Applications of Synthetic Organic Tetrasulfides as H2S Donors

Thioglucose-derived tetrasulfide, a unique polysulfide model compound

Polysulfides have received increased interest in redox biology due to their role as the precursors of H2S and persulfides. However, the compounds that are suitable for biological investigations are limited to cysteine- and glutathione-derived polysulfides. In this work, we report the preparation and evaluation of a novel polysulfide derived from thioglucose, which represents the first carbohydrate-based polysulfide. This compound, thioglucose tetrasulfide (TGS4), showed excellent stability and water solubility. H2S and persulfide production from TGS4, as well as its associated antioxidative property were also demonstrated. Additionally, TGS4 was demonstrated to significantly induce cellular sulfane sulfur level increase, in particular for the formation of hydropersulfides/trisulfides. These results suggest that TGS4 is a useful tool for polysulfide research.

Tetrasulfide bond boosts the anti-tumor efficacy of dimeric prodrug nanoassemblies

Dimeric prodrug nanoassemblies (DPNAs) stand out as promising strategies for improving the efficiency and safety of chemotherapeutic drugs. The success of trisulfide bonds (-SSS-) in DPNAs makes polysulfide bonds a worthwhile focus. Here, we explore the comprehensive role of tetrasulfide bonds (-SSSS-) in constructing superior DPNAs. Compared to trisulfide and disulfide bonds, tetrasulfide bonds endow DPNAs with superlative self-assembly stability, prolonged blood circulation, and high tumor accumulation. Notably, the ultra-high reduction responsivity of tetrasulfide bonds make DPNAs a highly selective "tumor bomb" that can be ignited by endogenous reducing agents in tumor cells. Furthermore, we present an "add fuel to the flames" strategy to intensify the reductive stress at tumor sites by replenishing exogenous reducing agents, making considerable progress in selective tumor inhibition. This work elucidates the crucial role of tetrasulfide bonds in establishing intelligent DPNAs, alongside the combination methodology, propelling DPNAs to new heights in potent cancer therapy.

Nickel-Catalyzed Acid Chlorides with Tetrasulfides for the Synthesis of Thioesters and Acyl Disulfides

Herein, we report a novel method for the synthesis of thioesters and acyl disulfides via nickel-catalyzed reductive cross-electrophile coupling of acid chlorides with tetrasulfides. This approach for the synthesis of thioesters and acyl disulfides is convenient and practical under mild reaction conditions, relying on easy availability. In addition, a wide range of thioesters and acyl disulfides were obtained in medium to good yields with good functional group tolerance. Moreover, thioesters and acyl disulfides can also be prepared at the gram scale, indicating that they have certain potential for industrial application.

Emergence of (hydro)persulfides as suppressors of lipid peroxidation and ferroptotic cell death

Recognition of the prevalence of hydropersulfides (RSSH) and characterization of their enhanced two-electron reactivity relative to thiols have led to their implication in maintaining cellular redox homeostasis, in addition to other potential roles. Recent attention on the one-electron reactivity of RSSH has uncovered their potent radical-trapping antioxidant activity, which enables them to inhibit phospholipid peroxidation and associated cell death by ferroptosis. Herein, we briefly review key aspects of the reactivity and underlying physicochemical properties of RSSH. We emphasize their reactivity to radicals-particularly lipid peroxyl radicals that propagate the lipid peroxidation chain reaction-and the recent recognition that this results in ferroptosis suppression. We highlight open questions related to recent developments in this area and, given that all living organisms possess the ability to synthesize persulfides endogenously, suggest they may be primordial radical scavengers that occurred early in evolution and still play a role today.

Sulfur-Polymer Nanoparticles: Preparation and Antibacterial Activity

High sulfur content polymers prepared by inverse vulcanization have many reported potential applications, including as novel antimicrobial materials. High sulfur content polymers usually have limited water-solubility and dispersibility due to their hydrophobic nature, which could limit the development of their applications. Herein, we report the formulation of high sulfur content polymeric nanoparticles by a nanoprecipitation and emulsion-based method. High sulfur content polymeric nanoparticles were found to have an inhibitory effect against important bacterial pathogens, including Gram-positive methicillin-resistant Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa. Salt-stable particles were formulated with the addition of a surfactant, which did not inhibit the antibacterial activity of the polymeric particles. Furthermore, the polymeric nanoparticles were found to inhibit S. aureus biofilm formation and exhibited low cytotoxicity against mammalian liver cells. Interaction of the polymeric particles with cellular thiols could be a potential mechanism of action against bacterial cells, as demonstrated by reaction with cysteine as a model thiol. The findings presented demonstrate methods of preparing aqueous dispersions of high sulfur content polymeric nanoparticles that could have useful biological applications.

Redox-Responsive Drug Delivery Systems: A Chemical Perspective

With the widespread global impact of cancer on humans and the extensive side effects associated with current cancer treatments, a novel, effective, and safe treatment is needed. Redox-responsive drug delivery systems (DDSs) have emerged as a potential cancer treatment with minimal side effects and enhanced site-specific targeted delivery. This paper explores the physiological and biochemical nature of tumors that allow for redox-responsive drug delivery systems and reviews recent advances in the chemical composition and design of such systems. The five main redox-responsive chemical entities that are the focus of this paper are disulfide bonds, diselenide bonds, succinimide-thioether linkages, tetrasulfide bonds, and platin conjugates. Moreover, as disulfide bonds are the most commonly used entities, the review explored disulfide-containing liposomes, polymeric micelles, and nanogels. While various systems have been devised, further research is needed to advance redox-responsive drug delivery systems for cancer treatment clinical applications.

