Effects of Methoxy Substituents on the Glutathione Peroxidase-like Activity of Cyclic Seleninate Esters

Introduction of Methyl Group in Substituted Isoselenazolones: Catalytic and Mechanistic Study

Copper-catalyzed direct selenation of substituted 2-bromo-N-phenylbenzamide substrates with elemental selenium powder provided a series of methoxy-substituted isoselenazolones via the C-Se and Se-N bond formations. Phenolic substituted isoselenazolones have been obtained by O-demethylation of the corresponding methoxy-substituted analogues using boron tribromide. Some isoselenazolones have been structurally characterized by X-ray single-crystal analysis. The glutathione peroxidase (GPx)-like antioxidant activity of isoselenazolones has been evaluated both in thiophenol and coupled-reductase assays. All isoselenazolones showed good GPx-like activities in the coupled-reductase assay. The ferric-reducing antioxidant power of phenolic antioxidants has also been evaluated. The best phenolic antioxidants were found to be good ferric-reducing antioxidant power agents. The single electron transfer, hydrogen atom transfer, and proton-coupled electron transfer mechanisms for the antioxidant properties of all catalysts have been supported by density functional theory calculations. The catalytic cycle was proposed for one of the phenolic isoselenazolones involving diselenide, selenenyl sulfide, selenol, and selenenic acid as intermediates using ⁷⁷Se{¹H} NMR spectroscopy.

Synthesis, Reactions, and Antioxidant Properties of Bis(3-amino-1-hydroxybenzyl)diselenide

Bis(3-amino-1-hydroxybenzyl)diselenide containing two ortho groups was synthesized from 7-nitro-3H-2,1-benzoxaselenole and in situ generated sodium benzene tellurolate (PhTeNa). One-pot synthesis of 1,3-benzoselenazoles was achieved from bis(3-amino-1-hydroxybenzyl)diselenide and aryl aldehydes using acetic acid as a catalyst. The X-ray crystal structure of chloro-substituted benzoselenazole revealed a planar structure with T-shaped geometry around the Se atom. Both natural bond orbital and atoms in molecules calculations confirmed the presence of secondary Se···H interactions in bis(3-amino-1-hydroxybenzyl)diselenide and Se···O interactions in benzoselenazoles, respectively. The glutathione peroxidase (GPx)-like antioxidant activities of all compounds were evaluated using a thiophenol assay. Bis(3-amino-1-hydroxybenzyl)diselenide and benzoselenazoles showed better GPx-like activity compared to that of the diphenyl diselenide and ebselen, used as references, respectively. Based on ⁷⁷Se{¹H} NMR spectroscopy, a catalytic cycle for bis(3-amino-1-hydroxybenzyl)diselenide using thiophenol and hydrogen peroxide was proposed involving selenol, selenosulfide, and selenenic acid as intermediates. The potency of all GPx mimics was confirmed by their in vitro antibacterial properties against the biofilm formation of Bacillus subtilis and Pseudomonas aeruginosa. Additionally, molecular docking studies were used to evaluate the in silico interactions between the active sites of the TsaA and LasR-based proteins found in Bacillus subtilis and Pseudomonas aeruginosa.

Synthesis, Catalytic GPx-like Activity, and SET Reactions of Conformationally Constrained 2,7-Dialkoxy-Substituted Naphthalene-1,8-peri-diselenides

Several 2,7-dialkoxy-substituted naphthalene-1,8-peri-diselenides were prepared and tested for catalytic antioxidant activity in an NMR-based assay employing the reduction of hydrogen peroxide with stoichiometric amounts of benzyl thiol. Acidic conditions enhanced their catalytic activity, whereas basic conditions suppressed it. The highest activity was observed with a 2,7-bis(triethyleneglycol) derivative. These compounds serve as mimetics of the antioxidant selenoenzyme glutathione peroxidase. Studies based on NMR peak-broadening effects and EPR spectroscopy indicated that a thiol-dependent SET reaction occurs under the conditions of the assay, which can be reversed by the addition of triethylamine. In contrast, peak broadening induced by proton-catalyzed electron transfer during the treatment of naphthalene-1,8-peri-diselenides with trifluoroacetic acid can be suppressed by the addition of excess thiol. These observations provide new insights into the redox mechanisms of these processes.

