Synthesis and Glutathione Peroxidase-Like Activities of Isoselenazolines

Glutathione Peroxidase-like Antioxidant Activity of 1,3-Benzoselenazoles: Synthesis and In Silico Molecular Docking Studies as Pancreatic Lipase Inhibitors

The synthesis of 1,3-benzoselenazoles was achieved by the reaction of corresponding bis[3-amino-N-(p-tolyl)benzamide-2-yl] diselenide, bis[3-amino-N-(4-methoxyphenyl)benzamide-2-yl] diselenide, and bis[3-amino-N-(4-(dimethylamino)phenyl) benzamide-2-yl] diselenide with aryl aldehydes. The 1,3-benzoselenazoles continued to exist as planar molecules due to the presence of secondary Se···O interactions as revealed by the single-crystal X-ray analysis. The presence of secondary Se···O interactions in 1,3-benzoselenazoles was confirmed using natural bond orbital (NBO) and atoms in molecules (AIM) calculations. Nucleus-independent chemical shift (NICS) values suggested the presence of aromatic character in a five-membered benzoselenazole heterocyclic ring. The glutathione peroxidase (GPx)-like antioxidant activity of all 1,3-benzoselenazoles was assessed using a thiophenol assay, exhibiting greater antioxidant activity than Ph2Se2 used as a reference. The most active catalyst carrying a strong electron-donating group (-NMe2) at the ortho-position to the benzoselenazole ring was further investigated at different concentrations of thiophenol, H2O2, and 1,3-benzoselenazoles as catalyst for determining their catalytic parameters. Moreover, the potential applications of all 1,3-benzoselenazoles against pancreatic lipase (PL) have been identified using in silico interactions between the active sites of the 1LPB protein as evaluated using a molecular docking study.

Synthesis and glutathione peroxidase (GPx)-like activity of selenocystine (SeC) bioconjugates of biotin and lipoic acid

The conjugation of active biomolecules provides insight into their bioreactivity, leading to many applications in biotechnology and materials science. Herein, we report L-selenocystine (SeC) bioconjugates of lipoic acid (universal antioxidant) and biotin (Vitamin-H). The SeC-bioconjugates, SeC-Biotin (1) and SeC-Lipoic acid (2) were synthesized using solid phase peptide synthesis (SPPS) method and were characterized by multinuclear 1D (1H, 13C, 77Se) and 2D (1H-1H COSY and 1H-13C TOCSY) NMR spectroscopy, ESI-MS spectrometry, and RP-HPLC. The GPx-like enzyme mimicking activity of the SeC-bioconjugates 1 and 2 has been investigated through the coupled reductase assay method for the catalytic reductions of hydrogen peroxide into water. A significant enhancement in GPx-like enzymatic activity was observed for both novel bioconjugates SeC-Biotin (1) and SeC-Lipoic acid (2) as compared to diphenyl diselenide (Ph2Se2), L-selenocystine (SeC), biotin, lipoic acid, and ebselen.

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 and antioxidant activities of N-thiophenyl ebselenamines: a 77Se{1H} NMR mechanistic study

Synthesis of N-thiophenyl ebselenamines and selenenyl sulphides as efficient radical-trapping and hydroperoxide-decomposing antioxidants, respectively has been described.

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.