Antioxidant action of persulfides and polysulfides against free radical-mediated lipid peroxidation

Hydrogen sulfide, hydropersulfides, and hydropolysulfides have been revealed to play important physiological roles such as cell signaling and protection against oxidative stress, but the underlying mechanisms and dynamics of action remain elusive. It is generally accepted that these species act by two-electron redox mechanisms, while the involvement of one-electron redox chemistry has received less attention. In this study, the radical-scavenging activity of hydrogen persulfide, hydrogen polysulfides (HS_nH n = 2-4), and diallyl- or dialkyl-sulfides (RS_nR, n = 1-4) was measured. Furthermore, their antioxidant effects against free radical-mediated human plasma lipid peroxidation were assessed by measuring lipid hydroperoxides. It was found that disodium disulfide, trisulfide, and tetrasulfide acted as potent peroxyl radical scavengers, the rate constant for scavenging peroxyl radical being 3.5 × 10⁵, 4.0 × 10⁵, and 6.0 × 10⁵ M^-1 s^-1 in PBS pH 7.4 at 37 °C respectively and that they inhibited plasma lipid peroxidation efficiently, the efficacy is increased with the catenation number. Disodium tetrasulfide was 1.5 times as reactive as Trolox toward peroxyl radical and inhibited plasma lipid peroxidation more efficiently than ascorbate and Trolox. On the other hand, diallyl- and dialkyl-sulfides did not exert significant radical-scavenging activity, nor did they inhibit lipid peroxidation efficiently, except for diallyl tetrasulfide, which suppressed plasma lipid peroxidation, despite less significantly than disodium tetrasulfide. Collectively, this study shows that hydrogen persulfide and hydrogen polysulfides act as potent radical-scavenging antioxidants and that, in addition to two-electron redox mechanisms, one electron redox reaction may also play important role in the in vivo defense against deleterious oxidative stress.

Hydropersulfides Inhibit Lipid Peroxidation and Protect Cells from Ferroptosis

Hydropersulfides (RSSH) are believed to serve important roles in vivo, including as scavengers of damaging oxidants and electrophiles. The α-effect makes RSSH not only much better nucleophiles than thiols (RSH), but also much more potent H-atom transfer agents. Since HAT is the mechanism of action of the most potent small-molecule inhibitors of phospholipid peroxidation and associated ferroptotic cell death, we have investigated their reactivity in this context. Using the fluorescence-enabled inhibited autoxidation (FENIX) approach, we have found RSSH to be highly reactive toward phospholipid-derived peroxyl radicals (k_inh = 2 × 10⁵ M^-1 s^-1), equaling the most potent ferroptosis inhibitors identified to date. Related (poly)sulfide products resulting from the rapid self-reaction of RSSH under physiological conditions (e.g., disulfide, trisulfide, H₂S) are essentially unreactive, but combinations from which RSSH can be produced in situ (i.e., polysulfides with H₂S or thiols with H₂S₂) are effective. In situ generation of RSSH from designed precursors which release RSSH via intramolecular substitution or hydrolysis improve the radical-trapping efficiency of RSSH by minimizing deleterious self-reactions. A brief survey of structure-reactivity relationships enabled the design of new precursors that are more efficient. The reactivity of RSSH and their precursors translates from (phospho)lipid bilayers to cell culture (mouse embryonic fibroblasts), where they were found to inhibit ferroptosis induced by inactivation of glutathione peroxidase-4 (GPX4) or deletion of the gene encoding it. These results suggest that RSSH and the pathways responsible for their biosynthesis may act as a ferroptosis suppression system alongside the recently discovered FSP1/ubiquinone and GCH1/BH₄/DHFR systems.

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.

Efficient inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by sulfuration with solubilized elemental sulfur

Hydrogen sulfide (H₂S), carbon monoxide (CO), and nitric oxide (NO) have garnered increasing scientific interest in recent decades due to their classifications as members of the gasotransmitter family of signaling molecules. Due to the versatility of sulfur redox chemistry in biological systems, H₂S specifically is being studied for its ability to modulate cellular redox environments, particularly through the downstream production of oxidized sulfur species. A major mechanism of this regulation is through a posttranslational modification known as persulfidation, where oxidized sulfur atoms are appended to free cysteine in proteins. Currently, it is difficult to discern the activity of H₂S itself versus these oxidized sulfur species, particularly sulfane sulfur (S⁰). We have previously developed a method of solvating S₈, a source of pure S⁰, to more accurately study persulfidation and sulfuration in general. Here, we apply this pure S⁰ to glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which has previously been shown to be inhibited by S⁰-containing polysulfides via persulfidation. Using solvated S⁰, we demonstrate that native, reduced GAPDH can be completely inhibited by sulfuration with S⁰. Further, oxidized GAPDH activity cannot be rescued using S⁰, demonstrating that it is the oxidation of reduced GAPDH by S⁰ that curtails its activity. We also compare inhibition of GAPDH by pure S⁰ to different polysulfides and demonstrate the modulating effects that pendant alkyl groups have on GAPDH inhibition. These results highlight the promise of this novel, simplified system for the study of S⁰.

