Thermal and Photoinduced Copper-Promoted C-Se Bond Formation: Synthesis of 2-Alkyl-1,2-benzisoselenazol-3(2H)-ones and Evaluation against Mycobacterium tuberculosis

Highly Electrophilic Intermediates in the Bypass Mechanism of Glutathione Peroxidase: Synthesis, Reactivity, and Structures of Selenocysteine-Derived Cyclic Selenenyl Amides

Selenocysteine (Sec)-derived cyclic selenenyl amides, formed by the intramolecular cyclization of Sec selenenic acids (Sec-SeOHs), have been postulated to function as protective forms in the bypass mechanism of glutathione peroxidase (GPx). However, their chemical properties have not been experimentally elucidated in proteins or small-molecule systems. Recently, we reported the first nuclear magnetic resonance observation of Sec-SeOHs and their cyclization to the corresponding cyclic selenenyl amides by using selenopeptide model systems incorporated in a molecular cradle. Herein, we elucidate the structures and reactivities of Sec-derived cyclic selenenyl amides. The crystal structures and reactions toward a cysteine thiol or a 1,3-diketone-type chemical probe indicated the highly electrophilic character of cyclic selenenyl amides. This suggests that they can serve not only as protective forms to suppress the inactivation of Sec-SeOHs in GPx but also as highly electrophilic intermediates in the reactions of selenoproteins.

Identification of Ebselen derivatives as novel SARS-CoV-2 main protease inhibitors: Design, synthesis, biological evaluation, and structure-activity relationships exploration

The main protease (Mpro) represents one of the most effective and attractive targets for designing anti-SARS-CoV-2 drugs. In this study, we designed and synthesized a novel series of Ebselen derivatives by incorporating privileged fragments from different pockets of the Mpro active site. Among these compounds, 11 compounds showed submicromolar activity in the FRET-based SARS-CoV-2 Mpro inhibition assay, with IC50 values ranging from 233 nM to 550 nM. Notably, compound 3a displayed submicromolar Mpro activity (IC50 = 364 nM) and low micromolar antiviral activity (EC50 = 8.01 µM), comparable to that of Ebselen (IC50 = 339 nM, EC50 = 3.78 µM). Time-dependent inhibition assay confirmed that these compounds acted as covalent inhibitors. Taken together, our optimization campaigns thoroughly explored the structural diversity of Ebselen and verified the impact of specific modifications on potency against Mpro.

Organoselenium Compounds: Chemistry and Applications in Organic Synthesis

Selenium, originally described as a toxin, turns out to be a crucial trace element for life that appears as selenocysteine and its dimer, selenocystine. From the point of view of drug developments, selenium-containing drugs are isosteres of sulfur and oxygen with the advantage that the presence of the selenium atom confers antioxidant properties and high lipophilicity, which would increase cell membrane permeation leading to better oral bioavailability. In this article, we have focused on the relevant features of the selenium atom, above all, the corresponding synthetic approaches to access a variety of organoselenium molecules along with the proposed reaction mechanisms. The preparation and biological properties of selenosugars, including selenoglycosides, selenonucleosides, selenopeptides, and other selenium-containing compounds will be treated. We have attempted to condense the most important aspects and interesting examples of the chemistry of selenium into a single article.

Facile synthesis, crystal structure, quantum calculation, and biological evaluations of novel selenenyl sulfide compounds as potential agrochemicals

BACKGROUND

In order to design compounds with fresh molecular skeleton to break through the limitation of available agrochemicals, a series of 36 novel selenenyl sulfide compounds were chemically synthesized, and their biological activities were fully evaluated against tobacco mosaic virus (TMV), 14 plant pathogenic fungi, three insect species and plant acetohydroxyacid synthase (AHAS).

