Chemical synthesis and functional evaluation of glycopeptides and glycoproteins containing rare glycosyl amino acid linkages

Covering: 1987 to 2023Naturally existing glycoproteins through post-translational protein glycosylation are highly heterogeneous, which not only impedes the structure-function studies, but also hinders the development of their potential medical usage. Chemical synthesis represents one of the most powerful tools to provide the structurally well-defined glycoforms. Being the key step of glycoprotein synthesis, glycosylation usually takes place at serine, threonine, and asparagine residues, leading to the predominant formation of the O- and N-glycans, respectively. However, other amino acid residues containing oxygen, nitrogen, sulfur, and nucleophilic carbon atoms have also been found to be glycosylated. These diverse glycoprotein linkages, occurring from microorganisms to plants and animals, play also pivotal biological roles, such as in cell-cell recognition and communication. The availability of these homogenous rare glycopeptides and glycoproteins can help decipher the glyco-code for developing therapeutic agents. This review highlights the chemical approaches for assembly of the functional glycopeptides and glycoproteins bearing these "rare" carbohydrate-amino acid linkages between saccharide and canonical amino acid residues and their derivatives.

Selective Anomeric Acetylation of Unprotected Sugars with Acetic Anhydride in Water

Unprotected sugars are selectively acetylated simply by stirring in aqueous solution in the presence of acetic anhydride and a weak base such as sodium carbonate. The reaction is selective for acetylation of the anomeric hydroxyl group of mannose, 2-acetamido, and 2-deoxy sugars and can be performed on a large scale. Competitive intramolecular migration of the 1-O-acetate to the 2-hydroxyl group when these two substituents are cis causes over-reaction and the formation of product mixtures.

Late-Stage Functionalisation of Peptides on the Solid Phase by an Iodination-Substitution Approach

The functionalisation of peptides at a late synthesis stage holds great potential, for example, for the synthesis of peptide pharmaceuticals, fluorescent biosensors or peptidomimetics. Here we describe an on-resin iodination-substitution reaction sequence on homoserine that is also suitable for peptide modification in a combinatorial format. The reaction sequence is accessible to a wide range of sulfur nucleophiles with various functional groups including boronic acids, hydroxy groups or aromatic amines. In this way, methionine-like thioethers or thioesters and thiosulfonates are accessible. Next to sulfur nucleophiles, selenols, pyridines and carboxylic acids were successfully used as nucleophiles, whereas phenols did not react. The late-stage iodination-substitution approach is not only applicable to short peptides but also to the more complex 34-amino-acid WW domains. We applied this strategy to introduce 7-mercapto-4-methylcoumarin into a switchable ZnII responsive WW domain to design an iFRET-based ZnII sensor.

Protecting group free glycosylation: one-pot stereocontrolled access to 1,2-trans glycosides and (1→6)-linked disaccharides of 2-acetamido sugars

Unprotected 2-acetamido sugars may be directly converted into their oxazolines using 2-chloro-1,3-dimethylimidazolinium chloride (DMC), and a suitable base, in aqueous solution. Freeze drying and acid catalysed reaction with an alcohol as solvent produces the corresponding 1,2-trans-glycosides in good yield. Alternatively, dissolution in an aprotic solvent system and acidic activation in the presence of an excess of an unprotected glycoside as a glycosyl acceptor, results in the stereoselective formation of the corresponding 1,2-trans linked disaccharides without any protecting group manipulations. Reactions using aryl glycosides as acceptors are completely regioselective, producing only the (1→6)-linked disaccharides.

Streamlined Iterative Assembly of Thio-Oligosaccharides by Aqueous S-Glycosylation of Diverse Deoxythio Sugars

A streamlined iterative assembly of thio-oligosaccharides was developed by aqueous glycosylation. Facile syntheses of various deoxythio sugars with the sulfur on different positions from commercially available starting materials were described. These syntheses featured efficient chemical methods including our recently reported BTM-catalyzed site-selective acylation. The resulting deoxythio sugars could then be used for the Ca(OH)2 -promoted protecting group-free S-glycosylation in water at room temperature. The aqueous glycosylation reaction proceeded smoothly to afford the corresponding 1,2-trans S-glycosides in good yields with high chemo- and stereoselectivity. An appropriate choice of protecting groups for the thiol in the glycosyl donor was necessary for the development of iterative synthesis of thio-oligosaccharides. The aqueous glycosylation was then applied to the synthesis of a trimannoside moiety of N-linked glycans core region.