Stabilities of Three Key Biological Trisulfides with Implications for Their Roles in the Release of Hydrogen Sulfide and Bioaccumulation of Sulfane Sulfur

Trisulfides and higher polysulfides are important in the body due to their function as key reservoirs of sulfane sulfur and their rapid reactions to release persulfides. Recent work has shown that persulfides act as powerful antioxidants and release hydrogen sulfide, an emerging gasotransmitter with numerous therapeutic effects. Despite the important role of polysulfides, there is a lack of understanding of their stabilities in aqueous systems. To investigate the reactivity of trisulfides and polysulfides, three key biologically important trisulfides were synthesized from cysteine, glutathione, and N-acetylcysteine, and the tetrasulfide of N-acetylcysteine was synthesized as a representative polysulfide. The stabilities of sulfides were monitored in buffered D₂O using ¹H NMR spectroscopy under a range of conditions including high temperatures and acidic and alkaline environments. The tri- and tetrasulfides degraded rapidly in the presence of primary and tertiary amines to the corresponding disulfide and elemental sulfur. The half-lives of N-acetylcysteine tri- and tetrasulfides in the presence of butylamine were 53 and 1.5 min, respectively. These results were important because they suggest that tri- and tetrasulfide linkages are short-lived species in vivo due to the abundance of amines in the body. Under basic conditions, cysteine and glutathione trisulfides were unstable due to the deprotonation of the ammonium group, exposing an amine; however, N-acetylcysteine trisulfide was stable at all pH values tested. Hydrogen sulfide release of each polysulfide in the presence of cysteine was quantified using a hydrogen sulfide-sensitive electrode and ¹H NMR spectroscopy.

Esterase-Activated Perthiocarbonate Persulfide Donors Provide Insights into Persulfide Persistence and Stability

Persulfides (RSSH) are important reactive sulfur species (RSS) that are intertwined with the biological functions of hydrogen sulfide (H₂S). The direct study of persulfides is difficult, however, due to their both nucleophilic and electrophilic character, which leads to the generation of an equilibrium of different RSS. To investigate the effects of persulfides directly, especially in biological systems, persulfide donors are needed to generate persulfides in situ. Here, we report the synthesis of esterase-activated perthiocarbonate persulfide donors and investigate the effects of structural modifications on persulfide release. Although steric bulk of the ester did not significantly alter persulfide release kinetics, increased steric bulk of the thiol increased the persulfide release rate. In addition, we found that the steric bulk and identity of the thiol significantly impact persulfide persistence. Further mechanistic investigations into different competing reaction pathways from perthiocarbonates revealed that multiple RSS can be delivered (i.e., H₂S, COS, or RSSH) depending on the persulfide donor structure and activator identity.

A ROS-responsive, self-immolative and self-reporting hydrogen sulfide donor with multiple biological activities for the treatment of myocardial infarction

Myocardial infarction (MI), as one of the leading causes of global death, urgently needs effective therapies. Recently, hydrogen sulfide (H₂S) has been regarded as a promising therapeutic agent for MI, while its spatiotemporally controlled delivery remains a major issue limiting clinical translation. To address this limitation, we designed and synthesized a novel H₂S donor (HSD-R) that can produce H₂S and emit fluorescence in response to reactive oxygen species (ROS) highly expressed at diseased sites. HSD-R can specifically target mitochondria and provide red fluorescence to visualize and quantify H₂S release in vitro and in vivo. Therapeutically, HSD-R significantly promoted the reconstruction of cardiac structure and function in a rat MI model. Mechanistically, myocardial protection is achieved by reducing cardiomyocyte apoptosis, attenuating local inflammation, and promoting angiogenesis. Furthermore, inhibition of typical pro-apoptotic genes (Bid, Apaf-1, and p53) played an important role in the anti-apoptotic effect of HSD-R to achieve cardioprotection, which were identified as new therapeutic targets of H₂S against myocardial ischemia injury. This ROS-responsive, self-immolative, and fluorescent H₂S donor can serve as a new theranostic agent for MI and other ischemic diseases.

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A reactive oxygen species-responsive and self-reporting H₂S donor (HSD-R) is developed for controlled H₂S delivery.

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HSD-R shows desirable fluorescence for imaging H₂S release upon triggering by reactive oxygen species.

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HSD-R displays significant cardioprotective effects in rats.

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HSD-R exhibits multiple biological activities including anti-apoptotic, anti-inflammatory, and pro-angiogenic effects.

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Anti-apoptotic activity of HSD-R is due to inhibiting the expression of several pro-apoptotic factors.

Polymerization of Aniline Derivatives to Yield Poly[N,N-(phenylamino)disulfides] as Polymeric Auxochromes

Polymerizations of phenylamines with a disulfide transfer reagent to yield poly[N,N-(phenylamino) disulfides] (poly-NADs) were investigated due to their unique repeat units that resulted in conjugation along the backbone that was perturbed by the aromatic rings and gave different colors for the polymers. These polymers were synthesized from 10 different anilines and sulfur monochloride in a step-growth polymerization. The polymers were characterized by nuclear magnetic resonance spectroscopy, size exclusion chromatography-multiangle light scattering, and UV-vis spectroscopy. These polymers possessed a polymeric backbone solely consisting of nitrogen and sulfur [-N(R)SS-], which was conjugated and yielded polymers of moderate molecular weight. Most notably, these polymers were an array of colors ranging from pale yellow to a deep purple depending on the substitution of the aromatic ring. The more electron-poor systems produced lighter yellow polymers, while the electron-rich systems gave orange, green, red, and even purple polymers.