Janus -faced oxidant and antioxidant profiles of organo diselenides

To date, organoseleniums are pre-eminent for peroxide decomposition and radical quenching antioxidant activities. On the contrary, here, a series of Janus-faced aminophenolic diselenides have been prepared from substituted 2-iodoaniline and selenium powder using copper-catalyzed methodology. Subsequently, condensation with substituted salicylaldehyde afforded the Schiff base, which on reduction, yielded the desired substituted aminophenolic diselenides in 72%-88% yields. The generation of reactive oxygen species (ROS) from oxygen gas by the synthesized aminophenolic diselenides was studied by analyzing the oxidation of dichlorofluorescein diacetate (DCFDA) dye and para-nitro-thiophenol by fluorescence and UV-Visible spectroscopic methods. Furthermore, density functional theory calculations and crystal structure analysis revealed the role of functional amine and hydroxyl sites present in the Janus-faced organoselenium catalyst for the activation of molecular oxygen, where NH and phenolic groups bring the oxygen molecule close to the catalyst by N-H⋯O and O-H⋯O intermolecular interactions. Additionally, these functionalities stabilize the selenium-centered radical in the formed transition states. Antioxidant activities of the synthesized diselenides have been explored as the catalyst for the decomposition of hydrogen peroxide using benzenethiol sacrificial co-reductant by a well-established thiol assay. Radical quenching antioxidant activity was studied by the quenching of DPPH radicals at 516 nm by UV-Visible spectroscopy. The structure activity correlation suggests that the electron-rich phenol and electron-rich and sterically hindered selenium center enhance the oxidizing property of the aminophenolic diselenides. Janus-faced diselenides were also evaluated for their cytotoxic effect on HeLa cancer cells via MTT assay, which suggests that the compounds are effective at 15-18 μM concentration against cancer cells. Moreover, the combination with therapeutic anticancer drugs Erlotinib and Doxorubicin showed promising cytotoxicity at the nanomolar concentration (8-28 nM), which is sufficient to suppress the growth of the cancer cells.

Recent advances in computational liquid-phase 77Se NMR

This review aims to highlight significant progress in the calculation of 77Se NMR chemical shifts and spin – spin coupling constants involving selenium substantiated with a vast amount of experimental data. The material is arranged in two basic sections: the first one dealing with the calculation of 77Se NMR chemical shifts and the second one dealing with the computation of spin – spin coupling constants involving 77Se nucleus, namely 77Se–1H, 77Se–13C and 77Se–77Se together with some more exotic types of couplings, 77Se – 15N, 77Se–19F, 77Se–29Si and 77Se–31P. A special attention is focused on the stereoelectronic effects involving selenium atom and their manifestation in the 77Se NMR spectra of organoselenium compounds studied with the aid of the modern calculation of 77Se NMR parametres in combination with experimental results.

The bibliography includes 114 references.

Chalcogen-Nitrogen Bond: Insights into a Key Chemical Motif

Chalcogen-nitrogen chemistry deals with systems in which sulfur, selenium, or tellurium is linked to a nitrogen nucleus. This chemical motif is a key component of different functional structures, ranging from inorganic materials and polymers, to rationally designed catalysts, to bioinspired molecules and enzymes. The formation of a selenium–nitrogen bond, typically occurring upon condensation of an amine and the unstable selenenic acid, often leading to intramolecular cyclizations, and its disruption, mainly promoted by thiols, are rather common events in organic Se-catalyzed processes. In this work, focusing on examples taken from selenium organic chemistry and biochemistry, the selenium–nitrogen bond is described, and its strength and reactivity are quantified using accurate computational methods applied to model molecular systems. The intermediate strength of the Se–N bond, which can be tuned to necessity, gives rise to significant trends when comparing it to the stronger S– and weaker Te–N bonds, reaffirming also in this context the peculiar and valuable role of selenium in chemistry and life.

Catalytic and highly regenerable aminic organoselenium antioxidants with cytoprotective effects

N-Methyl ebselenamines carrying an aminic group in very close proximity to selenium as excellent chain-breaking and glutathione peroxidase-like antioxidants could reduce the production of ROS in C6 astroglial cell lines with minimal toxic effects.

The Se…S/N interactions as a possible mechanism of δ-aminolevulinic acid dehydratase enzyme inhibition by organoselenium compounds: A computational study

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DPDS and PSA interacts with cysteine residues from AlaD active site.

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The Se…S interactions could be involved in the δ-AlaD inhibition.

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δ-AlaD from Cucumis sativus does not present cysteine residues in the active site.

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Se…N interactions could be involved in the organoselenium action.

