Glycomethanethiosulfonates: powerful reagents for protein glycosylation

Site- and Stereoselective Glycomodification of Biomolecules through Carbohydrate-Promoted Pictet-Spengler Reaction

Carbohydrates play pivotal roles in an array of essential biological processes and are consequently involved in many diseases. To meet the needs of glycobiology research, chemical enzymatic and non-enzymatic methods have been developed to generate glycoconjugates with well-defined structures. Herein, harnessing the unique properties of C6-oxidized glycans, we report a straightforward and robust strategy for site- and stereoselective glycomodification of biomolecules with N-terminal tryptophan residues by a carbohydrate-promoted Pictet-Spengler reaction, which is not adapted to typical aldehyde substrates under biocompatible conditions. This method reliably delivers highly homogeneous glycoconjugates with stable linkages and thus has great potential for functional modulation of peptides and proteins in glycobiology research. Moreover, this reaction can be performed at the glycosites of glycopeptides, glycoproteins and living-cell surfaces in a site-specific manner. Control experiments indicated that the protected α-O atom of aldehyde donors and free N-H bond of the tryptamine motif are crucial for this reaction. Mechanistic investigations demonstrated that the reaction exhibited a first-order dependence on both tryptophan and glycan, and deprotonation/rearomatization of the pentahydro-β-carbolinium ion intermediate might be the rate-determining step.

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.

Glycosyl disulfides: importance, synthesis and application to chemical and biological systems

This review explores methodologies for the preparation of glycosyl disulfides, their utility as intermediates in carbohydrate synthesis, and evaluates their biological impact in glycoscience and beyond.

Direct, stereoselective thioglycosylation enabled by an organophotoredox radical strategy

While strategies involving a 2e- transfer pathway have dictated glycosylation development, the direct glycosylation of readily accessible glycosyl donors as radical precursors is particularly appealing because of high radical anomeric selectivity and atom- and step-economy. However, the development of the radical process has been challenging owing to notorious competing reduction, elimination and/or SN side reactions of commonly used, labile glycosyl donors. Here we introduce an organophotocatalytic strategy through which glycosyl bromides can be efficiently converted into corresponding anomeric radicals by photoredox mediated HAT catalysis without a transition metal or a directing group and achieve highly anomeric selectivity. The power of this platform has been demonstrated by the mild reaction conditions enabling the synthesis of challenging α-1,2-cis-thioglycosides, the tolerance of various functional groups and the broad substrate scope for both common pentoses and hexoses. Furthermore, this general approach is compatible with both sp2 and sp3 sulfur electrophiles and late-stage glycodiversification for a total of 50 substrates probed.

Glycoprotein Mimics with Tunable Functionalization through Global Amino Acid Substitution and Copper Click Chemistry

Glycoproteins and their mimics are challenging to produce because of their large number of polysaccharide side chains that form a densely grafted protein-polysaccharide brush architecture. Herein a new approach to protein bioconjugate synthesis is demonstrated that can approach the functionalization densities of natural glycoproteins through oligosaccharide grafting. Global amino acid substitution is used to replace the methionine residues in a methionine-enriched elastin-like polypeptide with homopropargylglycine (HPG); the substitution was found to replace 93% of the 41 methionines in the protein sequence as well as broaden and increase the thermoresponsive transition. A series of saccharides were conjugated to the recombinant protein backbones through copper(I)-catalyzed alkyne-azide cycloaddition to determine reactivity trends, with 83-100% glycosylation of HPGs. Only an acetyl-protected sialyllactose moiety showed a lower level of 42% HPG glycosylation that is attributed to steric hindrance. The recombinant glycoproteins reproduced the key biofunctional properties of their natural counterparts such as viral inhibition and lectin binding.

A Universal Photochemical Method to Prepare Carbohydrate Sensors Based on Perfluorophenylazide Modified Polydopamine for Study of Carbohydrate-Lectin Interactions by QCM Biosensor

A universal photochemical method to prepare carbohydrate sensors based on perfluorophenylazide (PFPA) modified polydopamine (PDA) for the study of carbohydrate–lectin interactions by a quartz crystal microbalance (QCM) biosensor was developed. The PFPA was immobilized on PDA-coated gold sensors via Schiff base reactions. Upon light irradiation, the underivatized carbohydrates were inserted into the sensor surface, including mannose, galactose, fucose and N-acetylglucosamine (GlcNAc). Carbohydrate sensors were evaluated for the binding to a series of plant lectins. A kinetic study of the interactions between mannose and concanavalin A (Con A), fucose and Ulex europaeus agglutinin I (UEA-I) were performed. This method can eliminate the tedious modification of carbohydrates, improve the experimental efficiency, and reduce the experimental cost, which is of great significance for the development of QCM biosensors and the study of biomolecular interactions.