Tetravalent Spiroselenurane Catalysts: Intramolecular Se···N Chalcogen Bond-Driven Catalytic Disproportionation of H2O2 to H2O and O2 and Activation of I2 and NBS

Chalcogen-bonding interactions have recently gained considerable attention in the field of synthetic chemistry, structure, and bonding. Here, three organo-spiroselenuranes, having a Se(IV) center with a strong intramolecular Se···N chalcogen-bonded interaction, have been isolated by the oxidation of the respective bis(2-benzamide) selenides derived from an 8-aminoquinoline ligand. Further, the synthesized spiroselenuranes, when assayed for their antioxidant activity, show disproportionation of hydrogen peroxide into H₂O and O₂ with first-order kinetics with respect to H₂O₂ for the first time by any organoselenium molecules as monitored by ¹H NMR spectroscopy. Electron-donating 5-methylthio-benzamide ring-substituted spiroselenurane disproportionates hydrogen peroxide at a high rate of 15.6 ± 0.4 × 10³ μM min^-1 with a rate constant of 8.57 ± 0.50 × 10^-3 s^-1, whereas 5-methoxy and unsubstituted-benzamide spiroselenuranes catalyzed the disproportionation of H₂O₂ at rates of 7.9 ± 0.3 × 10³ and 2.9 ± 0.3 × 10³ μM min^-1 with rate constants of 1.16 ± 0.02 × 10^-3 and 0.325 ± 0.025 × 10^-3 s^-1, respectively. The evolved oxygen gas from the spiroselenurane-catalyzed disproportion of H₂O₂ has also been confirmed by a gas chromatograph-thermal conductivity detector (GCTCD) and a portable digital polarographic dissolved O₂ probe. Additionally, the synthesized spiroselenuranes exhibit thiol peroxidase antioxidant activities for the reduction of H₂O₂ by a benzenethiol co-reductant monitored by UV-visible spectroscopy. Next, the Se···N bonded spiroselenuranes have been explored as catalysts in synthetic oxidation iodolactonization and bromination of arenes. The synthesized spiroselenurane has activated I₂ toward the iodolactonization of alkenoic acids under base-free conditions. Similarly, efficient chemo- and regioselective monobromination of various arenes with NBS catalyzed by chalcogen-bonded synthesized spiroselenuranes has been achieved. Mechanistic insight into the spiroselenuranes in oxidation reactions has been gained by ⁷⁷Se NMR, mass spectrometry, UV-visible spectroscopy, single-crystal X-ray structure, and theoretical (DFT, NBO, and AIM) studies. It seems that the highly electrophilic nature of the selenium center is attributed to the presence of an intramolecular Se···N interaction and a vacant coordination site in spiroselenuranes is crucial for the activation of H₂O₂, I₂, and NBS. The reaction of H₂O₂, I₂, and NBS with tetravalent spiroselenurane would lead to an octahedral-Se(VI) intermediate, which is reduced back to Se(IV) due to thermodynamic instability of selenium in its highest oxidation state and the presence of a strong intramolecular N-donor atom.

Synthesis of alpha-methyl selenocysteine and its utilization as a glutathione peroxidase mimic

Selenocysteine (Sec) is the 21st amino acid in the genetic code where this amino acid is primarily involved in redox reactions in enzymes because of its high reactivity toward oxygen and related reactive oxygen species. Sec has found wide utility in synthetic peptides, especially as a replacement for cysteine. One limitation of using Sec in synthetic peptides is that it can undergo β-syn elimination reactions after oxidation, rendering the peptide inactive due to loss of selenium. This limitation can be overcome by substituting Cα-H with a methyl group. The resulting Sec derivative is α-methylselenocysteine ((αMe)Sec). Here, we present a new strategy for the synthesis of (αMe)Sec by alkylation of an achiral methyl malonate through the use of a selenium-containing alkylating agent synthesized in the presence of dichloromethane. The seleno-malonate was then subjected to an enzymatic hydrolysis utilizing pig liver esterase followed by a Curtius rearrangement producing a protected derivative of (αMe)Sec that could be used in solid-phase peptide synthesis. We then synthesized two peptides: one containing Sec and the other containing (αMe)Sec, based on the sequence of glutathione peroxidase. This is the first reported incorporation of (αMe)Sec into a peptide as well as the first reported biochemical application of this unique amino acid. The (αMe)Sec-containing peptide had superior stability as it could not undergo β-syn elimination and it also avoided cleavage of the peptide backbone, which we surprisingly found to be the case for the Sec-containing peptide when it was incubated for 96 hours in oxygenated buffer at pH 8.0.