Organoselenium compounds present many pharmacological properties and are promising drugs. However, toxicological effects associated with inhibition of thiol-containing enzymes, such as the δ-aminolevulinic acid dehydratase (δ-AlaD), have been described. The molecular mechanism(s) by which they inhibit thiol-containing enzymes at the atomic level, is still not well known. The use of computational methods to understand the physical–chemical properties and biological activity of chemicals is essential to the rational design of new drugs. In this work, we propose an in silico study to understand the δ-AlaD inhibition mechanism by diphenyl diselenide (DPDS) and its putative metabolite, phenylseleninic acid (PSA), using δ-AlaD enzymes from Homo sapiens (Hsδ-AlaD), Drosophila melanogaster (Dmδ-AlaD) and Cucumis sativus (Csδ-AlaD). Protein modeling homology, molecular docking, and DFT calculations are combined in this study. According to the molecular docking, DPDS and PSA might bind in the Hsδ-AlaD and Dmδ-AlaD active sites interacting with the cysteine residues by Se^…S interactions. On the other hand, the DPDS does not access the active site of the Csδ-AlaD (a non-thiol protein), while the PSA interacts with the amino acids residues from the active site, such as the Lys291. These interactions might lead to the formation of a covalent bond, and consequently, to the enzyme inhibition. In fact, DFT calculations (mPW1PW91/def2TZVP) demonstrated that the selenylamide bond formation is energetically favored. The in silico data showed here are in accordance with previous experimental studies, and help us to understand the reactivity and biological activity of organoselenium compounds.

Binding motif of ebselen in solution: chalcogen and hydrogen bonds team up

Ebselen, a compound active against SARS-CoV-2, forms a bifurcated supramolecular synthon thanks to chalcogen bond and hydrogen bond cooperation.

Organoselenium small molecules as catalysts for the oxidative functionalization of organic molecules

This perspective highlights the critical analysis of the challenges, in the past decade, which led to the development of organoselenium compounds and their use as versatile catalysts in organic synthesis towards the oxidation of olefins and C–H bonds. Furthermore, the emphasis here differs from previous reviews of the field by classifying the various types of catalyses and the diverse strategies.

Adaptive responses of sterically confined intramolecular chalcogen bonds

The existence of intramolecular chalcogen bonds (IChBs) in 2,6-disubstituted arylchalcogen derivatives is determined by the substituents and the sigma hole donor behavior of the chalcogen atom in the molecule.

The responsive behavior of an entity towards its immediate surrounding is referred to as an adaptive response. The adaptive responses of a noncovalent interaction at the molecular scale are reflected from its structural and functional roles. Intramolecular chalcogen bonding (IChB), an attractive interaction between a heavy chalcogen E (E = Se or Te) centered sigma hole and an ortho-heteroatom Lewis base donor D (D = O or N), plays an adaptive role in defining the structure and reactivity of arylchalcogen compounds. In this perspective, we describe the adaptive roles of a chalcogen centered Lewis acid sigma hole and a proximal Lewis base (O or N) in accommodating built-in steric stress in 2,6-disubstituted arylchalcogen compounds. From our perspective, the IChB components (a sigma hole and the proximal Lewis base) act in synergism to accommodate the overwhelming steric force. The adaptive responses of the IChB components are inferred from the observed molecular structures and reactivity. These include (a) adaptation of a conformation without IChBs, (b) adaptation of a conformation with weak IChBs, (c) twisting the skeletal aryl ring while maintaining IChBs, (d) ionization of the E–X bond (e.g., X = Br) to relieve stress and (e) intramolecular cyclization to relieve steric stress. A comprehensive approach, involving X-ray data analysis, density functional theory (DFT) calculations, reaction pattern analysis and principal component analysis (PCA), has been employed to rationalize the adaptive behaviors of IChBs in arylchalcogen compounds. We believe that the perception of ChB as an adaptive/stimulus responsive interaction would profit the futuristic approaches that would utilise ChB as self-assembly and molecular recognition tools.

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.

Nitro-, Azo-, and Amino Derivatives of Ebselen: Synthesis, Structure, and Cytoprotective Effects

Novel azo-bis-ebselen compounds 7 were prepared by reduction of 7-nitro-2-aryl-1,2-benzisoselenazol-3(2H)-ones 3 and 6 with sodium benzenetellurolate, NaTeC₆H₅, and by reaction of 2-bromo-3-nitrobenzamides with Na₂Se₂. The X-ray structure of 7b showed that the molecule, due to strong intramolecular secondary Se···N interactions, is completely planar. Azo-compounds 7 upon further reaction with NaTeC₆H₅ were reductively cleaved to provide 2 equiv of the corresponding aromatic amine. The weak Se-N bond was not stable enough to survive the reaction conditions, and diselenides 8 were isolated after workup. Whereas azo-bis-ebselens 7 were poor mimics of the glutathione peroxidase (GPx)-enzymes, nitroebselens 3, 6, and 11b and diselenides 8 were 3-6-fold more active than ebselen. Based on ⁷⁷Se NMR spectroscopy, a catalytic cycle for diselenide 8b, involving aminoebselen 14, was proposed. As assessed by chemiluminescence measurements, the good GPx-mimics could reduce production of reactive oxygen species (ROS) in stimulated human mononuclear cells more efficiently than Trolox. No toxic effects of the compounds were seen in MC3T3-cells at 25 μM.