One-pot synthesis of glycosyl phenylthiosulfonates from sulfinate, S and glycosyl bromides

Glycosyl phenylthiosulfonates are reagents which are valuable for the S-glycosylation decoration of organic compounds and proteins. Here, one-pot multiple-component synthesis of glycosyl phenylthiosulfonates from sulfinate, sulfur powder and glycosyl bromides is reported. The reactions afford glycosyl phenylthiosulfonates in good yields under mild conditions. Further application and exploration of glycosyl phenylthiosulfonates are still on underway in our group.

Synthesis of O-1- O-6 Substituted Positional Isomers of d-Glucose-Thioether Ligands and Their Ruthenium Polypyridyl Conjugates

A library of positional isomers of d-glucose (O-1–O-6) as ligands and their 11 light-active ruthenium conjugates has been synthesized. A protecting group strategy without the necessity of using palladium on carbon for the modification for the 2-O and 4-O position allows for the incorporation of sulfur donor atoms as ligands for transition metal complexes.

GLUT1-mediated selective tumor targeting with fluorine containing platinum(II) glycoconjugates

Increased glycolysis and overexpression of glucose transporters (GLUTs) are physiological characteristics of human malignancies. Based on the so-called Warburg effect, 18flurodeoxyglucose-positron emission tomography (FDG-PET) has successfully developed as clinical modality for the diagnosis and staging of many cancers. To leverage this glucose transporter mediated metabolic disparity between normal and malignant cells, in the current report, we focus on the fluorine substituted series of glucose, mannose and galactose-conjugated (trans-R,R-cyclohexane-1,2-diamine)-2-flouromalonato-platinum(II) complexes for a comprehensive evaluation on their selective tumor targeting. Besides highly improved water solubility, these sugar-conjugates presented improved cytotoxicity than oxaliplatin in glucose tranporters (GLUTs) overexpressing cancer cell lines and exhibited no cross-resistance to cisplatin. For the highly water soluble glucose-conjugated complex (5a), two novel in vivo assessments were conducted and the results revealed that 5a was more efficacious at a lower equitoxic dose (70% MTD) than oxaliplatin (100% MTD) in HT29 xenograft model, and it was significantly more potent than oxaliplatin in leukemia-bearing DBA/2 mice as well even at equimolar dose levels (18% vs 90% MTD). GLUT inhibitor mediated cell viability analysis, GLUT1 knockdown cell line-based cytotoxicity evaluation, and platinum accumulation study demonstrated that the cellular uptake of the sugar-conjugates was regulated by GLUT1. The higher intrinsic DNA reactivity of the sugar-conjugates was confirmed by kinetic study of platinum(II)-guanosine adduct formation. The mechanistic origin of the antitumor effect of the fluorine complexes was found to be forming the bifunctional Pt-guanine-guanine (Pt-GG) intrastrand cross-links with DNA. The results provide a rationale for Warburg effect targeted anticancer drug design.

Deciphering the origins of molecular toxicity of combretastatin A4 and its glycoconjugates: interactions with major drug transporters and their safety profiles in vitro and in vivo

Cellular uptake and transport mechanisms directly correlate with the drug-like profiles of lead compounds. To decipher the molecular origin of the toxicity of combretastatin A4 (CA4), an important microtubule targeting agent, we investigated the interactions between CA4 and six key drug transporters, namely hOAT1, hOAT3, hOCT1, hOCT2, hOATP1B3, and hOATP2B1. Three combretastatin-based glycoconjugates, namely Glu-CA4, Man-CA4, and Gal-CA4 with glucose, mannose, and galactose respectively, were synthesized and their in vitro and in vivo biological characteristics were evaluated. CA4 exhibited significant inhibition against hOAT3 and hOATP2B1, moderate inhibition of hOAT1 and hOCT2, and weak inhibitory effects on hOCT1 and hOATP1B3. Compared to CA4, the inhibitory activities of Glu-CA4 on the six transporters were minimal. The glycoconjugates were found to have a superior safety profile with their maximum tolerated dose (MTD) values exhibiting a 16-34-fold increase compared to CA4. Given the drawbacks of CA4, the enhanced solubility and safety profiles of CA4 glycoconjugates augur well for further investigation into these intriguing candidates' in vivo efficacy.