Probing the Formation of a Seleninic Acid in Living Cells by the Fluorescence Switching of a Glutathione Peroxidase Mimetic

Glutathione peroxidase (GPx) is a selenoenzyme that protects cells against oxidative damage. Although the formation of a seleninic acid (-SeO₂ H) by this enzyme during oxidative stress has been proposed, a selenic acid has not been identified in cells. Herein, we report that the formation of a seleninic acid can be monitored in living cells by using a redox-active ebselen analogue with a naphthalimide fluorophore. The probe reacts with H₂ O₂ to generate the highly fluorescent seleninic acid. The electron withdrawing nature of the -SeO₂ H moiety and strong Se⋅⋅⋅O interactions, which prevent the photoinduced electron transfer, are responsible for the fluorescence.

Organoselenium and organotellurium compounds containing chalcogen-oxygen bonds in organic synthesis or related processes

This chapter emphasizes aspects related to the role of organochalcogen (Se, Te) compounds with single E‒O and/or double E=O (E=Se, Te) bonds in organic synthesis, as reagents, intermediates, or catalysts, and it gives a larger extent mainly to data reported in the field during the last ten years. For each of these two heavier chalcogens the material is structured according to the oxidation state of the chalcogen and, for the same oxidation state, in sections dedicated to a particular type of compounds. Functionalization or cyclization reactions in which the organochalcogen compounds take part as nucleophiles, electrophiles or radicals, employed in various synthetic transformations, are discussed and, where available, the mechanistic aspects are outlined. New chiral species and new strategies were developed during last years in order to increase the yield, the reaction rate and the stereoselectivity in specific organic transformations, i.e. addition, oxidation, elimination, cyclization or rearrangement reactions. A notably attention was devoted to easily accessible and environmental friendly catalysts, re-usable and “green” solvents, as well as waste-free procedures.

Recent progress in selenium-catalyzed organic reactions

Selenium-based catalysts, including organo- and inorganoselenium ones, in organic synthesis in the recent decade are reviewed.

Atom-economical selenation of electron-rich arenes and phosphonates with molecular oxygen at room temperature

An environmentally benign selenation of electron-rich arenes and phosphonates is developed adopting a novel recycle–reuse–reduce strategy for selenol by oxygen.

Organoselenium compounds as mimics of selenoproteins and thiol modifier agents

Selenium is an essential trace element for animals and its role in the chemistry of life relies on a unique functional group: the selenol (-SeH) group. The selenol group participates in critical redox reactions. The antioxidant enzymes glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) exemplify important selenoproteins. The selenol group shares several chemical properties with the thiol group (-SH), but it is much more reactive than the sulfur analogue. The substitution of S by Se has been exploited in organic synthesis for a long time, but in the last 4 decades the re-discovery of ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) and the demonstration that it has antioxidant and therapeutic properties has renovated interest in the field. The ability of ebselen to mimic the reaction catalyzed by GPx has been viewed as the most important molecular mechanism of action of this class of compound. The term GPx-like or thiol peroxidase-like reaction was previously coined in the field and it is now accepted as the most important chemical attribute of organoselenium compounds. Here, we will critically review the literature on the capacity of organoselenium compounds to mimic selenoproteins (particularly GPx) and discuss some of the bottlenecks in the field. Although the GPx-like activity of organoselenium compounds contributes to their pharmacological effects, the superestimation of the GPx-like activity has to be questioned. The ability of these compounds to oxidize the thiol groups of proteins (the thiol modifier effects of organoselenium compounds) and to spare selenoproteins from inactivation by soft-electrophiles (MeHg⁺, Hg²⁺, Cd²⁺, etc.) might be more relevant for the explanation of their pharmacological effects than their GPx-like activity. In our view, the exploitation of the thiol modifier properties of organoselenium compounds can be harnessed more rationally than the use of low mass molecular structures to mimic the activity of high mass macromolecules that have been shaped by millions to billions of years of evolution.

