Peptide Functionalization Through the Generation of Selenocysteine Electrophile

Gold-Promoted Biocompatible Selenium Arylation of Small Molecules, Peptides and Proteins

A low pKa (5.2), high polarizable volume (3.8 Å), and proneness to oxidation under ambient conditions make selenocysteine (Sec, U) a unique, natural reactive handle present in most organisms across all domains of life. Sec modification still has untapped potential for site-selective protein modification and probing. Herein we demonstrate the use of a cyclometalated gold(III) compound, [Au(bnpy)Cl2 ], in the arylation of diselenides of biological significance, with a scope covering small molecule models, peptides, and proteins using a combination of multinuclear NMR (including 77 Se NMR), and LC-MS. Diphenyl diselenide (Ph-Se)2 and selenocystine, (Sec)2 , were used for reaction optimization. This approach allowed us to demonstrate that an excess of diselenide (Au/Se-Se) and an increasing water percentage in the reaction media enhance both the conversion and kinetics of the C-Se coupling reaction, a combination that makes the reaction biocompatible. The C-Se coupling reaction was also shown to happen for the diselenide analogue of the cyclic peptide vasopressin ((Se-Se)-AVP), and the Bos taurus glutathione peroxidase (GPx1) enzyme in ammonium acetate (2 mM, pH=7.0). The reaction mechanism, studied by DFT revealed a redox-based mechanism where the C-Se coupling is enabled by the reductive elimination of the cyclometalated Au(III) species into Au(I).

Selenium chemistry for spatio-selective peptide and protein functionalization

The ability to construct a peptide or protein in a spatio-specific manner is of great interest for therapeutic and biochemical research. However, the various functional groups present in peptide sequences and the need to perform chemistry under mild and aqueous conditions make selective protein functionalization one of the greatest synthetic challenges. The fascinating paradox of selenium (Se) - being found in both toxic compounds and also harnessed by nature for essential biochemical processes - has inspired the recent exploration of selenium chemistry for site-selective functionalization of peptides and proteins. In this Review, we discuss such approaches, including metal-free and metal-catalysed transformations, as well as traceless chemical modifications. We report their advantages, limitations and applications, as well as future research avenues.

Palladium-Catalyzed Direct Selanylation of Chalcogenophenes and Arenes Assisted by 2-(Methylthio)amide

The direct and selective conversion of a C-H bond into a C-Se bond remains a significant challenge, which is even more intricate with substrates having an innate regioselectivity under several reaction conditions, such as chalcogenophenes. We overrode their selectivity toward selanylation using palladium, copper, and the 2-(methylthio)amide directing group. This chelation-assisted direct selanylation was also suitable for mono and double ortho functionalization of arenes. The mechanistic studies indicate high-valent Pd(IV) species in the catalytic cycle, a reversible C-H activation step, and Cu(II) as a sequestering agent for organoselenide byproducts.

Selenium in Peptide Chemistry

In recent years, researchers have been exploring the potential of incorporating selenium into peptides, as this element possesses unique properties that can enhance the reactivity of these compounds. Selenium is a non-metallic element that has a similar electronic configuration to sulfur. However, due to its larger atomic size and lower electronegativity, it is more nucleophilic than sulfur. This property makes selenium more reactive toward electrophiles. One of the most significant differences between selenium and sulfur is the dissociation of the Se-H bond. The Se-H bond is more easily dissociated than the S-H bond, leading to higher acidity of selenocysteine (Sec) compared to cysteine (Cys). This difference in acidity can be exploited to selectively modify the reactivity of peptides containing Sec. Furthermore, Se-H bonds in selenium-containing peptides are more susceptible to oxidation than their sulfur analogs. This property can be used to selectively modify the peptides by introducing new functional groups, such as disulfide bonds, which are important for protein folding and stability. These unique properties of selenium-containing peptides have found numerous applications in the field of chemical biology. For instance, selenium-containing peptides have been used in native chemical ligation (NCL). In addition, the reactivity of Sec can be harnessed to create cyclic and stapled peptides. Other chemical modifications, such as oxidation, reduction, and photochemical reactions, have also been applied to selenium-containing peptides to create novel molecules with unique biological properties.

Nonyl Acridine Orange as a Prospective Photocatalyst in Chalcogenylation of Coumarins and Quinolinones

A mild and efficient method for preparation of 3-sulfenyl and 3-selenyl coumarins and quinolinones mediated by artificial light or sunlight is presented. The elaborated protocol highlights the use of nonyl acridine orange as a photocatalyst to generate a sulfenyl radical from thiols that is further trapped by a heterocycle. The utility of the protocol is justified by a diverse scope of thiols, including short cysteine-containing peptides. The same reaction conditions can be applied for preparation of 3-selenyl coumarins and quinolinones. Various protected and unprotected selenocysteine-containing peptides were successfully utilized demonstrating high tolerance for amino acids with sensitive groups (Arg, Lys, Trp, His, and Tyr).