RESULTS

All the target compounds were characterized by proton nuclear magnetic resonance (¹ H-NMR), carbon-13 (¹³ C)-NMR, selenium-77 (⁷⁷ Se)-NMR, and high-resolution mass spectrometry (HRMS). The crystal structure of 10j indicated that the Se-S bond was successfully constructed. Compounds 10d, 10h, 10s, 10u, 10aa, 10ac, 10ae, 10ag, and 10ai exhibited 40%, 43%, 39%, 41%, 47%, 46%, 47%, 42%, and 39% anti-TMV activities at 500 mg L^-1 , better than that of ribavirin. The median effective concentration (EC₅₀ ) against Sclerotinia sclerotiorum of 10ac was 6.69 mg L^-1 and EC₅₀ values against Physalospora piricola and Pyricularia grisea of 10z were 12.25 mg L^-1 and 15.27 mg L^-1 , respectively, superior to the corresponding values of chlorothalonil. Compounds 10c and 10v demonstrated 100% larvicidal activity against Culex pipiens pallens at 5 mg L^-1 , while 10a displayed 100% insecticidal activity against Mythimna separata at 200 mg L^-1 . Compounds 10c, 10j, and 10o showed > 60% inhibitions against plant AHAS at 10 μmol L^-1 . From the quantum calculation, highest occupied molecular orbital (HOMO) was considered as a factor that affects the anti-TMV activity.

CONCLUSION

The preliminary results suggested that more efforts should be devoted to exploring the selenenyl sulfides for the discovery of new leads of antiviral agent, fungicide, insecticide or AHAS inhibitors as potential agrochemicals for crop protection. © 2023 Society of Chemical Industry.

CuBr2-Catalyzed Annulation of 2-Bromo-N-Arylbenzimidamide with Se/S8 Powder for the Synthesis of Benzo[d]isoselenazole and Benzo[d]isothiazole

A CuBr₂-catalyzed annulation of 2-bromo-N-arylbenzimidamide with selenium/sulfur powder for the synthesis of benzo[d]isoselenazole and benzo[d]isothiazole in generally good yields was investigated. This synthetic strategy features good substrate scope and functional group tolerance. Furthermore, the corresponding products could be converted into N-aryl indoles via rhodium^III-catalyzed ortho C-H activation of the N-phenyl ring, providing an efficient approach for axial aromatic molecules.

Small-Molecule Ebselen Binds to YTHDF Proteins Interfering with the Recognition of N 6-Methyladenosine-Modified RNAs

YTHDF proteins bind the N 6-methyladenosine (m6A)-modified mRNAs, influencing their processing, stability, and translation. Therefore, the members of this protein family play crucial roles in gene regulation and several physiological and pathophysiological conditions. YTHDF proteins contain a hydrophobic pocket that accommodates the m6A embedded in the RRACH consensus sequence on mRNAs. We exploited the presence of this cage to set up an m6A-competitive assay and performed a high-throughput screen aimed at identifying ligands binding in the m6A pocket. We report the organoselenium compound ebselen as the first-in-class inhibitor of the YTHDF m6A-binding domain. Ebselen, whose interaction with YTHDF proteins was validated via orthogonal assays, cannot discriminate between the binding domains of the three YTHDF paralogs but can disrupt the interaction of the YTHDF m6A domain with the m6A-decorated mRNA targets. X-ray, mass spectrometry, and NMR studies indicate that in YTHDF1 ebselen binds close to the m6A cage, covalently to the Cys412 cysteine, or interacts reversibly depending on the reducing environment. We also showed that ebselen engages YTHDF proteins within cells, interfering with their mRNA binding. Finally, we produced a series of ebselen structural analogs that can interact with the YTHDF m6A domain, proving that ebselen expansion is amenable for developing new inhibitors. Our work demonstrates the feasibility of drugging the YTH domain in YTHDF proteins and opens new avenues for the development of disruptors of m6A recognition.

Synthesis of Spiro[5.5]trienones- and Spiro[4.5]trienones-Fused Selenocyanates via Electrophilic Selenocyanogen Cyclization and Dearomative Spirocyclization

A practical strategy for the synthesis of spiro[5.5]trienones-fused selenocyanates and spiro[4.5]trienones-fused selenocyanates through electrophilic selenocyanogen cyclization and dearomative spirocyclization is reported. This approach was conducted under mild conditions with broad substrate scope and good functional group tolerance. The utility of this procedure is exhibited in the late-stage functionalization of nature product and drug molecules.