Insights into the catalytic mechanism of synthetic glutathione peroxidase mimetics

Glutathione Peroxidase (GPx) is a key selenoenzyme that protects biomolecules from oxidative damage. Extensive research has been carried out to design and synthesize small organoselenium compounds as functional mimics of GPx. While the catalytic mechanism of the native enzyme itself is poorly understood, the synthetic mimics follow different catalytic pathways depending upon the structures and reactivities of various intermediates formed in the catalytic cycle. The steric as well as electronic environments around the selenium atom not only modulate the reactivity of these synthetic mimics towards peroxides and thiols, but also the catalytic mechanisms. The catalytic cycle of small GPx mimics is also dependent on the nature of peroxides and thiols used in the study. In this review, we discuss how the catalytic mechanism varies with the substituents attached to the selenium atom.

Design, synthesis and evaluation of clioquinol–ebselen hybrids as multi-target-directed ligands against Alzheimer's disease

A novel series of compounds obtained by fusing the metal-chelating agent clioquinol and the antioxidant ebselen were designed, synthesized and evaluated as multi-target-directed ligands against Alzheimer's disease (AD).

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.

Heterologous expression and characterization of human cellular glutathione peroxidase mutants

Cellular glutathione peroxidase (GPx1; EC1.11.1.9) is a major intracellular antioxidant selenoenzyme in mammals. However, the complicated expression mechanism of selenocysteine (Sec)-containing protein increases the difficulty of expressing human GPx1 (hGPx1) in Escherichia coli (E. coli). In this study, hGPx1 gene was cloned from a cDNA library of the human hepatoma cell line HepG2. The codon UGA encoding Sec49 of hGPx1 was first mutated to UGC encoding cysteine (Cys) and then biosynthetically converted to Sec during expression in an E. coli BL21(DE3)cys auxotrophic system. Seleno-GPx1^Sec displayed a low GPx activity of 522 U/μmol. To improve the activity, the other five Cys residues (C2, C78, C115, C156, C202) were mutated to serine (Ser) in one hGPx1 molecule. The mutant seleno-hGPx1^Ser showed a high activity of 5278 U/μmol, which was more than 10-fold enhanced as compared with seleno-GPx1^Sec . The activity was the highest among all of those seleno-proteins obtained by this method so far. Kinetic analysis of seleno-hGPx1^Ser showed a typical ping-pong mechanism, which was similar to those of natural GPxs. This research will be of value in overcoming the problem of limited sources of natural GPx and substantially promotes the research of the characterization of GPx. © 2014 IUBMB Life, 66(3):212-219, 2014.

Synthesis and biological evaluation of a new series of ebselen derivatives as glutathione peroxidase (GPx) mimics and cholinesterase inhibitors against Alzheimer's disease

A series of ebselen derivatives were designed, synthesised and evaluated as inhibitors of cholinesterases (ChEs) and glutathione peroxidase (GPx) mimics. Most of the compounds were found to be potent against AChEs and BuChE, compounds 5e and 5i, proved to be the most potent against AChE with IC₅₀ values of 0.76 and 0.46 μM, respectively. Among these hybrids, most of the compounds were found to be good GPx mimics compare with ebselen. The selected compounds 5e and 5i were also used to determine the catalytic parameters and in vitro hydrogen peroxide scavenging activity. The results indicate that compounds 5e and 5i may be excellent multifunctional agents for the treatment of AD.

An ebselen like catalyst with enhanced GPx activity via a selenol intermediate

Benzamide ring-substituted, quinine-derived ebselen analogue is synthesized which exists in selenol form upon addition of PhSH. It catalyses oxidation of PhSH with H₂O₂ faster (10³-fold) than ebselen.