Expedient syntheses of neoglycoproteins using phase transfer catalysis and reductive amination as key reactions

Phase-Transfer Catalyzed Microfluidic Glycosylation: A Small Change in Concentration Results in a Dramatic Increase in Stereoselectivity

Phase-transfer catalysis (PTC) is widely used in glycochemistry for the preparation of aryl glycosides by the glycosylation reaction. While investigating the possibility of synthesis of 4-(3-chloropropoxy)phenyl sialoside (Neu5Ac-OCPP) from N-acetylsialyl chloride with O-acetyl groups (1), we have recently discovered a strong dependence of the PTC glycosylation outcome on the mixing mode: under batch conditions, only α-anomer of Neu5Ac-OCPP was obtained, albeit in low yield (13%), while under microfluidic conditions the yield of Neu5Ac-OCPP increased to 36%, although stereoselectivity decreased (α/β ≤ 6.2). Here, we report that the outcome of this reaction, performed under microfluidic conditions using a Comet X-01 micromixer (at 2 μL/min flow rate), non-linearly depends on the concentration of N-acetylsialyl chloride 1 (5–200 mmol/L). The target Neu5Ac-OCPP was obtained in a noticeably higher yield (up to 66%) accompanied by enhanced stereoselectivity (α/β = 17:1–32:1) in the high concentration range (C > 50 mmol/L), whereas the yield (10–36%) and especially, stereoselectivity (α/β = 0.9:1–6.2:1) were lower in the low concentration range (C ≤ 50 mmol/L). This dramatic stepwise increase in stereoselectivity above critical concentration (50 mmol/L) is apparently related to the changes in the presentation of molecules on the surface of supramers of glycosyl donor, which exist in different concentration ranges.

Non-Native Amino Acid Click Chemistry-Based Technology for Site-Specific Polysaccharide Conjugation to a Bacterial Protein Serving as Both Carrier and Vaccine Antigen

Surface-expressed bacterial polysaccharides are important vaccine antigens but must be conjugated to a carrier protein for efficient antigen presentation and development of strong memory B cell and antibody responses, especially in young children. The commonly used protein carriers include tetanus toxoid (TT), diphtheria toxoid (DT), and its derivative CRM197, but carrier-induced epitopic suppression and bystander interference may limit the expanded use of the same carriers in the pediatric immunization schedule. Recent efforts to develop a vaccine against the major human pathogen group A Streptococcus (GAS) have sought to combine two promising vaccine antigens-the universally conserved group A cell wall carbohydrate (GAC) with the secreted toxin antigen streptolysin O (SLO) as a protein carrier; however, standard reductive amination procedures appeared to destroy function epitopes of the protein, markedly diminishing functional antibody responses. Here, we couple a cell-free protein synthesis (CFPS) platform, allowing the incorporation of non-natural amino acids into a C-terminally truncated SLO toxoid for the precise conjugation to the polyrhamnose backbone of GAC. The combined immunogen generated functional antibodies against both conserved GAS virulence factors and provided protection against systemic GAS challenges. CFPS may represent a scalable method for generating pathogen-specific carrier proteins for multivalent subunit vaccine development.

Per-glycosylation of the Surface-Accessible Lysines: One-Pot Aqueous Route to Stabilized Proteins with Native Activity

Chemical glycosylation of proteins is a powerful tool applied widely in biomedicine and biotechnology. However, it is a challenging undertaking and typically relies on recombinant proteins and site-specific conjugations. The scope and utility of this nature-inspired methodology would be broadened tremendously by the advent of facile, scalable techniques in glycosylation, which are currently missing. In this work, we investigated a one-pot aqueous protocol to achieve indiscriminate, surface-wide glycosylation of the surface accessible amines (lysines and/or N-terminus). We reveal that this approach afforded minimal if any change in the protein activity and recognition events in biochemical and cell culture assays, but at the same time provided a significant benefit of stabilizing proteins against aggregation and fibrillation - as demonstrated on serum proteins (albumins and immunoglobulin G, IgG), an enzyme (uricase), and proteins involved in neurodegenerative disease (α-synuclein) and diabetes (insulin). Most importantly, this highly advantageous result was achieved via a one-pot aqueous protocol performed on native proteins, bypassing the use of complex chemical methodologies and recombinant proteins.

Glycoconjugations of Biomolecules by Chemical Methods

Bioconjugations under benign aqueous conditions have the most promise to covalently link carbohydrates onto chosen molecular and macromolecular scaffolds. Chemical methodologies relying on C-C and C-heteroatom bond formations are the methods of choice, coupled with the reaction conditions being under aqueous milieu. A number of methods, including metal-mediated, as well as metal-free azide-alkyne cyclo-addition, photocatalyzed thiol-ene reaction, amidation, reductive amination, disulfide bond formation, conjugate addition, nucleophilic addition to vinyl sulfones and vinyl sulfoxides, native chemical ligation, Staudinger ligation, olefin metathesis, and Suzuki-Miyaura cross coupling reactions have been developed, in efforts to conduct glycoconjugation of chosen molecular and biomolecular structures. Within these, many methods require pre-functionalization of the scaffolds, whereas methods that do not require such pre-functionalization continue to be few and far between. The compilation covers synthetic methodology development for carbohydrate conjugation onto biomolecular and biomacromolecular scaffolds. The importance of such glycoconjugations on the functional properties is also covered.

