Quantum Chemical and Experimental Studies of an Unprecedented Reaction Pathway of Nucleophilic Substitution of 2-Bromomethyl-1,3-thiaselenole with 1,3-Benzothiazole-2-thiol Proceeding Stepwise at Three Different Centers of Seleniranium Intermediates

The results of quantum chemical and experimental studies of the reaction of 2-bromomethyl-1,3-thiaselenole with 1,3-benzothiazole-2-thiol made it possible to discover the unprecedented pathway of this reaction, which proceeds stepwise at three different centers of seleniranium intermediates. The first stage includes an attack of thiolate anion at the selenium atom of the seleniranium cation accompanied by ring opening with the formation of (Z)-2-[(1,3-benzothiazol-2-ylsulfanyl)selanyl]ethenyl vinyl sulfide, which is converted to six-membered heterocycle, 2-(2,3-dihydro-1,4-thiaselenin-2-ylsulfanyl)-1,3-benzothiazole, in a 99% yield. The latter compound undergoes rearrangement with ring contraction producing five-membered heterocycle, 2-[(1,3-thiaselenol-2-ylmethyl)sulfanyl]-1,3-benzothiazole, in a 99% yield (the thermodynamic product). The formation of 1,2-bis[(Z)-2-(vinylsulfanyl)ethenyl] diselenide is the result of the disproportionation of (Z)-2-[(1,3-benzothiazol-2-ylsulfanyl)selanyl]ethenyl vinyl sulfide. Thus, based on the quantum chemical and experimental studies, a regioselective synthesis of the reaction products in high yields was developed.

Effect of Selenium on TNAZ Molecule - A DFT Treatment

The present treatment deals with an unusual composite of TNAZ that is TNAZ+ nSe(n:1,2) within the constraints of density functional theory at the level of UB3LYP/6-31++G(d,p). TNAZ is an insensitive high explosive material. Since, selenium atom in its ground state has two unpaired electrons, the composites are considered in their singlet, triplet and quintet states. Selenium and TNAZ interact at different extents and the systems are electronically stable but TNAZ+2Se (singlet) structurally decomposes by the elongation of one of the geminally substituted nitro groups. Modeling studies indicate that the N-O bond elongation in the composite mentioned occurs only if azetidine ring is present with or without the nitramine bond. For the composites various structural, electronic and quantum chemical data have been harvested and discussed.

Selenoproteome Expression Studied by Non-Radioactive Isotopic Selenium-Labeling in Human Cell Lines

Selenoproteins, in which the selenium atom is present in the rare amino acid selenocysteine, are vital components of cell homeostasis, antioxidant defense, and cell signaling in mammals. The expression of the selenoproteome, composed of 25 selenoprotein genes, is strongly controlled by the selenium status of the body, which is a corollary of selenium availability in the food diet. Here, we present an alternative strategy for the use of the radioactive 75Se isotope in order to characterize the selenoproteome regulation based on (i) the selective labeling of the cellular selenocompounds with non-radioactive selenium isotopes (76Se, 77Se) and (ii) the detection of the isotopic enrichment of the selenoproteins using size-exclusion chromatography followed by inductively coupled plasma mass spectrometry detection. The reliability of our strategy is further confirmed by western blots with distinct selenoprotein-specific antibodies. Using our strategy, we characterized the hierarchy of the selenoproteome regulation in dose–response and kinetic experiments.

Seleno-Functionalization of Quercetin Improves the Non-Covalent Inhibition of Mpro and Its Antiviral Activity in Cells against SARS-CoV-2

The development of new antiviral drugs against SARS-CoV-2 is a valuable long-term strategy to protect the global population from the COVID-19 pandemic complementary to the vaccination. Considering this, the viral main protease (Mpro) is among the most promising molecular targets in light of its importance during the viral replication cycle. The natural flavonoid quercetin 1 has been recently reported to be a potent Mpro inhibitor in vitro, and we explored the effect produced by the introduction of organoselenium functionalities in this scaffold. In particular, we report here a new synthetic method to prepare previously inaccessible C-8 seleno-quercetin derivatives. By screening a small library of flavonols and flavone derivatives, we observed that some compounds inhibit the protease activity in vitro. For the first time, we demonstrate that quercetin (1) and 8-(p-tolylselenyl)quercetin (2d) block SARS-CoV-2 replication in infected cells at non-toxic concentrations, with an IC50 of 192 μM and 8 μM, respectively. Based on docking experiments driven by experimental evidence, we propose a non-covalent mechanism for Mpro inhibition in which a hydrogen bond between the selenium atom and Gln189 residue in the catalytic pocket could explain the higher Mpro activity of 2d and, as a result, its better antiviral profile.