Glycosylation of Recombinant Antigenic Proteins from Mycobacterium tuberculosis: In Silico Prediction of Protein Epitopes and Ex Vivo Biological Evaluation of New Semi-Synthetic Glycoconjugates

Tuberculosis is still one of the most deadly infectious diseases worldwide, and the use of conjugated antigens, obtained by combining antigenic oligosaccharides, such as the lipoarabinomannane (LAM), with antigenic proteins from Mycobacterium tuberculosis (MTB), has been proposed as a new strategy for developing efficient vaccines. In this work, we investigated the effect of the chemical glycosylation on two recombinant MTB proteins produced in E. coli with an additional seven-amino acid tag (recombinant Ag85B and TB10.4). Different semi-synthetic glycoconjugated derivatives were prepared, starting from mannose and two disaccharide analogs. The glycans were activated at the anomeric position with a thiocyanomethyl group, as required for protein glycosylation by selective reaction with lysines. The glycosylation sites and the ex vivo evaluation of the immunogenic activity of the different neo-glycoproteins were investigated. Glycosylation does not modify the immunological activity of the TB10.4 protein. Similarly, Ag85B maintains its B-cell activity after glycosylation while showing a significant reduction in the T-cell response. The results were correlated with the putative B- and T-cell epitopes, predicted using a combination of in silico systems. In the recombinant TB10.4, the unique lysine is not included in any T-cell epitope. Lys30 of Ag85B, identified as the main glycosylation site, proved to be the most important site involved in the formation of T-cell epitopes, reasonably explaining why its glycosylation strongly influenced the T-cell activity. Furthermore, additional lysines included in different epitopes (Lys103, -123 and -282) are also glycosylated. In contrast, B-cell epitopic lysines of Ag85B were found to be poorly glycosylated and, thus, the antibody interaction of Ag85B was only marginally affected after coupling with mono- or disaccharides.

Herbicidal ionic liquids derived from renewable sources

A new family of herbicidal ionic liquids based on d-glucose and MCPA or 2,4-D anions has been synthesized and the physicochemical, surface active and herbicidal properties of the obtained salts were characterized.

Developments and recent advancements in the field of endogenous amino acid selective bond forming reactions for bioconjugation

Bioconjugation methodologies have proven to play a central enabling role in the recent development of biotherapeutics and chemical biology approaches. Recent endeavours in these fields shed light on unprecedented chemical challenges to attain bioselectivity, biocompatibility, and biostability required by modern applications. In this review the current developments in various techniques of selective bond forming reactions of proteins and peptides were highlighted. The utility of each endogenous amino acid-selective conjugation methodology in the fields of biology and protein science has been surveyed with emphasis on the most relevant among reported transformations; selectivity and practical use have been discussed.

Selective chemical protein modification

Chemical modification of proteins is an important tool for probing natural systems, creating therapeutic conjugates and generating novel protein constructs. Site-selective reactions require exquisite control over both chemo- and regioselectivity, under ambient, aqueous conditions. There are now various methods for achieving selective modification of both natural and unnatural amino acids--each with merits and limitations--providing a 'toolkit' that until 20 years ago was largely limited to reactions at nucleophilic cysteine and lysine residues. If applied in a biologically benign manner, this chemistry could form the basis of true Synthetic Biology.

Thiol–Thiosulfonate Chemistry in Polymer Science: Simple Functionalization of Polymers via Disulfide Linkages

Herein we highlight the reaction of thiols with thiosulfonates yielding asymmetric disulfides. The chapter begins with an overview of the synthesis and reactivity of functional thiosulfonates and is followed by a review of polymeric thiosulfonates. We then emphasize the novel use of thiosulfonates as trapping/functionalization agents for macromolecular thiols obtained from parent (co)polymers prepared by reversible addition‐fragmentation chain transfer (RAFT) radical polymerization. We also note how such facile disulfide‐forming chemistries can be readily employed simultaneously with other highly efficient coupling chemistries with an emphasis on the concurrent reaction of activated esters with amines in the presence of thiosulfonates. Finally, we discuss the use of methyl disulfide (SSMe) functional/end‐modified (co)polymers as reagents for the formation of polymeric self‐assembled monolayers (polymer brushes) on metal surfaces such as nanoparticles and quantum dots.