Chemical (neo)glycosylation of biological drugs

Biological drugs, specifically proteins and peptides, are a privileged class of medicinal agents and are characterized with high specificity and high potency of therapeutic activity. However, biologics are fragile and require special care during storage, and are often modified to optimize their pharmacokinetics in terms of proteolytic stability and blood residence half-life. In this review, we showcase glycosylation as a method to optimize biologics for storage and application. Specifically, we focus on chemical glycosylation as an approach to modify biological drugs. We present case studies that illustrate the success of this methodology and specifically address the highly important question: does connectivity within the glycoconjugate have to be native or not? We then present the innovative methods of chemical glycosylation of biologics and specifically highlight the emerging and established protecting group-free methodologies of glycosylation. We discuss thermodynamic origins of protein stabilization via glycosylation, and analyze in detail stabilization in terms of proteolytic stability, aggregation upon storage and/or heat treatment. Finally, we present a case study of protein modification using sialic acid-containing glycans to avoid hepatic clearance of biological drugs. This review aims to spur interest in chemical glycosylation as a facile, powerful tool to optimize proteins and peptides as medicinal agents.

Applications of Shoda's reagent (DMC) and analogues for activation of the anomeric centre of unprotected carbohydrates

2-Chloro-1,3-dimethylimidazolinium chloride (DMC, herein also referred to as Shoda's reagent) and its derivatives are useful for numerous synthetic transformations in which the anomeric centre of unprotected reducing sugars is selectively activated in aqueous solution. As such unprotected sugars can undergo anomeric substitution with a range of added nucleophiles, providing highly efficient routes to a range of glycosides and glycoconjugates without the need for traditional protecting group manipulations. This mini-review summarizes the development of DMC and some of its derivatives/analogues, and highlights recent applications for protecting group-free synthesis.

One-Step Synthesis of Sugar Nucleotides

The chemical synthesis of sugar nucleotides requires a multistep procedure to ensure a selective reaction. Herein, sugar nucleotides were synthesized in one step using 2-chloro-1,3-dimethylimidazolinium chloride as the condensation reagent. The products were obtained in yields of 12-30%, and the yields were increased to 35-47% by the addition of a tuning reagent. NMR identification of the sugar nucleotides showed that mainly 1,2-trans-glycosides were present. The reported method represents a one-step route to sugar nucleotides from commercially available materials.

Scope of the DMC mediated glycosylation of unprotected sugars with phenols in aqueous solution

Activation of reducing sugars in aqueous solution using DMC and triethylamine in the presence of phenols allows direct stereoselective conversion to the corresponding 1,2-trans aryl glycosides without the need for any protecting groups.

On-Bead Peptoid Dimerization Induced by Incorporation of Glycosylated Bridging Units in Submonomer Solid-Phase Approach to Glycopeptoids

A study on submonomer solid-phase synthesis of S-glycopeptoids has been carried out by screening different parameters. Dimeric species, featuring glycosylated bridging amino monomers, were found under suitable conditions. These dimers arise from an on-resin cross-linking reaction occurring with the incorporation of a glycoamino submonomer into the growing chain and subsequent nucleophilic attack of the resulting secondary amine to a still unreacted bromoacetylated unit. The arising byproduct can be regarded as a novel dimeric peptoid type.

Synthesis of aryl-thioglycopeptides through chemoselective Pd-mediated conjugation

We describe herein a Pd-catalyzed methodology for the thioglycoconjugation of iodoaryl peptides and aminoacids. This operationally simple process occurs under semi-aqueous conditions and displays wide substrate scope. The strategy has been successfully applied to both the thioglycosylation of unprotected peptides and the generation of thioglyco-aminoacid building blocks, including those suitable for solid phase peptide synthesis. To demonstrate the broad potential of this technique for late stage functionalization, we successfully incorporated challenging unprotected β-S-GlcNAc- and α-S-GalNAc-derivatives into very long unprotected peptides. This study opens the way to new applications in chemical biology, considering the well-recognized advantages of S-glycosides over O-glycosides in terms of resistance towards both enzymatic and chemical degradation.

Strategies toward protecting group-free glycosylation through selective activation of the anomeric center

Glycosylation is an immensely important biological process and one that is highly controlled and very efficient in nature. However, in a chemical laboratory the process is much more challenging and usually requires the extensive use of protecting groups to squelch reactivity at undesired reactive moieties. Nonetheless, by taking advantage of the differential reactivity of the anomeric center, a selective activation at this position is possible. As a result, protecting group-free strategies to effect glycosylations are available thanks to the tremendous efforts of many research groups. In this review, we showcase the methods available for the selective activation of the anomeric center on the glycosyl donor and the mechanisms by which the glycosylation reactions take place to illustrate the power these techniques.