Magnetic mesocellular foams with nickel complexes: as efficient and reusable nanocatalysts for the synthesis of symmetrical and asymmetrical diaryl chalcogenides

In this work, magnetic mesocellular foam (M-MCF) silica nanoparticles were prepared via inserting magnetic nanoparticles into the pores of mesocellular foams, the inner surface of which was functionalized with a methionine-nickel complex (M-MCF@Met-Ni). The structure of the as-prepared nanocatalysts was studied by FT-IR spectroscopy, BET, TGA, VSM, SEM, HR-TEM, EDS, WDX, XRD, and ICP-OES techniques. Thereafter, this nanocatalyst was used as a new, effective, and magnetically reusable catalyst for C-S and C-Se bond formation under mild conditions. All corresponding products were prepared with good yields and appropriate turnover number (TON) and turnover frequency (TOF), which reveals the high activity of this magnetic nanocatalyst in both reactions. In addition, the recovery and hot filtration tests indicated that this catalyst could be simply separated from the reaction mixture using an outside magnet and reused five consecutive times without any significant loss of its catalyst activity or metal leaching.

Ligand-free access to benzisothiazolones and benzisoselenazolones through NiFe2O4 catalyzed concomitant annulation of 2-halobenzanilides with chalcogens and their late-stage transformations

A practical non-hazardous one-pot protocol for the synthesis of benzisothiazolones and benzisoselenazolones, involving magnetically retrievable nano-nickel ferrite catalyzed tandem annulation between 2-halobenzanilides and elemental S8 or Se.

The Mpro structure-based modifications of ebselen derivatives for improved antiviral activity against SARS-CoV-2 virus

The main protease (Mpro or 3CLpro) of SARS-CoV-2 virus is a cysteine enzyme critical for viral replication and transcription, thus indicating a potential target for antiviral therapy. A recent repurposing effort has identified ebselen, a multifunctional drug candidate as an inhibitor of Mpro. Our docking of ebselen to the binding pocket of Mpro crystal structure suggests a noncovalent interaction for improvement of potency, antiviral activity and selectivity. To test this hypothesis, we designed and synthesized ebselen derivatives aimed at enhancing their non-covalent bonds within Mpro. The inhibition of Mpro by ebselen derivatives (0.3 μM) was screened in both HPLC and FRET assays. Nine ebselen derivatives (EBs) exhibited stronger inhibitory effect on Mpro with IC₅₀ of 0.07–0.38 μM. Further evaluation of three derivatives showed that EB2-7 exhibited the most potent inhibition of SARS-CoV-2 viral replication with an IC₅₀ value of 4.08 µM in HPAepiC cells, as compared to the prototype ebselen at 24.61 μM. Mechanistically, EB2-7 functions as a noncovalent Mpro inhibitor in LC-MS/MS assay. Taken together, our identification of ebselen derivatives with improved antiviral activity may lead to developmental potential for treatment of COVID-19 and SARS-CoV-2 infection.

Synthesis of Benzoisoselenazolones via Rh(III)-Catalyzed Direct Annulative Selenation by Using Elemental Selenium

Isoselenazolone derivatives have attracted significant research interest because of their potent therapeutic activities and indispensable applications in organic synthesis. Efficient construction of functionalized isoselenazolone scaffolds is still challenging, and thus new synthetic approaches with improved operational simplicity have been of particular interest. In this manuscript, we introduce a rhodium-catalyzed direct selenium annulation by using stable and tractable elemental selenium. A series of benzamides as well as acrylamides were successfully coupled with selenium under mild reaction conditions, and the obtained isoselenazolones could be pivotal synthetic precursors for several organoselenium compounds. Based on the designed control experiments and X-ray absorption spectroscopy measurements, we propose an unprecedented selenation mechanism involving a highly electrophilic Se(IV) species as the reactive selenium donor. The reaction mechanism was further verified by a computational study.