The role of methoxy substituents in regulating the activity of selenides that serve as spirodioxyselenurane precursors and glutathione peroxidase mimetics

A series of o-(hydroxymethyl)phenyl selenides containing single or multiple methoxy substituents was synthesized, and the rate at which each compound catalyzed the oxidation of benzyl thiol to its disulfide with excess hydrogen peroxide was measured. This assay provided the means for comparing the relative abilities of the selenides to mimic the antioxidant selenoenzyme glutathione peroxidase. The mechanism for catalytic activity involves oxidation of the selenides to their corresponding selenoxides with hydrogen peroxide, cyclization to spirodioxyselenuranes, followed by reduction with two equivalents of thiol to regenerate the original selenide with concomitant disulfide formation. A single p-methoxy group on each aryl moiety afforded the highest catalytic activity, while methoxy groups in the meta position had little effect compared to the unsubstituted selenide, and o-methoxy groups suppressed activity. The installation of multiple methoxy groups on each aryl moiety provided no improvement. These results can be rationalized on the basis of dominating mesomeric and steric effects of the p- and o-substituents, respectively.

Synthesis, Structure and Antioxidant Activity of Cyclohexene-Fused Selenuranes and Related Derivatives

Synthesis, structure and antioxidant activity of new cyclohexene-fused spiroselenuranes and a spirotellurane is reported. Oxidation reactions of bis(o-formylcyclohex-1-ene)selenide/bis(2-hydroxymethylcyclohex-1-ene)selenide provide the corresponding spiroselenuranes. The glutathione peroxidase-like activity of the newly synthesized compounds has been evaluated.

Oxidation of Disulfides to Thiolsulfinates with Hydrogen Peroxide and a Cyclic Seleninate Ester Catalyst

Cyclic seleninate esters function as mimetics of the antioxidant selenoenzyme glutathione peroxidase. They catalyze the reduction of harmful peroxides with thiols, which are converted to disulfides in the process. The possibility that the seleninate esters could also catalyze the further oxidation of disulfides to thiolsulfinates and other overoxidation products under these conditions was investigated. This has ramifications in potential medicinal applications of seleninate esters because of the possibility of catalyzing the unwanted oxidation of disulfide-containing spectator peptides and proteins. A variety of aryl and alkyl disulfides underwent facile oxidation with hydrogen peroxide in the presence of catalytic benzo-1,2-oxaselenolane Se-oxide affording the corresponding thiolsulfinates as the principal products. Unsymmetrical disulfides typically afforded mixtures of regioisomers. Lipoic acid and N,N′-dibenzoylcystine dimethyl ester were oxidized readily under similar conditions. Although isolated yields of the product thiolsulfinates were generally modest, these experiments demonstrate that the method nevertheless has preparative value because of its mild conditions. The results also confirm the possibility that cyclic seleninate esters could catalyze the further undesired oxidation of disulfides in vivo.

Developments in Synthetic Application of Selenium(IV) Oxide and Organoselenium Compounds as Oxygen Donors and Oxygen-Transfer Agents

A variety of selenium compounds were proven to be useful reagents and catalysts for organic synthesis over the past several decades. The most interesting aspect, which emerged in recent years, concerns application of hydroperoxide/selenium(IV) oxide and hydroperoxide/organoselenium catalyst systems, as "green reagents" for the oxidation of different organic functional groups. The topic of oxidations catalyzed by organoselenium derivatives has rapidly expanded in the last fifteen years This paper is devoted to the synthetic applications of the oxidation reactions mediated by selenium compounds such as selenium(IV) oxide, areneseleninic acids, their anhydrides, selenides, diselenides, benzisoselenazol-3(2H)-ones and other less often used other organoselenium compounds. All these compounds have been successfully applied for various oxidations useful in practical organic syntheses such as epoxidation, 1,2-dihydroxylation, and α-oxyfunctionalization of alkenes, as well as for ring contraction of cycloalkanones, conversion of halomethyl, hydroxymethyl or active methylene groups into formyl groups, oxidation of carbonyl compounds into carboxylic acids and/or lactones, sulfides into sulfoxides, and secondary amines into nitrones and regeneration of parent carbonyl compounds from their azomethine derivatives. Other reactions such as dehydrogenation and aromatization, active carbon-carbon bond cleavage, oxidative amidation, bromolactonization and oxidation of bromide for subsequent reactions with alkenes are also successfully mediated by selenium (IV) oxide or organoselenium compounds. The oxidation mechanisms of ionic or free radical character depending on the substrate and oxidant are discussed. Coverage of the literature up to early 2015 is provided. Links have been made to reviews that summarize earlier literature and to the methods of preparation of organoselenium reagents and catalysts.

