Convergent synthesis of higher-order oligosaccharides corresponding to the cell-wall polysaccharide of the beta-hemolytic Streptococci group A. A branched hexasaccharide hapten

Synthetic Glycans to Improve Current Glycoconjugate Vaccines and Fight Antimicrobial Resistance

Antimicrobial resistance (AMR) is emerging as the next potential pandemic. Different microorganisms, including the bacteria Acinetobacter baumannii, Clostridioides difficile, Escherichia coli, Enterococcus faecium, Klebsiella pneumoniae, Neisseria gonorrhoeae, Pseudomonas aeruginosa, non-typhoidal Salmonella, and Staphylococcus aureus, and the fungus Candida auris, have been identified by the WHO and CDC as urgent or serious AMR threats. Others, such as group A and B Streptococci, are classified as concerning threats. Glycoconjugate vaccines have been demonstrated to be an efficacious and cost-effective measure to combat infections against Haemophilus influenzae, Neisseria meningitis, Streptococcus pneumoniae, and, more recently, Salmonella typhi. Recent times have seen enormous progress in methodologies for the assembly of complex glycans and glycoconjugates, with developments in synthetic, chemoenzymatic, and glycoengineering methodologies. This review analyzes the advancement of glycoconjugate vaccines based on synthetic carbohydrates to improve existing vaccines and identify novel candidates to combat AMR. Through this literature survey we built an overview of structure-immunogenicity relationships from available data and identify gaps and areas for further research to better exploit the peculiar role of carbohydrates as vaccine targets and create the next generation of synthetic carbohydrate-based vaccines.

The study of the reaction of terminated oligomerization in the synthesis of oligo-(β1-6)-glucosamines

The applicability of terminated oligomerization to the synthesis of oligo-(beta1-6)-glycosamines, fragments of the intercellular polysaccharide adhesin of staphylococci, was studied. The reactions of terminated oligomerization were carried out with mono-, di-, and trisaccharide monomers and N-protected aminopropanol; and spacered mono- and disaccharides as terminating molecules were also attempted. The primary formation of cyclic products of monomer intramolecular glycosylation was observed in almost all the reactions. Only the experiments with the monomer based on the disaccharide bromide under the conditions of the Helferich reaction led to reduced yields (30%) of the cyclic products. However, even in this case, the desired terminated oligosaccharides were generated in approximately 10% yield and mainly were the products of single glycosylation of the terminator by the monomer. These experiments allow the conclusion that, under the examined conditions, the reaction of terminated oligomerization could not result in the synthesis of oligoglucosamines with a high molecular mass.

Synthesis of the pentasaccharide repeating unit of lactosillan [correction of latosillan]

A pentasaccharide, beta-D-Man-(1-->2)-[beta-D-GlcNAc-(1-->4)]-alpha-L-Rha-(1-->4)-alpha-L-Rha-(1-->4)-alpha-L-Rha-1-OC8H17, representing the repeating unit of latosillan, was convergently synthesized from the building blocks, ethyl 2,3-O-isopropylidene-1-thio-alpha-l-rhamnopyranoside, 2-O-acetyl-3,4,6-tri-O-benzyl-beta-d-glucopyranosyl trichloroacetimidate, and 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-beta-d-glucopyranosyl trichloroacetimidate under standard glycosylation conditions. The target pentasaccharide showed acceptable differentiation-inducing activity on HL-60 cell lines at the dosages of 10-50 microg/mL.

Synthesis of a hexasaccharide fragment of group E streptococci polysaccharide and the tetrasaccharide repeating unit of E. coli O7:K98:H6

Syntheses of a hexasaccharide, the dimer of the repeating unit of the group E streptococci polysaccharide, and a tetrasaccharide, the repeating unit of the E. coli O7:K98:H6, were achieved by constructing alternate alpha-L-(1-->2)- and alpha-L-(1-->3)-linked L-rhamnopyranose backbones and substituting with beta-linked D-glucopyranose side chains for the former, and a D-glucopyranosyluronate branch for the latter, respectively, at O-2 of the L-rhamnose ring.