Ebselen and Analogues: Pharmacological Properties and Synthetic Strategies for Their Preparation

Ebselen is the leader of selenorganic compounds, and starting from its identification as mimetic of the key antioxidant enzyme glutathione peroxidase, several papers have appeared in literature claiming its biological activities. It was the subject of several clinical trials and it is currently in clinical evaluation for the treatment of COVID-19 patients. Given our interest in the synthesis and pharmacological evaluation of selenorganic derivatives with this review, we aimed to collect all the papers focused on the biological evaluation of ebselen and its close analogues, covering the timeline between 2016 and most of 2021. Our analysis evidences that, even if it lacks specificity when tested in vitro, being able to bind to every reactive cysteine, it proved to be always well tolerated in vivo, exerting no sign of toxicity whatever the administered doses. Besides, looking at the literature, we realized that no review article dealing with the synthetic approaches for the construction of the benzo[d][1,2]-selenazol-3(2H)-one scaffold is available; thus, a section of the present review article is completely devoted to this specific topic.

Inhibition of Pseudomonas aeruginosa Alginate Synthesis by Ebselen Oxide and Its Analogues

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that is frequently found in the airways of cystic fibrosis (CF) patients due to the dehydrated mucus that collapses the underlying cilia and prevents mucociliary clearance. During this life-long chronic infection, P. aeruginosa cell accumulates mutations that lead to inactivation of the mucA gene that results in the constitutive expression of algD-algA operon and the production of alginate exopolysaccharide. The viscous alginate polysaccharide further occludes the airways of CF patients and serves as a protective matrix to shield P. aeruginosa from host immune cells and antibiotic therapy. Development of inhibitors of alginate production by P. aeruginosa would reduce the negative impact from this viscous polysaccharide. In addition to transcriptional regulation, alginate biosynthesis requires allosteric activation by bis (3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP) binding to an Alg44 protein. Previously, we found that ebselen (Eb) and ebselen oxide (EbO) inhibited diguanylate cyclase from synthesizing c-di-GMP. In this study, we show that EbO, Eb, ebsulfur (EbS), and their analogues inhibit alginate production. Eb and EbS can covalently modify the cysteine 98 (C98) residue of Alg44 and prevent its ability to bind c-di-GMP. However, P. aeruginosa with Alg44 C98 substituted with alanine or serine was still inhibited for alginate production by Eb and EbS. Our results indicate that EbO, Eb, and EbS are lead compounds for reducing alginate production by P. aeruginosa. Future development of these inhibitors could provide a potential treatment for CF patients infected with mucoid P. aeruginosa.

Design and Use of Electrophilic Thiocyanating and Selenocyanating Reagents: An Interesting Trend for the Construction of SCN- and SeCN-Containing Compounds

Organothiocyanate and organoselenocyanate compounds are of paramount importance in organic chemistry as they are key intermediates to access sulfur- and selenium-containing compounds. Therefore, among the different synthetic pathways to get SCN- and SeCN-containing molecules, original methodologies using electrophilic reagents have recently been explored. This Minireview will showcase the recent advances that have been made. In particular, the design of several electrophilic sources and their applications for the thiocyanation and the selenocyanation of various classes of compounds will be highlighted and discussed.

Synthesis and evaluation of multi-target-directed ligands with BACE-1 inhibitory and Nrf2 agonist activities as potential agents against Alzheimer's disease