Effect of Methoxy Substituents on the Activation Barriers of the Glutathione Peroxidase-Like Mechanism of an Aromatic Cyclic Seleninate

Density functional theory (DFT) models including explicit water molecules have been used to model the redox scavenging mechanism of aromatic cyclic seleninates. Experimental studies have shown that methoxy substitutions affect the rate of scavenging of reactive oxygen species differently depending upon the position. Activities are enhanced in the para position, unaffected in the meta, and decreased in the ortho. DFT calculations show that the activation barrier for the oxidation of the selenenyl sulfide, a proposed key intermediate, is higher for the ortho methoxy derivative than for other positions, consistent with the low experimental conversion rate.

Xerogel-sequestered silanated organochalcogenide catalysts for bromination with hydrogen peroxide and sodium bromide

While H₂O₂ is a powerful oxidant, decomposing into environmentally benign H₂O and O₂, a catalyst is often required for reactions with H₂O₂ to proceed at synthetically useful rates. Organotellurium and organoselenium compounds catalyze the oxidation of halide salts to hypohalous acids using H₂O₂. When sequestered into xerogel monoliths, the xerogel-chalcogenide combinations have demonstrated increased catalytic activity relative to the organochalcogen compound alone in solution for the oxidation of halide salts to hypohalous acids with H₂O₂. Diorganotellurides, diorganoselenides, and diorganodiselenides bearing triethoxysilane functionalities were sequestered into xerogel monoliths and their catalytic activity and longevity were investigated. The longevity of the catalyst-xerogel combinations was examined by isolating and recycling the catalyst-xerogel combination. It was found tellurium-containing catalyst 3 and selenium-containing catalyst 8 maintained their catalytic activity through three recycling trials and adding electron-donating substituents to catalyst 3 also increased the catalytic rate. The presence of organotellurium and organoselenium groups in the +4 oxidation state was determined by X-ray photoelectron spectroscopy.

Insight into the mechanism revealing the peroxidase mimetic catalytic activity of quaternary CuZnFeS nanocrystals: colorimetric biosensing of hydrogen peroxide and glucose

Artificial enzyme mimetics have attracted immense interest recently because natural enzymes undergo easy denaturation under environmental conditions restricting practical usefulness. We report for the first time chalcopyrite CuZnFeS (CZIS) alloyed nanocrystals (NCs) as novel biomimetic catalysts with efficient intrinsic peroxidase-like activity. Novel peroxidase activities of CZIS NCs have been evaluated by catalytic oxidation of the peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2). CZIS NCs demonstrate the synergistic effect of elemental composition and photoactivity towards peroxidase-like activity. The quaternary CZIS NCs show enhanced intrinsic peroxidase-like activity compared to the binary NCs with the same constituent elements. Intrinsic peroxidase-like activity has been correlated with the energy band position of CZIS NCs extracted using scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy. Kinetic analyses indicate Michaelis-Menten enzyme kinetic model catalytic behavior describing the rate of the enzymatic reaction by correlating the reaction rate with substrate concentration. Typical color reactions arising from the catalytic oxidation of TMB over CZIS NCs with H2O2 have been utilized to establish a simple and sensitive colorimetric assay for detection of H2O2 and glucose. CZIS NCs are recyclable catalysts showing high efficiency in multiple uses. Our study may open up the possibility of designing new photoactive multi-component alloyed NCs as enzyme mimetics in biotechnology applications.

Organoselenium-catalyzed, hydroxy-controlled regio- and stereoselective amination of terminal alkenes: efficient synthesis of 3-amino allylic alcohols

An efficient route to prepare 3-amino allylic alcohols in excellent regio- and stereoselectivity in the presence of bases by orangoselenium catalysis has been developed. In the absence of bases α,β-unsaturated aldehydes were formed in up to 97% yield. Control experiments reveal that the hydroxy group is crucial for the direct amination.

Introduction of a catalytic triad increases the glutathione peroxidase-like activity of diaryl diselenides

Glutathione peroxidase-like antioxidant activity of amine and amide-based diselenides is described.

Computer-assisted designed “selenoxy–chinolin”: a new catalytic mechanism of the GPx-like cycle and inhibition of metal-free and metal-associated Aβ aggregation

Using support from rational computer-assisted design, a novel series of hybrids designed by fusing the metal-chelating agent CQ and the antioxidant ebselen were synthesized and evaluated as multitarget-directed ligands.