Modeling Selenoprotein Se-Nitrosation: Synthesis of a Se-Nitrososelenocysteine with Persistent Stability

Reusable Selenenyl Iodide-Initiated Cascade Cyclization of Polyenes with N-terminating Groups

Although cascade cyclization of polyenes has advanced remarkably, most reported examples are limited to carbon- or oxygen-terminating groups, with nitrogen-terminating cyclizations remaining rare. In this work, we developed a cascade cyclization of polyenes with N-terminating groups, initiated by an isolable selenenyl iodide (RSeI) bearing a cavity-shaped substituent. Acid-labile substrates were successfully employed in the cascade cyclization. Compared with commonly used organoselenium reagents, a selenenyl iodide, characterized by its unique soft electrophilic nature, proved to be the most effective in promoting the cascade reaction. Furthermore, the stabilizing effect of the cavity-shaped substituent enabled the isolation of a selenenic acid (RSeOH), which was generated via oxidative β-selenoxide elimination from the cyclized product during its derivatization to an olefin, as a stable compound. We also developed a method for regenerating the starting selenenyl iodide from the selenenic acid. The reusability of the selenenyl iodide renders the series of molecular transformations environmentally benign by reducing unwanted organoselenium waste.

New Fluorescent Chemodosimetric Mechanism for Selective Recognition of Selenocysteine by Dansyl-Appended Ruthenium Nitrosyl Complexes

Selenocysteine (Sec) is a biologically essential amino acid that serves as a crucial component in selenoproteins that play a key role in various cellular functions. Thus, developing a reliable and rapid method for detecting Sec in physiological media is of paramount importance. This report introduces for the first time a novel fluorescent chemodosimetric mechanism for the selective recognition of Sec using dansyl-appended ruthenium nitrosyl complexes. These complexes consist of a tetradentate ligand featuring a π-extended system (L = N,N'-bis(2-hydroxy-1-naphthylidene)-1,2-phenylenediamine) and a monodentate ligand derived from the conjugated dansyl group, which acts as a strong fluorescent signaling unit (ID = dansyl-imidazole, BD = dansyl-benzimidazole). The reaction between Sec and the complexes {RuNO}6 = [RuL(NO)(ID)]Cl or [RuL(NO)(BD)]Cl in an aqueous phase enhances fluorescence; as a result, it releases NO• that has been demonstrated through fluorimetric titrations, UV-vis titrations, 77Se NMR, EPR, IR, MS, and electronic density calculations. [RuL(NO)(ID)]Cl and [RuL(NO)(BD)]Cl quantitatively detect Sec within a micromolar concentration range, achieving the limit of detection as low as 0.31 and 0.12 μM, respectively, within just 5 min. Remarkably, these chemodosimeters can also be conveniently employed to detect Sec in living Saccharomyces cerevisiae cells.

Selenocysteine-Activatable Near-Infrared Fluorescent Probe for Screening of Anti-inflammatory Components in Herbs

Inflammation, a central process in numerous diseases, plays a crucial role in hepatic disorders, arthritis, cardiac conditions, and neurodegenerative ailments. Given the lack of effective anti-inflammatory drugs, it is imperative to assess inflammation severity and explore novel therapeutics. Selenocysteine (Sec), a key mediator of selenium's biological function, is closely involved in anti-inflammatory responses. We have synthesized a novel near-infrared fluorescent probe, Sec-BDP, which can image Sec dynamics in vivo with high selectivity and sensitivity. Sec-BDP detects Sec at concentrations as low as 0.085 μM. Utilizing this probe, we visualized Sec levels in cell, zebrafish, and mouse inflammation models, enabling a clear assessment of inflammation severity. To screen for drug candidates, Sec-BDP was integrated with ultrahigh performance liquid chromatography quadrupole time-of-flight mass spectrometry to identify potent anti-inflammatory compounds in Astragalus membranaceus, such as 5-O-methylvisammioside. Imaging of Sec with Sec-BDP provides insights into Sec-related diseases and aids in discovering new treatments. This probe advances selenium biology and promises more targeted therapeutic strategies.