[Synthesis of heteroaromatic N-beta-glycosides of N-acetylghicosamine under phase transfer conditions. III. 1,2,4-triazoIin-3-ones glucosaminides]

The catalytic phase transfer reactions of 2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-a-D-glucopyranosyl chloride with thiosubstituted 4-phenyl-5-R-thio-Delta2-1,2,4-triazolin-3-ones in solid alkali-organic solvent and in aqueous alkali-organic solvent were studied. The main products ofglucosaminilation reaction are the appropriate N-beta-glucosaminides. The effect of reaction conditions on the yield and composition of products formed was elucidated on the example of the reaction alpha-D-glucosaminyl chloride with 5-methylthio-4-phenyl-Delta2-1,2,4-triazolin-3-one. The optimal conditions of phase transfer glycosylation were established. N-1,2-trans-grycosidic bond formation was proved by 1H NMR and IR data as well as a comparison with spectral data obtained previously for the glycosides of similar structure.

Convenient preparation and characterization of a monoclonal antibody for the N-linked sugar chain of a glycoprotein using a microbial endoglycosidase

We attempted to obtain the monoclonal antibody specific for the N-linked complex-type sialo-oligosaccharide in glycoproteins. We first synthesized a chimeric immunoantigen having an N-linked complex-type of oligosaccharide of glycopeptide, which was bound to a p-formylphenyl compound and conjugated with phosphatidylethanolamine dimyristoyl using the transglycosylation activity of a microbial endoglycosidase (Endo-M) and a reductive amination reaction. This preparative method was convenient and provided a good yield. By immunizing mice with this chimeric neoglycolipid, the monoclonal antibody for the complex-type of sialo-oligosaccharide was obtained in the culture fluid of the cell line even though it was relatively unstable. The monoclonal antibody reacted with various glycoproteins having complex-type sialo-oligosaccharides, but not with those having complex-type asialo-oligosaccharides and high mannose types of oligosaccharides, or with any glycosphingolipids. One of epitopes of this monoclonal antibody seemed to be an alpha-2,6-linked sialic acid at the non-reducing end of the sialo-oligosaccharide of the glycoprotein.

Chemoenzymatic synthesis and application of a sialoglycopolymer with a chitosan backbone as a potent inhibitor of human influenza virus hemagglutination

Sialoglycopeptide (SGP) is referred as the glycopeptide in hen's egg yolk, which has an N-linked, complex-type, disialyl biantennary oligosaccharide with an alpha-(2-->6)-sialyl N-acetyllactosamine residue. The residue is known as a binding ligand of type-A human influenza virus hemagglutinin. We describe herein a simple synthesis of a sialoglycopolymer with a chitosan backbone as a potent inhibitor of human influenza virus hemagglutination that makes use of the natural source ingredient, SGP, and the transglycosylation activity of endo-beta-N-acetylglucosaminidase from Mucor hiemalis (Endo-M). Its inhibitiory activity for influenza virus hemagglutination is 40 times higher than that of SGP, and its competitive inhibition is determined to be over 300 times higher than that of fetuin. These results indicate that a sialoglycopolymer having a multivalent sialo-oligosaccharide could potentially be used for the prevention of influenza virus infection.

Chemical modification of chitosan: preparation and lectin binding properties of alpha-galactosyl-chitosan conjugates. Potential inhibitors in acute rejection following xenotransplantation

Water-soluble alpha-galactosyl-chitosan conjugates were prepared by reductive amination of p-formylphenyl beta-melibioside on chitosan in good yield. Strong binding with phytohemagglutinin was demonstrated using Griffonia simplicifolia.

Preparations of antigens and immunoadsorbents corresponding to the Streptococcus group A cell-wall polysaccharide

The allyl glycosides of a tri-, penta- and hexasaccharide corresponding to the Streptococcus Group A cell-wall polysaccharide were coupled to solid or soluble supports to give immunoaffinity columns and neoglycoproteins, respectively. Cysteamine hydrochloride was added to the allyl glycosides and the resulting cysteamine adducts were used for subsequent coupling to linkers via the amine functionality. The tri- and penta-saccharide cysteamine adducts were coupled directly to the azalactone-derivatized 3M Emphase Biosupport Medium AB 1 to yield two affinity columns. The penta- and hexa- saccharides were coupled to bovine serum albumin or ovalbumin via the conjugate addition of the epsilon-amino groups of lysines on the proteins with the N-acryloylated sugars or the oligosaccharide-squarate adducts, derived in turn from the cysteamine adducts. The efficiency of the above methods is compared.