Photocatalytic Activation of Elemental Sulfur Enables a Chemoselective Three-Component Thioesterification

A mild and chemoselective three-component thioesterification using olefins, α-ketoacids, and elemental sulfur has been developed. The photocatalytic activation of elemental sulfur, a cheap and abundant sulfur source, enables the rapid installation of a sulfur atom into molecules, reactions that ordinarily would require the use of reactive and malodorous sulfur-containing compounds such as thiols and thioacids. This novel reaction is characterized by high yields and a broad substrate scope, which enables the introduction of thioester moieties into complex molecules including a steroid, a peptide, and a nonprotected glycoside. Mechanistic studies indicated that the success of this transformation depends on the multiple roles played by the elemental sulfur, including those of a sulfurizing agent, a terminal oxidant, and a HAT mediator.

A rare case of a 2:2:1 ternary cocrystal of pyridine sulfides and trithiocyanuric acid

We report a rare case of a 2:2:1 ternary cocrystal consisting of two trithiocyanuric acid molecules, two bis(pyridin-4-yl) sulfide molecules and 1,4-bis(pyridin-4-yl)tetrasulfane, namely, 1,3,5-triazinane-2,4,6-trithione-4-(pyridin-4-ylsulfanyl)pyridine-1,4-bis(pyridin-4-yl)tetrasulfane (2/2/1), 2C₃H₃N₃S₃·2C₁₀H₈N₂S·C₁₀H₈N₂S₄. This interesting crystal structure with five neutral molecules per asymmetric unit was synthesized and characterized by means of X-ray diffraction (XRD) experiments and quantum-chemical modelling. Among various specific interactions, hydrogen and halogen bridges have a significant role in stabilizing the crystal structure. In particular, the role played by stacking interactions has been revealed by structure analysis and theoretical calculations. Crystallization was spontaneous and reproducible. One of the components, 1,4-bis(pyridin-4-yl)tetrasulfane, has been characterized by XRD for the first time.

Synthesis and Hydrogen Sulfide Releasing Properties of Diaminodisulfides and Dialkoxydisulfides

Heterosubstituted disulfides are an understudied class of molecules that have been used in biological studies, but they have not been investigated for their ability to release hydrogen sulfide (H₂S). The synthesis of two sets of chemicals with the diaminodisulfide (NSSN) and dialkoxydisulfide (OSSO) functional groups was reported. These chemicals were synthesized from commercially available sulfur monochloride or a simple disulfur transfer reagent. Both the diaminodisulfide and dialkoxydisulfide functional groups were found to have rapid rates of H₂S release in the presence of excess thiol. The release of H₂S was complete with 10 min, and the only byproducts were conversion of the thiols into disulfides and the amines or alcohols originally used in the synthesis of the diaminodisulfide or dialkoxydisulfide functional groups. These results will allow the design of H₂S releasing chemicals that also release natural, biocompatible alcohols or amines. Chemicals with the diaminodisulfide and dialkoxydisulfide functional groups may find applications in medicine where a controlled, burst release of H₂S is needed.

S-desulfurization: A different covalent modification mechanism from persulfidation by GSH

Post-translational transformation of cysteine residues to persulfides, known as protein S-sulfhydration or persulfidation, is a beneficial H₂S signaling mechanism. In this paper, we found that GSH is bound to active cysteine sites of protein by S-desulfurization, which is a new covalent modification mechanism of protein, thus regulating catalytic activity. Here, we provide direct evidence that GSH modifies the reactive cysteine residues of four enzymes (alliinase/D-LDH/ADH/G6PD) and generates protein-SG or protein-SSG derivatives by S-desulfurization. S-desulfurization, α-carbon nucleophilic substitution or thiol-disulfide exchange occurs and H₂S is released as a by-product. S-desulfurization is the opposite of persulfidation in terms of H₂S production/consumption and enzyme inhibition/mitigation. Here, we elucidated the GSH mechanisms and H₂S mechanisms in the enzyme-metabolite system and the beneficial roles of persulfidation and S-desulfurization. These theoretical findings are now shedding light on understanding GSH and H₂S molecular functions and providing new theoretical basis for them in cell signaling pathways.

Trisulfide linked cholesteryl PEG conjugate attenuates intracellular ROS and collagen-1 production in a breast cancer co-culture model

The progression of cancer has been closely-linked with augmentation of cellular reactive oxygen species (ROS) levels and ROS-associated changes in the tumour microenvironment (TME), including alterations to the extracellular matrix and associated low drug uptake. Herein we report the application of a co-culture model to simulate the ROS based cell-cell interactions in the TME using fibroblasts and breast cancer cells, and describe how novel reactive polymers can be used to modulate those interactions. Under the co-culture conditions, both cell types exhibited modifications in behaviour, including significant overproduction of ROS in the cancer cells, and elevation of the collagen-1 secretion and stained actin filament intensity in the fibroblasts. To examine the potential of using reactive antioxidant polymers to intercept ROS communication and thereby manipulate the TME, we employed H2S-releasing macromolecular conjugates which have been previously demonstrated to mitigate ROS production in HEK cells. The specific conjugate used, mPEG-SSS-cholesteryl (T), significantly reduced ROS levels in co-cultured cancer cells by approximately 50%. This reduction was significantly greater than that observed with the other positive antioxidant controls. Exposure to T was also found to downregulate levels of collagen-1 in the co-cultured fibroblasts, while exhibiting less impact on cells in mono-culture. This would suggest a possible downstream effect of ROS-mitigation by T on stromal-tumour cell signalling. Since fibroblast-derived collagens modulate crucial steps in tumorigenesis, this ROS-associated effect could potentially be harnessed to slow cancer progression. The model may also be beneficial for interrogating the impact of antioxidants on naturally enhanced ROS levels, rather than relying on the application of exogenous oxidants to simulate elevated ROS levels.