50 Years of Organoselenium Chemistry, Biochemistry and Reactivity: Mechanistic Understanding, Successful and Controversial Stories

In 1973, two major discoveries changed the face of selenium chemistry: the identification of the first mammal selenoenzyme, glutathione peroxidase 1, and the discovery of the synthetic utility of the so-called selenoxide elimination. While the chemical mechanism behind the catalytic activity of glutathione peroxidases appears to be mostly unveiled, little is known about the mechanisms of other selenoproteins and, for some of them, even the function lies in the dark. In chemistry, the capacity of organoselenides of catalyzing hydrogen peroxide activation for the practical manipulation of organic functional groups has been largely explored, and some mechanistic details have been clearly elucidated. As a paradox, despite the long-standing experience in the field, the nature of the active oxidant in various reactions still remains matter of debate. While many successes characterize these fields, the pharmacological use of organoselenides still lacks any true application, and while some organoselenides were found to be non-toxic and safe to use, to date no therapeutically approved use was granted. In this review, some fundamental and chronologically aligned topics spanning organoselenium biochemistry, chemistry and pharmacology are discussed, focusing on the current mechanistic picture describing their activity as either bioactive compounds or catalysts.

Isolation and characterization of a two-coordinate phosphinidene oxide

Nitroso compounds, R-N=O, are common intermediates in organic synthesis, and are typically amenable to storage and manipulation at ambient temperature under aerobic conditions. By contrast, phosphorus-containing analogues, such as R-P=O (R = OH, CH3, OCH3, Ph), are extremely reactive and need to be studied in inert gas matrices at ultralow temperatures (3-15 K). These species are believed to be key intermediates in the degradation/combustion of organic phosphorus compounds, a class of chemicals that includes chemical warfare agents and flame retardants. Here we describe the isolation of a two-coordinate phosphorus(III) oxide under ambient conditions, enabled by the use of an extremely bulky amine ligand. Reactivity studies reveal that the phosphorus centre can be readily oxidized, and that in doing so, the P-O bond remains intact, an observation that is of interest to the proposed reactivity of transient phosphorus(III) oxides.

Self-Sulfhydrated, Nitro-Fixed Albumin Nanoparticles as a Potent Therapeutic Agent for the Treatment of Acute Liver Injury

Exogenous polysulfhydryls (R-SH) supplementation and nitric oxide (NO) gas molecules delivery provide essential antioxidant buffering pool components and anti-inflammatory species in cellular defense against injury, respectively. Herein, the intermolecular disulfide bonds in bovine serum albumin (BSA) molecules were reductively cleaved under native and mild conditions to expose multiple sulfhydryl groups (BSA-SH), then sulfhydryl-nitrosylated (R-SNO), and nanoprecipitated to form injectable self-sulfhydrated, nitro-fixed albumin nanoparticles (BSA-SNO NPs), allowing albumin to act as a NO donor reservoir and multiple sulfhydryl group transporter while also preventing unfavorable oxidation and self-cross-linking of polysulfhydryl groups. In two mouse models of ischemia/reperfusion-induced and endotoxin-induced acute liver injury (ALI), a single low dosage of BSA-SNO NPs (S-nitrosothiols: 4 μmol·kg-1) effectively attenuated oxidative stress and systemic inflammation cascades in the upstream pathophysiology of disease progression, thus rescuing dying hepatocytes, regulating host defense, repairing microcirculation, and restoring liver function. By mechanistically upregulating the antioxidative signaling pathway (Nrf-2/HO-1/NOQ1) and inhibiting the inflammatory cytokine storm (NF-κB/p-IκBα/TNF-α/IL-β), BSA-SNO NPs blocked the initiation of the mitochondrial apoptotic signaling pathway (Cyto C/Bcl-2 family/caspase-3) and downregulated the cell pyroptosis pathway (NLRP3/ASC/IL-1β), resulting in an increased survival rate from 26.7 to 73.3%. This self-sulfhydrated, nitro-fixed functionalized BSA nanoformulation proposes a potential drug-free treatment strategy for ALI.