Inhibition mechanism of SARS-CoV-2 main protease by ebselen and its derivatives

The SARS-CoV-2 pandemic has triggered global efforts to develop therapeutics. The main protease of SARS-CoV-2 (Mpro), critical for viral replication, is a key target for therapeutic development. An organoselenium drug called ebselen has been demonstrated to have potent Mpro inhibition and antiviral activity. We have examined the binding modes of ebselen and its derivative in Mpro via high resolution co-crystallography and investigated their chemical reactivity via mass spectrometry. Stronger Mpro inhibition than ebselen and potent ability to rescue infected cells were observed for a number of derivatives. A free selenium atom bound with cysteine of catalytic dyad has been revealed in crystallographic structures of Mpro with ebselen and MR6-31-2 suggesting hydrolysis of the enzyme bound organoselenium covalent adduct and formation of a phenolic by-product, confirmed by mass spectrometry. The target engagement with selenation mechanism of inhibition suggests wider therapeutic applications of these compounds against SARS-CoV-2 and other zoonotic beta-corona viruses.

Selective cysteine-to-selenocysteine changes in a [NiFe]-hydrogenase confirm a special position for catalysis and oxygen tolerance

In [NiFe]-hydrogenases, the active-site Ni is coordinated by four cysteine-S ligands (Cys; C), two of which are bridging to the Fe(CO)(CN)2 fragment. Substitution of a single Cys residue by selenocysteine (Sec; U) occurs occasionally in nature. Using a recent method for site-specific Sec incorporation into proteins, each of the four Ni-coordinating cysteine residues in the oxygen-tolerant Escherichia coli [NiFe]-hydrogenase-1 (Hyd-1) has been replaced by U to identify its importance for enzyme function. Steady-state solution activity of each Sec-substituted enzyme (on a per-milligram basis) is lowered, although this may reflect the unquantified presence of recalcitrant inactive/immature/misfolded forms. Protein film electrochemistry, however, reveals detailed kinetic data that are independent of absolute activities. Like native Hyd-1, the variants have low apparent KMH2 values, do not produce H2 at pH 6, and display the same onset overpotential for H2 oxidation. Mechanistically important differences were identified for the C576U variant bearing the equivalent replacement found in native [NiFeSe]-hydrogenases, its extreme O2 tolerance (apparent KMH2 and Vmax [solution] values relative to native Hyd-1 of 0.13 and 0.04, respectively) implying the importance of a selenium atom in the position cis to the site where exogenous ligands (H−, H2, O2) bind. Observation of the same unusual electrocatalytic signature seen earlier for the proton transfer-defective E28Q variant highlights the direct role of the chalcogen atom (S/Se) at position 576 close to E28, with the caveat that Se is less effective than S in facilitating proton transfer away from the Ni during H2 oxidation by this enzyme.

Novel inhibition mechanism of SARS-CoV-2 main protease by ebselen and its derivatives

The global emergence of SARS-CoV-2 has triggered numerous efforts to develop therapeutic options for COVID-19 pandemic. The main protease of SARS-CoV-2 (Mpro), which is a critical enzyme for transcription and replication of SARS-CoV-2, is a key target for therapeutic development against COVID-19. An organoselenium drug called ebselen has recently been demonstrated to have strong inhibition against Mpro and antiviral activity but its molecular mode of action is unknown preventing further development. We have examined the binding modes of ebselen and its derivative in Mpro via high resolution co-crystallography and investigated their chemical reactivity via mass spectrometry. Stronger Mpro inhibition than ebselen and potent ability to rescue infected cells were observed for a number of ebselen derivatives. A free selenium atom bound with cysteine 145 of Mpro catalytic dyad has been revealed by crystallographic studies of Mpro with ebselen and MR6-31-2 suggesting hydrolysis of the enzyme bound organoselenium covalent adduct, formation of a phenolic by-product is confirmed by mass spectrometry. The target engagement of these compounds with an unprecedented mechanism of SARS-CoV-2 Mpro inhibition suggests wider therapeutic applications of organo-selenium compounds in SARS-CoV-2 and other zoonotic beta-corona viruses.

