Allylic selenosulfide rearrangement: a method for chemical ligation to cysteine and other thiols

Thiol Catalysis of Selenosulfide Bond Cleavage by a Triarylphosphine

The arylthiol 4-mercaptophenylacetic acid (MPAA) is a powerful catalyst of selenosulfide bond reduction by the triarylphosphine 3,3',3″-phosphanetriyltris(benzenesulfonic acid) trisodium salt (TPPTS). Both reagents are water-soluble at neutral pH and are particularly adapted for working with unprotected peptidic substrates. Contrary to trialkylphosphines such as tris(2-carboxyethyl)phosphine hydrochloride (TCEP), TPPTS has the advantage of not inducing deselenization reactions. We believe that the work reported here will be of value for those manipulating selenosulfide bonds in peptidic or protein molecules.

α-Vinyl azide–cysteine click coupling reaction enabled bioorthogonal peptide/protein modification

α-Alkyl and α-aryl vinyl azides were found to be able to couple with cysteine-derived alkyl thiols chemoselectively under mild conditions, providing the corresponding β-ketosulfides with simultaneous extrusion of N2 and ammonia.

Labeling and Natural Post-Translational Modification of Peptides and Proteins via Chemoselective Pd-Catalyzed Prenylation of Cysteine

The prenylation of peptides and proteins is an important post-translational modification observed in vivo. We report that the Pd-catalyzed Tsuji-Trost allylation with a Pd/BIPHEPHOS catalyst system allows the allylation of Cys-containing peptides and proteins with complete chemoselectivity and high n/i regioselectivity. In contrast to recently established methods, which use non-native connections, the Pd-catalyzed prenylation produces the natural n-prenylthioether bond. In addition, a variety of biophysical probes such as affinity handles and fluorescent tags can be introduced into Cys-containing peptides and proteins. Furthermore, peptides containing two cysteine residues can be stapled or cyclized using homobifunctional allylic carbonate reagents.

Application of dehydroalanine as a building block for the synthesis of selenocysteine-containing peptides

Selenocysteine (Sec), the 21^st proteinogenic amino acid, is inserted co-translationally into number of natural proteins. It is coded by a dual function stop codon UGA (opal). It is a redox active amino acid found at the active sites of several enzymes that are involved in oxidation–reduction reactions. These enzymes include the three major mammalian selenoproteins glutathione peroxidase (GPx), thioredoxin reductase (TrxR), and iodothyronine deiodinase (Dio). Although Sec is structurally similar to its sulfur analogue cysteine (Cys), the lower pK_a of the selenol group in Sec as compared to that of Cys and the interesting redox properties of the selenium atom in peptides and proteins play crucial roles in redox catalysis. However, the chemical synthesis of Sec-containing peptides has been a difficult task. In this paper, we report on a new method for the synthesis of Sec and Sec-containing peptides using dehydroalanine (Dha) as a building block.

A new method for the synthesis of Sec and Sec-containing peptides using dehydroalanine (Dha) as a building block is described.

A thiol-free synthesis of alkynyl chalcogenides by the copper-catalyzed C–X (X = S, Se) cross-coupling of alkynyl carboxylic acids with Bunte salts

A thiol-free protocol is developed for the copper-catalyzed C–X (X = S, Se) cross-coupling of alkynyl carboxylic acids with Bunte salts.