Blockwise Approach to Fragments of the O-Specific Polysaccharide of Shigella flexneri Serotype 2a: Convergent Synthesis of a Decasaccharide Representative of a Dimer of the Branched Repeating Unit1

The D'A'B'(E')C'DAB(E)C decasaccharide representative of a dimer of a frame-shifted pentasaccharide repeating unit of the O-specific polysaccharide of Shigella flexneri 2a was synthesized as its methyl glycoside by condensing a pentasaccharide donor (D'A'B'(E')C') and a pentasaccharide acceptor (DAB(E)C-OMe). Several convergent routes to these two building blocks, involving either the AB linkage or the BC linkage as the disconnection site, were evaluated in comparison to the linear strategy. The latter was preferred. It is based on the use of the trichloroacetimidate chemistry. The target branched oligosaccharide was designed to probe the recognition at the molecular level of the natural polysaccharide by protective monoclonal antibodies.

A concise synthesis of two isomeric pentasaccharides, the O repeat units from the lipopolysaccharides of P. syringae pv. porri NCPPB 3364T and NCPPB 3365

A concise synthesis of two isomeric pentasaccharides, alpha-L-Rhap-(1-->2)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->3)-[beta-D-GlcpNAc-(1-->2)]-alpha-L-Rhap (A) and alpha-L-Rhap-(1-->2)-alpha-L-Rhap-(1-->3)-[beta-D-GlcpNAc-(1-->2)]-alpha-L-Rhap-(1-->3)-alpha-L-Rhap (B), the O repeats from the lipopolysaccharides of Pseudonomonas syringae pv. porri NCPPB 3364T and 3365 was achieved via assembly of the building blocks, allyl 3,4-di-O-benzoyl-alpha-L-rhamnopyranoside (1), 2,3,4-tri-O-benzoyl-alpha-L-rhamnopyranosyl trichloroacetimidate (2), allyl 4-O-benzoyl-3-O-chloroacetyl-alpha-L-rhamnopyranoside (6), 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-beta-D-glucopyranosyl trichloroacetimidate (7), and allyl 2,4-di-O-benzoyl-alpha-L-rhamnopyranoside (10). Coupling of 1 with 2 followed by deallylation and trichloroacetimidate formation gave the disaccharide donor 5, while condensation of 6 with 7, followed by dechloroacetylation, offered the disaccharide acceptor 9. Then, 5 was coupled with 10 to obtain the trisaccharide 11, and subsequent deallylation and trichloroacetimidate formation furnished the trisaccharide donor 13. Coupling of 9 with 13, followed by deprotection, afforded pentasaccharide 19, while condensation of 9 with 5, followed by deallylation and trichloroacetimidate formation, gave the tetrasaccharide donor 16, whose coupling with 10 and subsequent deprotection yielded another pentasaccharide 22.

Preparation of synthetic glycoconjugates as potential vaccines against Shigella flexneri serotype 2a disease

The synthesis of three neoglycopeptides incorporating carbohydrate haptens, differing in length, covalently linked to a non natural universal T helper peptide is disclosed. They were synthesized according to a blockwise strategy based on the condensation of appropriate di-, tri-, and tetrasaccharide trichloroacetimidate donors onto an azidoethyl 2-acetamido-2-deoxybeta-D-glucopyranoside acceptor. Use of thiol-maleimide coupling chemistry allowed site-selective efficient conjugation.

Efficient synthesis of a heptasaccharide, the repeating unit of the O-chain lipopolysaccharide produced by Xanthomonas campestris strain 642

alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->2)-alpha-L-Rhap-(1-->3)-[beta-D-Xylp-(1-->2)-][beta-D-Xylp-(1-->4)-]alpha-L-Rhap-(1-->3)-alpha-L-Rhap, the repeating unit of the O-chain lipopolysaccharide produced by Xanthomonas campestris strain 642 was synthesized as its methyl glycoside via 3-O-selective glycosylation of methyl alpha-L-rhamnopyranosyl-(1-->3)-2,4-di-O-benzoyl-alpha-L-rhamnopyranoside (9) with 2,3,4-tri-O-benzoyl-alpha-L-rhamnopyranosyl-(1-->3)-2,4-di-O-benzoyl-alpha-L-rhamnopyranosyl-(1-->2)-3,4-di-O-benzoyl-alpha-L-rhamnopyranosyl trichloroacetimidate (8), followed by dixylosylation with 2,3,4-tri-O-benzoyl-alpha,beta-D-xylopyranosyl trichloroacetimidate (12) and subsequent deacylation.