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.

Synergistic Activation of Nitrite and Thiocarbonyl Compounds Affords NO and Sulfane Sulfur via (Per)thionitrite (SNO- /SSNO- )

(Per)thionitrite (SNO- /SSNO- ) intermediates play vital roles in modulating nitric oxide (NO) and hydrogen sulfide (H2 S) dependent bio-signalling processes. Whilst the previous preparations of such intermediates involved reactive H2 S/HS- or sulfane sulfur (S0 ) species, the present report reveals that relatively stable thiocarbonyl compounds (such as carbon disulfide (CS2 ), thiocarbamate, thioacetic acid, and thioacetate) react with nitrite anion to yield SNO- /SSNO- . For instance, the reaction of CS2 and nitrite anion (NO2 - ) under ambient condition affords CO2 and SNO- /SSNO- . A detailed investigation involving UV/Vis, FTIR, HRMS, and multinuclear NMR studies confirm the formation of SNO- /SSNO- , which are proposed to form through an initial nucleophilic attack by nitrite anion followed by a transnitrosation step. Notably, reactions of CS2 and nitrite in the presence of thiol RSH show the formation of organic polysulfides R-Sn -R, thereby illustrating that the thiocarbonyls are capable of influencing the pool of bioavailable sulfane sulfurs. Furthermore, the availability of both NO2 - and thiocarbonyl motifs in the biological context hints at their synergistic metal-free activations leading to the generation of NO gas and various reactive sulfur species via SNO- /SSNO- .

Demonstration of the Formation of a Selenocysteine Selenenic Acid through Hydrolysis of a Selenocysteine Selenenyl Iodide Utilizing a Protective Molecular Cradle

Selenocysteine selenenic acids (Sec-SeOHs) and selenocysteine selenenyl iodides (Sec-SeIs) have long been recognized as crucial intermediates in the catalytic cycle of glutathione peroxidase (GPx) and iodothyronine deiodinase (Dio), respectively. However, the observation of these reactive species remained elusive until our recent study, where we successfully stabilized Sec-SeOHs and Sec-SeIs using a protective molecular cradle. Here, we report the first demonstration of the chemical transformation from a Sec-SeI to a Sec-SeOH through alkaline hydrolysis. A stable Sec-SeI derived from a selenocysteine methyl ester was synthesized using the protective cradle, and its structure was determined by crystallographic analysis. The alkaline hydrolysis of the Sec-SeI at -50 °C yielded the corresponding Sec-SeOH in an 89% NMR yield, the formation of which was further confirmed by its reaction with dimedone. The facile and nearly quantitative conversion of the Sec-SeI to the Sec-SeOH not only validates the potential involvement of this process in the catalytic mechanism of Dio, but also highlights its utility as a method for producing a Sec-SeOH.

Promotion of S-nitrosation of cysteine by a {Co(NO)2}10 complex

S-Nitrosothiols (SNOs) serve as endogenous carriers and donors of NO within living cells, releasing nitrosonium ions (NO+), NO, or other nitroso derivatives. In this study, we present a bioinspired {Co(NO)2}10 complex 1 that achieved S-nitrosation towards Cys residues. The incorporation of a ferrocenyl group in 1 allowed for fine-tuning of the nitrosation reaction, taking advantage of the redox ability of Cys residues. Complex 1 was synthesized and characterized, demonstrating its NO translation reactivity. Furthermore, complex 1 successfully converted Cys into S-nitrosocysteine (Cys-SNO), as confirmed by UV-Vis, IR, and XAS spectroscopy. This study presents a promising approach for S-nitrosation of Cys residues for further exploration in the modification of Cys-containing peptides.