The Reactive Sulfur Species Concept: 15 Years On

Fifteen years ago, in 2001, the concept of “Reactive Sulfur Species” or RSS was advocated as a working hypothesis. Since then various organic as well as inorganic RSS have attracted considerable interest and stimulated many new and often unexpected avenues in research and product development. During this time, it has become apparent that molecules with sulfur-containing functional groups are not just the passive “victims” of oxidative stress or simple conveyors of signals in cells, but can also be stressors in their own right, with pivotal roles in cellular function and homeostasis. Many “exotic” sulfur-based compounds, often of natural origin, have entered the fray in the context of nutrition, ageing, chemoprevention and therapy. In parallel, the field of inorganic RSS has come to the forefront of research, with short-lived yet metabolically important intermediates, such as various sulfur-nitrogen species and polysulfides (S_x²⁻), playing important roles. Between 2003 and 2005 several breath-taking discoveries emerged characterising unusual sulfur redox states in biology, and since then the truly unique role of sulfur-dependent redox systems has become apparent. Following these discoveries, over the last decade a “hunt” and, more recently, mining for such modifications has begun—and still continues—often in conjunction with new, innovative and complex labelling and analytical methods to capture the (entire) sulfur “redoxome”. A key distinction for RSS is that, unlike oxygen or nitrogen, sulfur not only forms a plethora of specific reactive species, but sulfur also targets itself, as sulfur containing molecules, i.e., peptides, proteins and enzymes, preferentially react with RSS. Not surprisingly, today this sulfur-centred redox signalling and control inside the living cell is a burning issue, which has moved on from the predominantly thiol/disulfide biochemistry of the past to a complex labyrinth of interacting signalling and control pathways which involve various sulfur oxidation states, sulfur species and reactions. RSS are omnipresent and, in some instances, are even considered as the true bearers of redox control, perhaps being more important than the Reactive Oxygen Species (ROS) or Reactive Nitrogen Species (RNS) which for decades have dominated the redox field. In other(s) words, in 2017, sulfur redox is “on the rise”, and the idea of RSS resonates throughout the Life Sciences. Still, the RSS story isn’t over yet. Many RSS are at the heart of “mistaken identities” which urgently require clarification and may even provide the foundations for further scientific revolutions in the years to come. In light of these developments, it is therefore the perfect time to revisit the original hypotheses, to select highlights in the field and to question and eventually update our concept of “Reactive Sulfur Species”.

A Minimal, Unstrained S-Allyl Handle for Pre-Targeting Diels-Alder Bioorthogonal Labeling in Live Cells

The unstrained S-allyl cysteine amino acid was site-specifically installed on apoptosis protein biomarkers and was further used as a chemical handle and ligation partner for 1,2,4,5-tetrazines by means of an inverse-electron-demand Diels-Alder reaction. We demonstrate the utility of this minimal handle for the efficient labeling of apoptotic cells using a fluorogenic tetrazine dye in a pre-targeting approach. The small size, easy chemical installation, and selective reactivity of the S-allyl handle towards tetrazines should be readily extendable to other proteins and biomolecules, which could facilitate their labeling within live cells.

Chemoselective synthesis of functional homocysteine residues in polypeptides and peptides

A procedure for conversion of methionine residue into functional homocysteine residue has been described.

Allyl Sulfides: Reactive Substrates for Olefin Metathesis

Allyl sulfides have gained traction in recent years as promoters for olefin metathesis. The high reactivity of allyl sulfides in olefin metathesis is remarkable, given that many sulfur-containing substrates are incompatible with ruthenium-based olefin metathesis catalysts. In stark contrast, allyl sulfides actually enhance the rate of metathesis in comparison with other alkenes, when matched with a suitable catalyst. This review examines how the high reactivity of allyl sulfides in olefin metathesis has been harnessed in diverse areas of synthesis. In the cases examined, allyl sulfides have been explicitly incorporated into substrates to promote olefin metathesis. Recent insights into catalyst considerations, applications in chemical and biochemical synthesis, and future opportunities are discussed.

Click chemistry in complex mixtures: bioorthogonal bioconjugation

The selective chemical modification of biological molecules drives a good portion of modern drug development and fundamental biological research. While a few early examples of reactions that engage amine and thiol groups on proteins helped establish the value of such processes, the development of reactions that avoid most biological molecules so as to achieve selectivity in desired bond-forming events has revolutionized the field. We provide an update on recent developments in bioorthogonal chemistry that highlights key advances in reaction rates, biocompatibility, and applications. While not exhaustive, we hope this summary allows the reader to appreciate the rich continuing development of good chemistry that operates in the biological setting.

Allenamides as orthogonal handles for selective modification of cysteine in peptides and proteins

In this study, a remarkably simple and direct strategy has been successfully developed to selectively label target cysteine residues in fully unprotected peptides and proteins. The strategy is based on the reaction between allenamides and the cysteine thiol, and proceeds swiftly in aqueous medium with excellent selectivity and quantitative conversion, thus forming a stable and irreversible conjugate. The combined simplicity and mildness of the process project allenamide as robust and versatile handles to target cysteines and has potential use in biological systems. Additionally, fluorescent-labeling studies demonstrated that the installation of a C-terminal allenamide moiety onto various molecules of interest may supply a new methodology towards the site-specific labeling of cysteine-containing proteins. Such a new labeling strategy may thus open a window for its application in the field of life sciences.