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.

Trisulfide-Bearing PEG Brush Polymers Donate Hydrogen Sulfide and Ameliorate Cellular Oxidative Stress

A potential approach to combat cellular dysfunction is to manipulate cell communication and signaling pathways to restore physiological functions while protecting unaffected cells. For instance, delivering the signaling molecule H₂S to certain cells has been shown to restore cell viability and re-normalize cell behavior. We have previously demonstrated the ability to incorporate a trisulfide-based H₂S-donating moiety into linear polymers with good in vitro releasing profiles and demonstrated their potential for ameliorating oxidative stress. Herein, we report two novel series of brush polymers decorated with higher numbers of H₂S-releasing segments. These materials contain two trisulfide-based monomers co-polymerized with oligo(ethylene glycol methyl ether methacrylate) via reversible addition-fragmentation chain-transfer polymerization. The macromolecules were characterized to have a range of trisulfide densities with similar, well-defined molecular weight distribution, good H₂S-releasing profiles, and high cellular tolerance. Using an amperometric technique, the H₂S liberated and total sulfide release were found to depend on concentrations and chemical nature of triggering molecules (glutathione and cysteine) and, importantly, the position of reactive groups within the brush structure. Notably, when introduced to cells at well-tolerated doses, two macromolecular donors which have the same proportion as of the H₂S-donating monomer (30%) but differ in releasing moiety location show similar cellular H₂S-releasing kinetics. These donors can restore reactive oxygen species levels to baseline values, when polymer pretreated cells are exposed to exogenous oxidants (H₂O₂). Our work opens up a new aspect in preparing H₂S macromolecule donors and their application to arresting cellular oxidative cascades.

SG1002 and Catenated Divalent Organic Sulfur Compounds as Promising Hydrogen Sulfide Prodrugs

Significance: Sulfur has a critical role in protein structure/function and redox status/signaling in all living organisms. Although hydrogen sulfide (H2S) and sulfane sulfur (SS) are now recognized as central players in physiology and pathophysiology, the full scope and depth of sulfur metabolome's impact on human health and healthy longevity has been vastly underestimated and is only starting to be grasped. Since many pathological conditions have been related to abnormally low levels of H2S/SS in blood and/or tissues, and are amenable to treatment by H2S supplementation, development of safe and efficacious H2S donors deserves to be undertaken with a sense of urgency; these prodrugs also hold the promise of becoming widely used for disease prevention and as antiaging agents. Recent Advances: Supramolecular tuning of the properties of well-known molecules comprising chains of sulfur atoms (diallyl trisulfide [DATS], S8) was shown to lead to improved donors such as DATS-loaded polymeric nanoparticles and SG1002. Encouraging results in animal models have been obtained with SG1002 in heart failure, atherosclerosis, ischemic damage, and Duchenne muscular dystrophy; with TC-2153 in Alzheimer's disease, schizophrenia, age-related memory decline, fragile X syndrome, and cocaine addiction; and with DATS in brain, colon, gastric, and breast cancer. Critical Issues: Mode-of-action studies on allyl polysulfides, benzyl polysulfides, ajoene, and 12 ring-substituted organic disulfides and thiosulfonates led several groups of researchers to conclude that the anticancer effect of these compounds is not mediated by H2S and is only modulated by reactive oxygen species, and that their central model of action is selective protein S-thiolation. Future Directions: SG1002 is likely to emerge as the H2S donor of choice for acquiring knowledge on this gasotransmitter's effects in animal models, on account of its unique ability to efficiently generate H2S without byproducts and in a slow and sustained mode that is dose independent and enzyme independent. Efficient tuning of H2S donation characteristics of DATS, dibenzyl trisulfide, and other hydrophobic H2S prodrugs for both oral and parenteral administration will be achieved not only by conventional structural modification of a lead molecule but also through the new "supramolecular tuning" paradigm.

Trisulfide bond-mediated doxorubicin dimeric prodrug nanoassemblies with high drug loading, high self-assembly stability, and high tumor selectivity

Rational design of nanoparticulate drug delivery systems (nano-DDS) for efficient cancer therapy is still a challenge, restricted by poor drug loading, poor stability, and poor tumor selectivity. Here, we report that simple insertion of a trisulfide bond can turn doxorubicin homodimeric prodrugs into self-assembled nanoparticles with three benefits: high drug loading (67.24%, w/w), high self-assembly stability, and high tumor selectivity. Compared with disulfide and thioether bonds, the trisulfide bond effectively promotes the self-assembly ability of doxorubicin homodimeric prodrugs, thereby improving the colloidal stability and in vivo fate of prodrug nanoassemblies. The trisulfide bond also shows higher glutathione sensitivity compared to the conventional disulfide bond, and this sensitivity enables efficient tumor-specific drug release. Therefore, trisulfide bond-bridged prodrug nanoassemblies exhibit high selective cytotoxicity on tumor cells compared with normal cells, notably reducing the systemic toxicity of doxorubicin. Our findings provide new insights into the design of advanced redox-sensitive nano-DDS for cancer therapy.