Cumulative evidence suggests that β-amyloid and oxidative stress are closely related with each other and play key roles in the process of Alzheimer's disease (AD). Multitarget regulation of both pathways might represent a promising therapeutic strategy. Here, a series of selenium-containing compounds based on ebselen and verubecestat were designed and synthesized. Biological evaluation showed that 13f exhibited good BACE-1 inhibitory activity (IC₅₀ = 1.06 μΜ) and potent GPx-like activity (ν₀ = 183.0 μM min^-1). Aβ production experiment indicated that 13f could reduce the secretion of Aβ1-40 in HEK APPswe 293T cells. Moreover, 13f exerted a cytoprotective effect against the H₂O₂ or 6-OHDA caused cell damage via alleviation of intracellular ROS, mitochondrial dysfunction, Ca²⁺ overload and cell apoptosis. The mechanism studies indicated that 13f exhibited cytoprotective effect by activating the Keap1-Nrf2-ARE pathway and stimulating downstream anti-oxidant protein including HO-1, NQO1, TrxR1, GCLC, and GCLM. In addition, 13f significantly reduced the production of NO and IL-6 induced by LPS in BV2 cells, which confirmed its anti-inflammatory activity as a Nrf2 activator. The BBB permeation assay predicted that 13f was able to cross the BBB. In summary, 13f might be a promising multi-target-directed ligand for the treatment of AD.

Neuroprotective and Anti-neuroinflammatory Properties of Ebselen Derivatives and Their Potential to Inhibit Neurodegeneration

Ebselen (EBS) is an organo-selenium-containing compound that has anti-inflammatory, antitumor, and antibacterial properties. EBS is being explored as a possible treatment for reperfusion injury and stroke and is under clinical trials as a mimetic of lithium for the treatment of bipolar disorder [Mota et al. Synapse 2020, 74 (7), 1-6] and noise-induced hearing loss as a result of these actives [Martini et al. J. Psychiatr. Res. 2019, 109, 107-117. Slusarczyk et al. Neural Regener. Res. 2019, 17 (7), 1255-1261. Thangamani et al. PLoS One 2015, 10 (7), e0133877. Kil et al. Lancet 2017, 390 (10098), 969-979]. However, we wanted to characterize derivatives of EBS as neuroprotective, anti-neuroinflammatory, and antioxidant compounds. Recently, we have reported on a new thermal and photoinduced copper-mediated cross-coupling between potassium selenocyanate (KSeCN) and N-substituted ortho-halobenzamides to form ebselen derivatives with increased synthetic efficiency [Thanna et al. J. Org. Chem. 2017, 82 (7), 3844-3854]. Our synthesis allows for the varying of the remote benzene ring with various substituents or replacing that ring with heterocyclic rings such as pyridine, pyrrole, thiophene, etc. In this study, we synthesized seven new heterocyclic EBS derivatives to further diversify our EBS library. These 21 compounds were then evaluated for their neuroprotective properties, with four compounds showing an equal or better neuroprotective profile than EBS. Compounds 5, 9, 23, and 27 showed 73, 86, 80, 84% cell viability, respectively, at a 10 μM concentration. These studies were performed using human neuroblastoma SH-SY5Y cells in an oxygen and glucose deprivation (OGD) model of ischemia. At the same concentration, these compounds significantly inhibited lipopolysaccharide-induced nitric oxide and tumor necrosis factor alpha release from Human microglia clone 3 microglial cells. Compounds 9 and 27 showed significantly increased cell viability (84 and 80%, respectively) for SH-SY5Y cells exposed to microglia-activated media. These compounds showed only mild GPx-like reductive activity, with compounds 2, 7, 12, and 14 (115, 96, 95, and 82%, respectively) showing a higher percent rate of oxidation of NADPH in a coupled reaction assay compared to ebselen. This research highlights several derivatives of ebselen that show improved activity as neuroprotective agents over the parent compound.