2-(Selenocyanatomethyl)-2-propenol — A convenient synthon for ligation via the deselenative allylic rearrangement of allyl selenosulfides: preparation, functional group compatibility, and application

The preparation and reactions of 2-(selenocyanatomethyl)-2-propenol are described and reveal the compatibility of the allylic selenocyanate group with a range of mild oxidizing and Lewis acidic conditions. 2-(Selenocyanatomethyl)-2-propenol and its derivatives are employed in the functionalization of simple and amino acid derived thiols in methanolic solution at room temperature to give 2-(hydroxymethyl)allyl sulfides in good to excellent yield.

Protecting group-free glycoligation by the desulfurative rearrangement of allylic disulfides as a means of assembly of oligosaccharide mimetics

2-(2-Pyridyldithio-3-butenyl) glycosides react with carbohydrate-based thiols in a two-step process involving sulfenyl transfer followed by desulfurative 2,3-allylic rearrangement, promoted by either triphenylphosphine or silver nitrate, to give novel saccharide mimetics. In an alternative embodiment of the same chemistry anomeric thiols are coupled with carbohydrates derivatized in the form of 2-(2-pyridyldithio-3-butenyl) ethers. This new method of glycoligation does not require protection of hydroxyl groups and is compatible with the presence of acetamides, azides, trichloroethoxycarbamates, and thioglycosides. Variations on the general theme enable the preparation of mimetics of reducing and nonreducing oligosaccharides as well as of nonglycosidically linked systems.

Protein chemical modification on endogenous amino acids

Chemical modification of protein is an arduous but fruitful task. Many chemical methods have been developed for such purpose by carefully balancing reactivity and selectivity. Now both chemists and biologists have in hand an arsenal of tools from which they can select a relevant reaction to tackle their problems. This review focuses on the various chemical transformations available for selective modification of proteins. It also provides a brief overview of some of their main applications, including detection of protein interactions, preparation of bioconjugates, and protein microarrays.

Chemoselective immobilization of peptides on abiotic and cell surfaces at controlled densities

We report herein a new and enabling approach for decorating both abiotic and cell surfaces with the extracellular matrix IKVAV peptide in a site-specific manner using strain promoted azide-alkyne cycloaddition. A cyclooctyne-derivatized IKVAV peptide was synthesized and immobilized on the surface of pancreatic islets through strain-promoted azide-alkyne cycloaddition with cell surface azides generated by the electrostatic adsorption of a cytocompatible poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) copolymer bearing azido groups (PP-N(3)). Both "one-pot" and sequential addition of PP-N(3) and a cyclooctyne-derivatized IKVAV peptide conjugate enabled efficient modification of the pancreatic islet surface in less than 60 min. The ability to bind peptides at controlled surface densities was demonstrated in a quantitative manner using microarrays. Additionally, the technique is remarkably rapid and highly efficient, opening new avenues for the molecular engineering of cellular interfaces and protein and peptide microarrays.

Silver-mediated allylic disulfide rearrangement for conjugation of thiols in protic media

Alkyl and aryl allyl disulfides are induced to undergo the desulfurative allylic rearrangement by silver nitrate in protic solvents at room temperature, thereby removing the necessity for the use of phosphines as thiophilic reagents. The silver-mediated reaction functions at ambient temperature in protic solvents and in the absence of protecting groups.

Chemical modification of proteins at cysteine: opportunities in chemistry and biology

Chemical modification of proteins is a rapidly expanding area in chemical biology. Selective installation of biochemical probes has led to a better understanding of natural protein modification and macromolecular function. In other cases such chemical alterations have changed the protein function entirely. Additionally, tethering therapeutic cargo to proteins has proven invaluable in campaigns against disease. For controlled, selective access to such modified proteins, a unique chemical handle is required. Cysteine, with its unique reactivity, has long been used for such modifications. Cysteine has enjoyed widespread use in selective protein modification, yet new applications and even new reactions continue to emerge. This Focus Review highlights the enduring utility of cysteine in protein modification with special focus on recent innovations in chemistry and biology associated with such modifications.

Sugars and proteins: New strategies in synthetic biology

The development of novel methodology for bond-forming processes that are compatible with biomolecules allows the assembly, alteration, or modification of proteins. Such synthetic proteins allow precise insight and investigation of function in a manner that has the potential for almost unlimited diversity.