Copper(II)-mediated C-H sulphenylation or selenylation of tryptophan enabling macrocyclization of peptides

Cu(II)-mediated C-H sulphenylation or selenylation of Trp indole by a derivative of cysteine or selenocysteine enables access to the tryptathionine unit or its selenium congener. The mechanism of these protocols, which allow macrocyclization of Trp-containing peptides, has been studied.

Chemoselective Copper-Mediated Modification of Selenocysteines in Peptides and Proteins

Highly valuable bioconjugated molecules must be synthesized through efficient, chemoselective chemical modifications of peptides and proteins. Herein, we report the chemoselective modification of peptides and proteins via a reaction between selenocysteine residues and aryl/alkyl radicals. In situ radical generation from hydrazine substrates and copper ions proceeds rapidly in an aqueous buffer at near neutral pH (5-8), providing a variety of Se-modified linear and cyclic peptides and proteins conjugated to aryl and alkyl molecules, and to affinity label tag (biotin). This chemistry opens a new avenue for chemical protein modifications.

Straightforward Functionalization of Sulfur-Containing Peptides via 5- and 6-endo-dig Cyclization Reactions

We present a simple and convenient method for the generation of sulfenyl electrophiles from peptides containing S–S or S–H bonds by employing N-chlorosuccinimide. The corresponding sulfenyl electrophiles are further utilized in 5- and 6-endo-dig cyclization reactions yielding indolizinium salts, indoles, benzo[b]furans, polyaromatic hydrocarbons (PAHs) and isocoumarins, as well as quinolinones bearing a glutathione moiety. PAH derivatives can be used as selective fluorescent dyes for the visualization of lipid droplets in living cells.

Diorganyl Dichalcogenides and Copper/Iron Salts: Versatile Cyclization System To Achieve Carbo- and Heterocycles from Alkynes

Organochalcogen-containing cyclic molecules have shown several promising pharmacological properties. Consequently, different strategies have been developed for their synthesis in the past few years. Particularly due to the low cost and environmental aspects, copper- and iron-promoted cyclization reactions of alkynyl substrates have been broadly and efficiently applied for this purpose. This short review presents an overview of the most recent advances in the synthesis of organochalcogen-containing carbo- and heterocycles by reacting diorganyl disulfides, diselenides, and ditellurides with alkyne derivatives in the presence of copper and iron salts to promote cyclization reactions.

1 Introduction

2 Synthesis of Carbo- and Heterocycles via Reactions of Alkynes with Diorganyl Dichalcogenides and Copper Salts

3 Synthesis of Carbo- and Heterocycles via Reactions of Alkynes with Diorganyl Dichalcogenides and Iron Salts

4 Conclusions

Strong σ-Hole Activation on Icosahedral Carborane Derivatives for a Directional Halide Recognition

Crystal engineering based on σ-hole interactions is an emerging approach for realization of new materials with higher complexity. Neutral inorganic clusters derived from 1,2-dicarba-closo-dodecaborane, substituted with -SeMe, -TeMe, and -I moieties on both skeletal carbon vertices are experimentally demonstrated herein as outstanding chalcogen- and halogen-bond donors. In particular, these new molecules strongly interact with halide anions in the solid-state. The halide ions are coordinated by one or two donor groups (μ₁ - and μ₂ -coordinations), to stabilize a discrete monomer or dimer motifs to 1D supramolecular zig-zag chains. Crucially, the observed chalcogen bond and halogen bond interactions feature remarkably short distances and high directionality. Electrostatic potential calculations further demonstrate the efficiency of the carborane derivatives, with V_s,max being similar or even superior to that of reference organic halogen-bond donors, such as iodopentafluorobenzene.

Selanyl and tellanyl electrophiles as a driving force in the construction of sophisticated polyaromatic hydrocarbons

Herein, we report the first examples of N-polyaromatic compounds bearing up to 13 fused aromatic rings, including 23H-benzo[12,1]tetrapheno[8,9-b]benzo[12,1]tetrapheno[9,8-h]carbazole derivatives.

Selenocysteinyl electrophiles efficiently promote the formation of coumarin and quinolinone cores by 6-endo-dig cyclization

Efficient methods have been disclosed for the construction of nitrogen and oxygen containing heterocyclic systems attached to selenocysteine or selenoglutathione.