate-limiting step of the reaction. These insights are further supported by complementary DFT calculations. We anticipate that the results from these investigations will allow for the further development of dithioesters as important chemical motifs for studying H2S chemical biology.