Polysulfuration via a Bilateral Thiamine Disulfurating Reagent

An efficient moduling disulfuration was developed for polysulfide construction via a bilateral six-membered thiamine disulfurating reagent. Under the control of energy release of ring strain, diverse unsymmetrical trisulfides and tetrasulfides were generated through the assembly of nucleophiles on both sides of the sulfur-sulfur motif. This strategy exhibits features of high efficiency, mild conditions, and general scope.

Modified cyclodextrins solubilize elemental sulfur in water and enable biological sulfane sulfur delivery

An important form of biological sulfur is sulfane sulfur, or S⁰, which is found in polysulfide and persulfide compounds as well as in elemental sulfur. Sulfane sulfur, often in the form of S₈, functions as a key energy source in the metabolic processes of thermophilic Archaean organisms found in sulfur-rich environments and can be metabolized both aerobically and anaerobically by different archaeons. Despite this importance, S₈ has a low solubility in water (∼19 nM), raising questions of how it can be made chemically accessible in complex environments. Motivated by prior crystallographic data showing S₈ binding to hydrophobic motifs in filamentous glycoproteins from the sulfur reducing Staphylothermus marinus anaerobe, we demonstrate that simple macrocyclic hydrophobic motifs, such as 2-hydroxypropyl β-cyclodextrin (2HPβ), are sufficient to solubilize S₈ at concentrations up to 2.0 ± 0.2 mM in aqueous solution. We demonstrate that the solubilized S₈ can be reduced with the common reductant tris(2-carboxyethyl)phosphine (TCEP) and reacts with thiols to generate H₂S. The thiol-mediated conversion of 2HPβ/S₈ to H₂S ranges from 80% to quantitative efficiency for Cys and glutathione (GSH). Moreover, we demonstrate that 2HPβ can catalyze the Cys-mediated reduction of S₈ to H₂S in water. Adding to the biological relevance of the developed systems, we demonstrate that treatment of Raw 264.7 macrophage cells with the 2HPβ/S₈ complex prior to LPS stimulation decreases NO₂ ^- levels, which is consistent with known activities of bioavailable H₂S and sulfane sulfur. Taken together, these investigations provide a new strategy for delivering H₂S and sulfane sulfur in complex systems and more importantly provide new insights into the chemical accessibility and storage of S⁰ and S₈ in biological environments.

Efficient Polysulfide-Based Nanotheranostics for Triple-Negative Breast Cancer: Ratiometric Photoacoustics Monitored Tumor Microenvironment-Initiated H2 S Therapy

The incidence of triple-negative breast cancer (TNBC) is difficult to predict, and TNBC has a high mortality rate among women worldwide. In this study, a theranostics approach is developed for TNBC with ratiometric photoacoustic monitored thiol-initiated hydrogen sulfide (H₂ S) therapy. The ratiometric photoacoustic (PA) probe (CY) with a thiol-initiated H₂ S donor (PSD) to form a nanosystem (CY-PSD nanoparticles) is integrated. In this theranostics approach, H₂ S generated from PSD is sensed by CY based on ratiometric PA signals, which simultaneously pinpoints the tumor region. Additionally, H₂ S is cytotoxic toward TNBC cells (MDA-MB 231), showing a tumor inhibition rate of 63%. To further verify its pharmacological mechanism, proteomics analysis is performed on tumors treated with CY-PSD nanoparticles. Cells are killed by the significant mitochondrial dysfunction via supressed energy supply and apoptosis initiation. Besides, the observed inhibition of oxidative stress also generates the cytotoxicity. Significant Kyoto Encyclopedia of Genes Genomes pathways related to TNBC are found to be inhibited. This H₂ S theranostics approach updates the current anticancer therapies which brings promise for women suffering malignant breast cancer.

A novel LC-HRMS method reveals cysteinyl and glutathionyl polysulfides in wine

A novel ultra high pressure liquid chromatography combined with high resolution mass spectrometry (UHPLC-HRMS) method was developed to study glutathionyl and cysteinyl polysulfides in wine. Different HPLC columns were investigated in order to optimise the chromatographic resolution of the polysulfide standard mixtures synthesised in-house. The optimisation of the chromatographic conditions when trying to separate glutathionylated and cysteinylated species containing from 3 to 5 sulfur atoms proved particularly challenging, with the cationic exchange column IonPac CS12A-MS resulting to be the best column for this task.The synergistic application of the newly developed methods together with the synthesised reference standard mixtures allowed the identification and the detection of 11 different glutathionyl and cysteinyl polysulfides. Moreover, analysing 15 young white wines was possible to confirm the presence of GSSSG in wine (GS = glutathione). More importantly, this study allowed for the first identification of several symmetric and asymmetric new polysulfides, namely: GSSSSG, CSSSC (CS = cysteine), CSSSSC, CSSSG, and CSSSSG. These molecules have not previously been identified in wine, raising the question on their biogenesis and role on wine quality.