Straightforward glycosylation of alcohols and amino acids mediated by ionic liquid

Green glycosylation of functionalized alcohols and α-amino acids, using an ionic liquid as a recyclable solvent, was performed in one step directly from the unprotected monosaccharide under scandium triflate or ferric chloride catalysis. Pure α- and β-glycosides could be obtained after specific enzymatic hydrolysis.

Photogenerated lectin sensors produced by thiol-ene/yne photo-click chemistry in aqueous solution

The photoinitiated radical reactions between thiols and alkenes/alkynes (thiol-ene and thiol-yne chemistry) have been applied to a functionalization methodology to produce carbohydrate-presenting surfaces for analyses of biomolecular interactions. Polymer-coated quartz surfaces were functionalized with alkenes or alkynes in a straightforward photochemical procedure utilizing perfluorophenylazide (PFPA) chemistry. The alkene/alkyne surfaces were subsequently allowed to react with carbohydrate thiols in water under UV-irradiation. The reaction can be carried out in a drop of water directly on the surface without photoinitiator, and any disulfide side products were easily washed away after the functionalization process. The resulting carbohydrate-presenting surfaces were evaluated in real-time studies of protein-carbohydrate interactions using a quartz crystal microbalance (QCM) flow-through system with recurring injections of selected lectins, with intermediate regeneration steps using low pH buffer. The resulting methodology proved fast, efficient and scalable to high-throughput analysis formats, and the produced surfaces showed significant protein binding with expected selectivities of the lectins used in the study.

A "tag-and-modify" approach to site-selective protein modification

Covalent modification can expand a protein's functional capacity. Fluorescent or radioactive labeling, for instance, allows imaging of a protein in real time. Labeling with an affinity probe enables isolation of target proteins and other interacting molecules. At the other end of this functional spectrum, protein structures can be naturally altered by enzymatic action. Protein-protein interactions, genetic regulation, and a range of cellular processes are under the purview of these post-translational modifications. The ability of protein chemists to install these covalent additions selectively has been critical for elucidating their roles in biology. Frequently the transformations must be applied in a site-specific manner, which demands the most selective chemistry. In this Account, we discuss the development and application of such chemistry in our laboratory. A centerpiece of our strategy is a "tag-and-modify" approach, which entails sequential installation of a uniquely reactive chemical group into the protein (the "tag") and the selective or specific modification of this group. The chemical tag can be a natural or unnatural amino acid residue. Of the natural residues, cysteine is the most widely used as a tag. Early work in our program focused on selective disulfide formation in the synthesis of glycoproteins. For certain applications, the susceptibility of disulfides to reduction was a limitation and prompted the development of several methods for the synthesis of more stable thioether modifications. The desulfurization of disulfides and conjugate addition to dehydroalanine are two routes to these modifications. The dehydroalanine tag has since proven useful as a general precursor to many modifications after conjugate addition of various nucleophiles; phosphorylated, glycosylated, peptidylated, prenylated, and even mimics of methylated and acetylated lysine-containing proteins are all accessible from dehydroalanine. While cysteine is a useful tag for selective modification, unnatural residues present the opportunity for bio-orthogonal chemistry. Azide-, arylhalide-, alkyne-, and alkene-containing amino acids can be incorporated into proteins genetically and can be specifically modified through various transformations. These transformations often rely on metal catalysis. The Cu-catalyzed azide-alkyne addition, Ru-catalyzed olefin metathesis, and Pd-catalyzed cross-coupling are examples of such transformations. In the course of adapting these reactions to protein modification, we learned much about the behavior of these reactions in water, and in some cases entirely new catalysts were developed. Through a combination of these bio-orthogonal transformations from the panel of tag-and-modify reactions, multiple and distinct modifications can be installed on protein surfaces. Multiple modifications are common in natural systems, and synthetic access to these proteins has enabled study of their biological role. Throughout these investigations, much has been learned in chemistry and biology. The demands of selective protein modification have revealed many aspects of reaction mechanisms, which in turn have guided the design of reagents and catalysts that allow their successful deployment in water and in biological milieu. With this ability to modify proteins, it is now possible to interrogate biological systems with precision that was not previously possible.