Synthesis of neoglycoconjugates by the desulfurative rearrangement of allylic disulfides

Two series of neoglycosyl donors are prepared on the basis of connection of an allylic disulfide motif to the anomeric center via a simple O-glycosyl linkage or N-glycosyl amide unit. Conjugation of both sets of donors to cysteine in peptides is demonstrated through classical disulfide exchange followed by the phosphine-mediated desulfurative allylic rearrangement resulting in neoglycopeptides characterized by a simple thioether spacer. The conjugation reaction functions in the absence of protecting groups on both the neoglycosyl donor and peptide in aqueous media at room temperature.

Dechalcogenative allylic selenosulfide and disulfide rearrangements: complementary methods for the formation of allylic sulfides in the absence of electrophiles. Scope, limitations, and application to the functionalization of unprotected peptides in aqueous media

Primary allylic selenosulfates (seleno Bunte salts) and selenocyanates transfer the allylic selenide moiety to thiols giving primary allylic selenosulfides, which undergo rearrangement in the presence of PPh3 with the loss of selenium to give allylically rearranged allyl alkyl sulfides. This rearrangement may be conducted with prenyl-type selenosulfides to give isoprenyl alkyl sulfides. Alkyl secondary and tertiary allylic disulfides, formed by sulfide transfer from allylic heteroaryl disulfides to thiols, undergo desulfurative allylic rearrangement on treatment with PPh3 in methanolic acetonitrile at room temperature. With nerolidyl alkyl disulfides this rearrangement provides an electrophile-free method for the introduction of the farnesyl chain onto thiols. Both rearrangements are compatible with the full range of functionality found in the proteinogenic amino acids, and it is demonstrated that the desulfurative rearrangement functions in aqueous media, enabling the derivatization of unprotected peptides. It is also demonstrated that the allylic disulfide rearrangement can be induced in the absence of phosphine at room temperature by treatment with piperidine, or simply by refluxing in methanol. Under these latter conditions the reaction is also applicable to allyl aryl disulfides, providing allylically rearranged allyl aryl sulfides in good yields.

Sigmatropic rearrangements as tools for amino acid and peptide modification: application of the allylic sulfur ylide rearrangement to the preparation of neoglycoconjugates and other conjugates

Reaction of S-allyl cysteine derivatives, generated by the selenocysteine ligation, with rhodium carbenoids, stabilized and unstabilized, enables the attachment of diverse functionality onto cysteine residues. The reaction is successfully applied to the introduction of lipid-like residues, a fluorous alkyl chain, and mono- and disaccharides.

Preparation of N-tBoc L-glutathione dimethyl and di-tert-butyl esters: versatile synthetic building blocks

The title l-glutathione derivatives, containing acid- and base-labile esters, respectively, were obtained in good overall yields. N-(t)Boc l-glutathione dimethyl ester was prepared via Fischer esterification of l-glutathione disulfide (GSSG) using HCl in dry methanol, protection of the amine with (t)Boc(2)O, and tributylphosphine cleavage of the disulfide in wet isopropanol. Alternatively, Fischer esterification and (t)Boc-protection of l-glutathione (GSH) also furnished N-(t)Boc glutathione dimethyl ester accompanied by a small amount of S-(t)Boc that was removed chromatographically. The di-tert-butyl ester was obtained by S-palmitoylation of GSH in TFA as solvent, N-(t)Boc-protection, esterification using (t)BuOH mediated by diisopropylcarbodiimide/copper(I) chloride, and saponification of the thioester. These l-glutathione derivatives are versatile synthetic building blocks for the preparation of S-glutathione adducts.

Allylic Disulfide Rearrangement and Desulfurization: Mild, Electrophile-Free Thioether Formation from Thiols

[reaction: see text] Secondary and tertiary allylic 2-pyridyl and 2-benzothiazolyl disulfides react with thiol groups at room temperature to give secondary and tertiary allyl alkyl disulfides. On the addition of a phosphine, a desulfurative sigmatropic rearrangement takes place at room temperature to give thioethers.

A novel neoglycopeptide linkage compatible with native chemical ligation

The straightforward synthesis of a novel class of neoglycopeptide and its fusion with a larger peptide thioester using sequential chemoselective ligations is described.