Synthesis, Spectroscopic, and Structural Characterization of Organyl Disulfanides and a Tetrasulfanide

Various room-temperature-stable monoorganylpolysulfanides of the form [X][RS_n] (X = [PPh₄]⁺, [PNP]⁺, [NEt₄]⁺; R = Ph, t-Bu, n ≥ 2) were synthesized in a simple and versatile one-step process starting from sodium thiolates and elemental sulfur. The compounds were characterized by X-ray crystal structure analysis, NMR spectroscopy, microelemental analysis, and electrospray mass ionization spectrometry including collision-induced dissociation experiments. While these salts are well-defined species as crystals, they undergo complex equilibria in solution. In one case, compounds ranging from n = 1-8 have been observed in solution. Structural features, dynamics in solution, as well as thermochromic properties of one of the compounds, [PPh₄][PhS₂], are investigated in detail by temperature- and pressure-dependent X-ray crystal structure analysis. The experimental data are complemented by periodic boundary density functional theory calculations on the crystal structures, as well as energy decomposition analyses.

The biothiol-triggered organotrisulfide-based self-immolative fluorogenic donors of hydrogen sulfide enable lysosomal trafficking

Biothiol-reactive organotrisulfide-based self-immolative fluorogenic donors of H2S are rationally designed for the efficient monitoring of intracellular and lysosomal trafficking of H2S with a concomitant turn-on fluorescence. The non-toxic nature of the donors with a sustained release of H2S will certainly be helpful for their biomedical applications in the future.

Radical Substitution Provides a Unique Route to Disulfides

Radical substitution on tetrasulfides is demonstrated to be a highly effective means to prepare unsymmetric disulfides. Alkyl and aryl radicals generated thermally or photochemically underwent substitution on readily prepared dialkyl, diaryl, and diacyl tetrasulfides to yield the corresponding disulfides in good to excellent yields. Classic and contemporary thermal and photochemical radical sources could be employed; while photoredox catalysis approaches led to either oxidation or reduction of the tetrasulfide, energy transfer photocatalysis was particularly useful. The success of the approach is driven by the thermodynamic stability of the perthiyl radicals formed upon substitution on the tetrasulfide; they simply combine under the reaction conditions to provide the starting tetrasulfide. Competition kinetic experiments reveal that alkyl radical substitution on tetrasulfides is a rapid reaction (6 × 10⁵ M^-1 s^-1) that is enhanced at least 6-fold upon moving from dialkyl tetrasulfide to diacyl tetrasulfide due to favorable polar effects. This unique and versatile reaction enables introduction of disulfide moieties from a variety of radical precursors and straightforward access to hydropersulfides.

H2S-Donating trisulfide linkers confer unexpected biological behaviour to poly(ethylene glycol)-cholesteryl conjugates

Inspired by the properties of the naturally occurring H₂S donor, diallyl trisulfide (DATS, extracted from garlic), the biological behaviour of trisulfide-bearing PEG-conjugates was explored. Specifically, three conjugates comprising an mPEG tail and a cholesteryl head were investigated: conjugates bridged by a trisulfide linker (T), a disulfide linker (D) or a carbamate linker (C), and a fourth comprising two mPEG tails bridged by a trisulfide linker (P). H₂S testing using both a fluorescent chemical probe in HEK293 cells and an amperometric sensor to monitor release in suspended cells, demonstrated the ability of the trisulfide conjugates, T and P, to release H₂S in the presence of cellular thiols. Cytotoxicity and cyto-protective capacity on HEK293 cells showed that T was the best tolerated of the conjugates studied, and remarkably more so than D or C. Moreover, it was noted that application of T conferred a protective effect to the cells, effectively abolishing the toxicity associated with co-administered C. The interaction of conjugates and combinations thereof with the cell membrane of HEK cells, as well as ROS generation were also investigated. It was found that C caused significant membrane perturbation, correlating with high losses in cell viability and pronounced generation of ROS, especially in the mitochondria. T, however, did not disturb the membrane and was able to mitigate the generation of ROS, especially in the mitochondria. The interplay of the cholesteryl group and H₂S donation for conferring cytoprotective effects was clearly demonstrated as P did not display the same beneficial characteristics as T.

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.

Role of Hydrogen Sulfide in Chronic Diseases

In the past, hydrogen sulfide (H₂S) was considered as a poisonous gas or waste of the body. Later, researchers found that H₂S-producing enzymes exist in mammals. Moreover, their findings indicated that endogenous H₂S was associated with the occurrence of many diseases. Therefore, endogenous H₂S is able to participate in the regulation of physiological and pathological functions of the body as a gas signaling molecule. In this review, we summarize the regulation mechanism of endogenous H₂S on the body, such as proliferation, apoptosis, migration, angiogenesis, as well as vasodilation/vasoconstriction. Furthermore, we also analyze the relationship between H₂S and some chronic diseases, including hypoxic pulmonary hypertension, myocardial infarction, ischemic perfusion kidney injury, diabetes, and chronic intestinal diseases. Finally, we discuss dietary restriction and drugs that target for H₂S. Hence, H₂S is expected to become a potential target for treatment of these chronic diseases.

An Appraisal of Developments in Allium Sulfur Chemistry: Expanding the Pharmacopeia of Garlic

Alliums and allied plant species are rich sources of sulfur compounds that have effects on vascular homeostasis and the control of metabolic systems linked to nutrient metabolism in mammals. In view of the multiple biological effects ascribed to these sulfur molecules, researchers are now using these compounds as inspiration for the synthesis and development of novel sulfur-based therapeutics. This research has led to the chemical synthesis and biological assessment of a diverse array of sulfur compounds representative of derivatives of S-alkenyl-l-cysteine sulfoxides, thiosulfinates, ajoene molecules, sulfides, and S-allylcysteine. Many of these synthetic derivatives have potent antimicrobial and anticancer properties when tested in preclinical models of disease. Therefore, the current review provides an overview of advances in the development and biological assessment of synthetic analogs of allium-derived sulfur compounds.