Selective anomeric acetylation of unprotected sugars in water

High yielding selective acetylation of only the anomeric hydroxyl of unprotected sugars is possible in aqueous solution using 2-chloro-1,3-dimethylimidazolinium chloride (DMC), thioacetic acid, and a suitable base. The reaction, which may be performed on a multi-gram scale, is stereoselective for sugars that possess a hydroxyl group at position-2, exclusively yielding the 1,2-trans products. The use of an iterative reagent addition procedure allows the use of sodium carbonate as the base, avoiding the formation of triethylammonium salts, which may hamper product purification. The glycosyl acetate products may be used as donor substrates for glycosidase-catalysed synthesis. The crude aqueous acetylation reaction mixture may also be used for this purpose.

Protecting group free synthesis of glycosyl thiols from reducing sugars in water; application to the production of N-glycan glycoconjugates

Un-protected 2-acetamido terminated reducing sugars may be converted into the corresponding glycosyl thiols in water, and conjugated to peptides using the thiol–ene click reaction without recourse to any protecting groups.

Protecting-Group-Free Synthesis of Glycopolymers Bearing Sialyloligosaccharide and Their High Binding with the Influenza Virus

Glycopolymers having pendant triazole-linked sialyloligosaccharides were successfully synthesized from free saccharides without any protection of the hydroxy and carboxy groups on the saccharides. The glycomonomers were synthesized by the direct azidation of free saccharides using 2-chloro-1,3-dimethylimidazolinium chloride as a condensing agent followed by copper(I)-catalyzed azide-alkyne cycloaddition. The resultant glycomonomers were copolymerized with acrylamide by a reversible addition-fragmentation chain transfer technique. Each of the glycopolymers were obtained and then immobilized on a gold-coated sensor of quartz crystal microbalance to analyze their binding behavior with the lectin. The glycopolymers strongly bound with the corresponding lectin without nonspecific adsorption in aqueous solution. In addition, the glycopolymer bearing a complex-type sialyl N-linked oligosaccharide was found to strongly bind with both human and avian influenza A viruses. The strong binding, observed using the hemagglutination inhibition assay, was attributed to the glycocluster effect of the glycopolymer and the biantennary structure of the N-linked oligosaccharide.

Protecting-group-free one-pot synthesis of glycoconjugates directly from reducing sugars

The conversion of sugars into glycomimetics typically involves multiple protecting-group manipulations. The development of methodology allowing the direct aqueous conversion of free sugars into glycosides, and mimics of oligosaccharides and glycoconjugates in a high-yielding and stereoselective process is highly desirable. The combined use of 2-azido-1,3-dimethylimidazolinium hexafluorophosphate and the Cu-catalyzed Huisgen cycloaddition allowed the synthesis of a range of glycoconjugates in a one-step reaction directly from reducing sugars under aqueous conditions. The reaction, which is completely stereoselective, may be applied to the convergent synthesis of triazole-linked glycosides, oligosaccharides, and glycopeptides. The procedure provides a method for the one-pot aqueous ligation of oligosaccharides and peptides bearing alkyne side chains.

PNA-encoded synthesis (PES) of a 10 000-member hetero-glycoconjugate library and microarray analysis of diverse lectins

Identification of selective and synthetically tractable ligands to glycan-binding proteins is important in glycoscience. Carbohydrate arrays have had a tremendous impact on profiling glycan-binding proteins and as analytical tools. We report a highly miniaturized synthetic format to access nucleic-acid-encoded hetero-glycoconjugate libraries with an unprecedented diversity in the combinations of glycans, linkers, and capping groups. Novel information about plant and bacterial lectin specificity was obtained by microarray profiling, and we show that a ligand identified on the array can be converted to a high-affinity soluble ligand by straightforward chemistry.

S-Michael additions to chiral dehydroalanines as an entry to glycosylated cysteines and a sulfa-Tn antigen mimic

Stereoselective sulfa-Michael addition of appropriately protected thiocarbohydrates to chiral dehydroalanines has been developed as a key step in the synthesis of biologically important cysteine derivatives, such as S-(β-D-glucopyranosyl)-D-cysteine, which has not been synthesized to date, and S-(2-acetamido-2-deoxy-α-D-galactopyranosyl)-L-cysteine, which could be considered as a mimic of Tn antigen. The corresponding diamide derivative was also synthesized and analyzed from a conformational viewpoint, and its bound state with a lectin was studied.

Solid phase oxime ligations for the iterative synthesis of polypeptide conjugates

All on-resin! An efficient C-to-N iterative strategy for solid phase chemical ligations (SPCL).