Selenium atom on phosphate enhances specificity and sensitivity of DNA polymerization and detection

DNA polymerization is of high specificity in vivo. However, its specificity is much lower in vitro, which limits the advanced applications of DNA polymerization in ultrasensitive nucleic acid detection. Herein...

The Thermochemical, Structural, and Spectroscopic Analyses of the Tautomers of Sulfur and Selenium Modified Emissive Nucleobases

In this study, density functional theory (DFT) and time dependent density functional theory (TD-DFT) are used to investigate the stabilities and spectral properties [IR, UV-vis, and two-photon absorption (2PA)] of two sets of modified RNA nucleobase tautomers. The modifications introduce either a sulfur or selenium atom to form an isothiazolo[4,3-d]pyrimidine or isoselenazolo[4,3-d]pyrimidine heterocylic core respectively. The relative stabilities of both sets of modified tautomers determined with B3LYP/6-31++G(d,p) reveal that in water (with a polarizable continuum model) the 6-keto-2-amino tautomer of guanine and the rare 4-imino-2-keto tautomer of cytosine may be present at significant populations while the 6-enol-2-amino tautomer of guanine is more common in the gas phase. The identification of these modified tautomers due the natural differences in their vibrational modes and hence IR spectra is possible. Furthermore, the photophysical properties of both these sets of modified tautomers indicate that excitation and emission energies are shifted relative to their more abundant form in both one photon absorption and emission, and two-photon absorption (2PA) spectra as determined at the B3LYP/6-31++G(d,p) and CAM-B3LYP/aug-cc-pVDZ levels of theory, respectively. Even though the 2PA cross sections in water for all of the species are small (0.3 - 2.3 GM), the modified cytosine tautomer shows promise as its cross section is larger than the more dominant form. The spectral separation between the dominant form and the tautomers of isoselenazole and isothiazole modified cytosine and guanine are relatively similar, suggesting both modifications could be useful in elucidating the tautomers from their more abundant counterparts.

Nanoparticle Conjugates of Selenium Compounds: Preparation, Characterisation and Electron Transfer

One of the important features influencing the biological applications of organoselenium compounds is their redox state, which in turn is affected by their interactions with nearby heteroatoms. To modulate the biological action of selenium in such compounds, researchers have designed new structural motifs and also developed new formulations using inorganic nanoparticles. Metal nanoparticles such as gold nanoparticles (GNPs) and magnetic nanoparticles (MNPs) like iron oxide (Fe3O4) have been extensively studied for conjugation with many heteroatoms (sulphur, nitrogen and oxygen) containing ligands. Selenium, being more polarisable than sulphur, can induce significant surface passivation, thereby providing easy modulations with physico-chemical properties. Considering this, we investigated the physico-chemical properties of a few selenium compounds conjugated to GNPs and MNPs. The GNP conjugates were characterised by spectroscopic and microscopic tools, such as optical absorption, Raman spectroscopy, dynamic light scattering (DLS), the zeta potential and transmission electron microscopy (TEM). The results confirmed that the selenium atom was covalently conjugated to GNPs and this conjugation not only increased their electron transfer ability, but also their antioxidant ability. In another study, asymmetric phenyl selenides were conjugated with MNPs and characterised byX-ray diffraction (XRD), TEM, DLS and zeta potential. The radical scavenging ability of the selenium compounds improved upon conjugation with the MNPs. Therefore, the above studies confirmed that the redox activities of selenium compounds can be modulated upon conjugation with inorganic nanoparticles, such as GNPs and MNPs, which in turn provides new avenues for delivering organoselenium compounds.