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.

Electronic structures and binding motifs of sodium polysulfide clusters NaSn - (n = 5-9): A joint negative ion photoelectron spectroscopy and computational investigation

We report a joint experimental and computational study on the electronic and geometric structures of a series of NaS_n ^- (n = 5-9) clusters. Cryogenic, size-selective, negative ion photoelectron spectroscopy was employed to obtain their photoelectron spectra, in which distinctive spectral features with electron binding energy (EBE) up to 6.4 eV are unraveled. The EBE of the first peak in each spectrum for NaS_n ^- (n = 5-9), assigned to the transition from the ground state of the anion to the ground state of each neutral radical, was observed to increase with cluster size. The vertical detachment energies (VDEs), measured from the first peak maximum, are 3.43 ± 0.02, 3.57 ± 0.02, 3.82 ± 0.03, 3.86 ± 0.02, and 4.00 ± 0.02 eV, and the adiabatic detachment energies (ADEs), determined from the onset of the first peak, are 3.27 ± 0.05, 3.44 ± 0.05, 3.65 ± 0.05, 3.75 ± 0.05, and 3.93 ± 0.05 eV, for n = 5-9, respectively. A number of low-lying isomers of the anions were screened and identified with density functional theory calculations, showing a structural preference of a chainlike polysulfide moiety electrostatically interacting with a sodium cation for all of the clusters. The CCSD(T)/aug-cc-pVTZ calculated VDEs and ADEs are in excellent agreement with the experimental results, confirming the identified isomers. Further analyses based on excited-state transitions, molecular orbitals, and natural population charges were performed, to assign and reveal the nature of all observed spectral bands. These computational results suggest that the electron detachment process and observed excitations are mainly derived from the polysulfide chain within each NaS_n ^- cluster. This work provides a fundamental understanding of the intrinsic molecular properties of sodium polysulfide systems, which widely exist in life science and sodium-sulfur cells.

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.

Fluorogenic hydrogen sulfide (H2S) donors based on sulfenyl thiocarbonates enable H2S tracking and quantification

Hydrogen sulfide (H2S) is an important cellular signaling molecule that exhibits promising protective effects. Although a number of triggerable H2S donors have been developed, spatiotemporal feedback from H2S release in biological systems remains a key challenge in H2S donor development. Herein we report the synthesis, evaluation, and application of caged sulfenyl thiocarbonates as new fluorescent H2S donors. These molecules rely on thiol cleavage of sulfenyl thiocarbonates to release carbonyl sulfide (COS), which is quickly converted to H2S by carbonic anhydrase (CA). This approach is a new strategy in H2S release and does not release electrophilic byproducts common from COS-based H2S releasing motifs. Importantly, the release of COS/H2S is accompanied by the release of a fluorescent reporter, which enables the real-time tracking of H2S by fluorescence spectroscopy or microscopy. Dependent on the choice of fluorophore, either one or two equivalents of H2S can be released, thus allowing for the dynamic range of the fluorescent donors to be tuned. We demonstrate that the fluorescence response correlates directly with quantified H2S release and also demonstrate the live-cell compatibility of these donors. Furthermore, these fluorescent donors exhibit anti-inflammatory effects in RAW 264.7 cells, indicating their potential application as new H2S-releasing therapeutics. Taken together, sulfenyl thiocarbonates provide a new platform for H2S donation and readily enable fluorescent tracking of H2S delivery in complex environments.

Effects of sulfane sulfur content in benzyl polysulfides on thiol-triggered H2S release and cell proliferation

Investigations into hydrogen sulfide (H2S) signaling pathways have demonstrated both the generation and importance of persulfides, which are reactive sulfur species that contain both reduced and oxidized sulfur. These observations have led researchers to suggest that oxidized sulfur species, including sulfane sulfur (S0), are responsible for many of the physiological phenomena initially attributed to H2S. A common method of introducing S0 to biological systems is the administration of organic polysulfides, such as diallyl trisulfide (DATS). However, prior reports have demonstrated that commercially-available DATS often contains a mixture of polysulfides, and furthermore a lack of structure-activity relationships for organic polysulfides has limited our overall understanding of different polysulfides and their function in biological systems. Advancing our interests in the chemical biology of reactive sulfur species including H2S and S0, we report here our investigations into the rates and quantities of H2S release from a series of synthetic, pure benzyl polysulfides, ranging from monosulfide to tetrasulfide. We demonstrate that H2S is only released from the trisulfide and tetrasulfide, and that this release requires thiol-mediated reduction in the presence of cysteine or reduced glutathione. Additionally, we demonstrate the different effects of trisulfides and tetrasulfides on cell proliferation in murine epithelial bEnd.3 cells.

Iterative synthetic strategies and gene deletant experiments enable the first identification of polysulfides in Saccharomyces cerevisiae

Polysulfides, potential signalling molecules, were synthesised and then found and explored for the first time in yeast.

Trisulfides over disulfides: highly selective synthetic strategies, anti-proliferative activities and sustained H2S release profiles

Highly selective synthesis of trisulfides over disulfides is demonstrated along with their potential as anti-proliferative agents and sustained donors of H₂S.