Repurposed drug candidates for antituberculosis therapy

Antibiotics have been a key part of clinical treatments for more than 70 years. Long-term use of antimicrobial treatments has led to the development of severe bacterial resistance, which has become increasingly serious due to antibiotic abuse, resulting in the treatment of bacterial infections becoming challenging. The repurposing of approved drugs presents a promising strategy to address current bottlenecks in the development of novel antibacterial agents. Drug repurposing is a cost-effective emerging strategy, which aims to treat resistant infectious diseases by identifying known drugs with predicted efficacy for diseases other than the target disease. This strategy has potential in the treatment of tuberculosis (TB), particularly drug-resistant TB. In recent years, a panel of drugs approved for clinical use or clinical trials, such as linezolid, vancomycin and celecoxib, have been found to have anti-TB activities. However, the utility of drug repurposing is limited by the number of candidate compounds and their low activities. The low activities of repurposed drugs have slowed the development of a drug-repurposing strategy for anti-TB drugs. The present review discusses progress in the discovery of new anti-TB agents through drug repurposing since 2014. We also discuss the challenges faced and analyze the innovative ways that are being used to overcome these difficulties. This review may provide a useful guide for researchers in the field of drug repurposing.

Lewis Acid-Catalyzed Asymmetric Selenocyanation of β-Ketoesters with N-Selenocyanatosaccharin

The first electrophilic asymmetric selenocyanation has been achieved in the presence of Ni(OTf)₂ and (R,R)-DBFOX/Ph using N-selenocyanatosaccharin as the new selenocyanation reagent. Thus, a series of α-selenocyanato-β-keto esters were synthesized with high yields (up to 99%) and good ee values (up to 92% ee). The readily preparation of the reagent and high enantioselectivity make this methodology much practical for the synthesis of chiral selenocyanates.

Development and Therapeutic Potential of Selenazo Compounds

Incorporation of selenium (Se) atom into small molecules can substantially enhance their antioxidant, anti-inflammatory, antimutagenic, antitumoral or chemopreventive, antiviral, antibacterial, antifungal, antiparasitic, and neuroprotective effects. Specifically, selenazo compounds have received great attention owing to their chemical properties, pharmaceutical applications, and low toxicity. In this Perspective, we compile extensive literature evidence with the description and discussion of the most recent advances in different selenazo and selenadiazo motifs as potential pharmacological candidates. We also provide some perspectives on the challenges and future directions in the advancement of these selenazo compounds, each of which could generate drug candidates for various diseases.

Auriferous alkynylselenolatoalkylidynes

Gold plating carbynes – The incorporation of gold(i) centres, either terminal or bridging, into alkynylselenolatocarbynes provides models for how such metallated carbon-wires might bind to metal surfaces.

Alkynylselenolatoalkylidynes (LnMC–Se–CCR) as building blocks for mixed metal/main-group extended frameworks

The reactions of [W(CBr)(CO)₂(Tp*)] (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) with lithium alkynylselenolates LiSeCCR (R = SiMe₃, SiⁱPr₃, ⁿBu, ^tBu, Ph, p-tolyl) afford the alkynylselenolatoalkylidyne complexes [W(CSeCCR)(CO)₂(Tp*)].

The Influence of O/S Exchange on the Biocatalytical Activity of Benzisoselenazol-3(2H)-ones

The crucial feature of organoselenium compounds, when considering them as promising drug candidates in cancer therapy, is their unique ability to alter the cellular redox regulations. Organic Se-molecules continue to demonstrate a positive therapeutic effect both in cancer prevention—as antioxidants, and treatment—as prooxidants. The growing interest in this field of research highlights the need to search for particular pharmacophore motifs, which could enhance the efficiency and selectivity, and decrease the toxicity of potential anticancer agents. Herein, a series of redox-active organoselenium derivatives—N-functionalized benzisoselenazol-3(2H)-thiones, has been designed and synthetized. A new synthetic pathway, with the application of Lawesson’s reagent, has been developed and efficiently applied. The key steps involving microwave irradiation facilitated performing the reaction in solvent-free conditions, shortening the reaction time and significantly improving the overall yield of the process. Six N-alkyl derivatives have been obtained and tested as antioxidant catalysts and anti-proliferative agents. The N-propyl benzisoselenazol-3(2H)-thione was the best peroxide scavenger and the N-cyclohexyl derivative exhibited the best cytotoxic activity towards prostate cancer cell line DU145.