An approach to the synthesis and attachment of scillabiose to steroids

Hellebrin and transvaalin are two naturally occurring saponins with biological activity. In the present paper, we describe a high yielding route to the synthesis and coupling of their shared glycone, scillabiose, to a model steroid. A convergent coupling strategy utilizing a scillabiose-based glycosyl donor was devised for the glycosylation. This convergent approach is appealing due to its high efficiency and simple deprotection procedure and may find further use in total synthesis of naturally occurring saponins and related compounds sharing the same glycone. Due to the widespread occurrence of this glycone in nature, the complete NMR spectroscopic characterization of all compounds prepared herein is provided as reference material. In addition, glycosylations were performed with the monosaccharide constituents of scillabiose, thereby providing a limited series of glycosylated steroids for potential future evaluation of the effects of the glycone on the overall biological activity.

Rewritable glycochips

We describe microarraying of carbohydrates for protein screening using either disulfide bridge or Schiff base imine immobilization chemistries on plasmachemical deposited functional nanolayers. The commonly observed issue of nonspecific background binding of proteins is overcome by spotting carbohydrates through a protein-resistant overlayer yielding spatially localized interaction with a reactive functional underlayer.

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.

Distinctive inhibition of O-GlcNAcase isoforms by an alpha-GlcNAc thiolsulfonate

O-GlcNAcase (OGA) promotes O-GlcNAc removal, and thereby plays a key role in O-GlcNAc metabolism, a feature of a variety of vital cellular processes. Two splice transcripts of human OGA encode "long OGA", which contains a distinct N-terminal O-GlcNAcase domain and a C-terminal histoneacetylferase (HAT) domain, and "short OGA", which lacks the HAT domain. The functional roles of long OGA are only beginning to be unraveled, and the characteristics of short OGA remain almost unknown. We find that short OGA, which possesses O-GlcNAcase catalysis machinery like that of long OGA, exhibits comparative resistance to previously described potent inhibitors of long OGA and lysosomal hexosaminidases, including PUGNAc and NAG-thiazoline, suggesting a role for the HAT domain in O-GlcNAcase catalysis. We also find that alpha-GlcNAc thiolsulfonate (2) is the most potent inhibitor of short OGA yet described (Ki = 10 microM), and exhibits some degree of selectivity versus long OGA and lysosomal hexosaminidases. In contrast to its mode of inhibition of short OGA, 2 acts as a irreversible inhibitor of long OGA by covalently modifying the enzyme as an S-GlcNAc derivative. Covalent attachment of GlcNAc to the HAT domain of long OGA dramatically changes its properties with respect to enzymatic activity and caspase-3 cleavage.

Synthesis of a novel ‘smart’ bifunctional chelating agent 1-(2-[β,d-galactopyranosyloxy]ethyl)-7-(1-carboxy-3-[4-aminophenyl]propyl)-4,10-bis(carboxymethyl)-1,4,7,10-tetraazacyclododecane (Gal-PA-DO3A-NH2) and its Gd(III) complex

A new synthetic pathway to 1-(2-[beta,D-galactopyranosyloxy]ethyl)-7-(1-carboxy-3-[4-aminophenyl]propyl)-4,10-bis(carboxymethyl)-1,4,7,10-tetraazacyclododecane (Gal-PA-DO3A-NH2) and 1-(2-[beta,D-galactopyranosyloxy]ethyl)-4,7,10-tris(carboxymethyl)-1, 4,7,10-tetraazacyclododecane (Gal-DO3A) chelating agents was developed involving full hydroxyl- and carboxyl-group protection in precursors to product. Two sequences of cyclen-N-functionalisation were subsequently investigated, one successfully, towards synthesis of the novel 'smart' bifunctional Gal-PA-DO3A-NH2 chelate. The longitudinal proton relaxivities of the neutral [Gd-(Gal-PA-DO3A-NH2)] and [Gd-(Gal-DO3A)] complexes were increased by 28% and 37% in the presence of beta-galactosidase, respectively.