Investigation of synergistic antimicrobial effects of the drug combinations of meropenem and 1,2-benzisoselenazol-3(2H)-one derivatives on carbapenem-resistant Enterobacteriaceae producing NDM-1

The worldwide prevalence of NDM-1-producing bacteria has drastically undermined the clinical efficacy of the last line antibiotic of carbapenems, prompting a need to devise effective strategy to preserve their clinical value. Our previous studies have shown that ebselen can restore the efficacy of meropenem against a laboratory strain that produces NDM-1. Here we report the construction of a focused compound library of 1,2-benzisoselenazol-3(2H)-one derivatives which comprise a total of forty-six candidate compounds. The structure-activity relationship of these compounds and their potential to serve as an adjuvant to enhance the antimicrobial efficacy of meropenem against a collection of clinical NDM-1-producing carbapenem-resistant Enterobacteriaceae isolates was examined. Drug combination assays indicated that these derivatives exhibited synergistic antimicrobial activity when used along with meropenem, effectively restoring the activity of carbapenems against the resistant strains tested in a Galleria mellonella larvae in vivo infection model. The mode of inhibition of one compound, namely 11_a38, which was depicted when tested on the purified NDM-1 enzyme, indicated that it could covalently bind to the enzyme and displaced one zinc ion from the active site. Overall, this study provides a novel 1,2-benzisoselenazol-3(2H)-one scaffold that exhibits strong synergistic antimicrobial activity with carbapenems, and low cytotoxicity. The prospect of application of such compounds as carbapenem adjuvants warrants further evaluation.

Copper-catalyzed generation of flavone selenide and thioether derivatives using KSeCN and KSCN via C–H functionalization

Two simple Cu-catalyzed methods using KSeCN and KSCN to construct C–Se and C–S bonds on flavone skeletal structures via C–H functionalization are developed.

Exploring Covalent Allosteric Inhibition of Antigen 85C from Mycobacterium tuberculosis by Ebselen Derivatives

Previous studies identified ebselen as a potent in vitro and in vivo inhibitor of the Mycobacterium tuberculosis (Mtb) antigen 85 (Ag85) complex, comprising three homologous enzymes required for the biosynthesis of the mycobacterial cell wall. In this study, the Mtb Ag85C enzyme was cocrystallized with azido and adamantyl ebselen derivatives, resulting in two crystallographic structures of 2.01 and 1.30 Å resolution, respectively. Both structures displayed the anticipated covalent modification of the solvent accessible, noncatalytic Cys209 residue forming a selenenylsulfide bond. Continuous difference density for both thiol modifiers allowed for the assessment of interactions that influence ebselen binding and inhibitor orientation that were unobserved in previous Ag85C ebselen structures. The k_inact/K_I values for ebselen, adamantyl ebselen, and azido ebselen support the importance of observed constructive chemical interactions with Arg239 for increased in vitro efficacy toward Ag85C. To better understand the in vitro kinetic properties of these ebselen derivatives, the energetics of specific protein-inhibitor interactions and relative reaction free energies were calculated for ebselen and both derivatives using density functional theory. These studies further support the different in vitro properties of ebselen and two select ebselen derivatives from our previously published ebselen library with respect to kinetics and protein-inhibitor interactions. In both structures, the α9 helix was displaced farther from the enzyme active site than the previous Ag85C ebselen structure, resulting in the restructuring of a connecting loop and imparting a conformational change to residues believed to play a role in substrate binding specific to Ag85C. These notable structural changes directly affect protein stability, reducing the overall melting temperature by up to 14.5 °C, resulting in the unfolding of protein at physiological temperatures. Additionally, this structural rearrangement due to covalent allosteric modification creates a sizable solvent network that encompasses the active site and extends to the modified Cys209 residue. In all, this study outlines factors that influence enzyme inhibition by ebselen and its derivatives while further highlighting the effects of the covalent modification of Cys209 by said inhibitors on the structure and stability of Ag85C. Furthermore, the results suggest a strategy for developing new classes of Ag85 inhibitors with increased specificity and potency.