Synthesis of beta-D-GlcpNAc-(1-->3)-alpha-L-Rhap-(1-->2)-[beta-L-Xylp-(1-->4)]-alpha-L-Rhap-(1-->3)-alpha-L-Rhap, the repeating unit of the O-antigen produced by Pseudomonas solanacearum ICMP 7942

An efficient synthesis of beta-D-GlcpNAc-(1-->3)-alpha-L-Rhap-(1-->2)-[beta-L-Xylp-(1-->4)]-alpha-L-Rhap-(1-->3)-alpha-L-Rhap, the repeating unit of the O-antigen produced by Pseudomonas solanacearum ICMP 7942 and its isomer beta-D-GlcpNAc-(1-->3)-alpha-L-Rhap-(1-->4)-[beta-L-Xylp-(1-->2)]-alpha-L-Rhap-(1-->3)-alpha-L-Rhap was achieved via sequential assembly of the building blocks, allyl 2,3-O-isopropylidene-alpha-L-rhamnopyranoside (2), allyl 3,4-O-isopropylidene-alpha-L-rhamnopyranoside (3), allyl 2,4-di-O-benzoyl-alpha-L-rhamnopyranoside (6), 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-beta-D-glucopyranosyl trichloroacetimidate (7), and 2,3,4-tri-O-benzoyl-beta-L-xylopyranosyl trichloroacetimidate (12). The process was carried out in a regio- and stereoselective manner using glycosyl trichloroacetimidates as donors and unprotected or partially protected rhamnopyranosides as acceptors in the presence of a catalytic amount of trimethylsilyl trifluoromethanesulfonate (TMSOTf).

Saturation transfer difference 1D-TOCSY experiments to map the topography of oligosaccharides recognized by a monoclonal antibody directed against the cell-wall polysaccharide of group A streptococcus

A new saturation transfer difference 1D-TOCSY NMR experiment that allows the investigation of complex ligands interacting with proteins and its application in the mapping of which portions of oligosaccharide ligands (epitope) interact with a complementary antibody are described. The interaction between trisaccharide and hexasaccharide ligands, corresponding to fragments of the cell-wall polysaccharide of Streptococcus Group A, and a monoclonal antibody directed against the polysaccharide is investigated at the molecular level. The polysaccharide consists of alternating alpha-(1-->2) and alpha-(1-->3) linked L-rhamnopyranose (Rha) residues with branching N-acetyl-D-glucopyranosylamine (GlcNAc) residues linked beta-(1-->3) to alternate rhamnopyranose rings. The epitope is proven to consist not only of the immunodominant GlcNAc sugar but also of an entire branched trisaccharide repeating unit. The experimental NMR data serve to check and validate the computed models of the oligosaccharide-antibody complexes.

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.

Application of two-dimensional NMR spectroscopy and molecular dynamics simulations to the conformational analysis of oligosaccharides corresponding to the cell-wall polysaccharide of Streptococcus group A

This paper describes the use of a protocol for conformational analysis of oligosaccharide structures related to the cell-wall polysaccharide of Streptococcus group A. The polysaccharide features a branched structure with an L-rhamnopyranose (Rhap) backbone consisting of alternating alpha-(1-->2) and alpha-(1-->3) links and D-N-acetylglucosamine (GlcpNAc) residues beta-(1-->3)-connected to alternating rhamnose rings: [formula: see text] Oligomers consisting of three to six residues have been synthesized and nuclear magnetic resonance (NMR) assignments have been made. The protocol for conformational analysis of the solution structure of these oligosaccharides involves experimental and theoretical methods. Two-dimensional NMR spectroscopy methods (TOCSY, ROESY and NOESY) are utilized to obtain chemical shift data and proton-proton distances. These distances are used as constraints in 100 ps molecular dynamics simulations in water using QUANTA and CHARMm. In addition, the dynamics simulations are performed without constraints. ROE build-up curves are computed from the averaged structures of the molecular dynamics simulations using the CROSREL program and compared with the experimental curves. Thus, a refinement of the initial structure may be obtained. The alpha-(1-->2) and the beta-(1-->3) links are unambiguously defined by the observed ROE cross peaks between the A-B',A'-B and C-B,C'-B' residues, respectively. The branch-point of the trisaccharide CBA' is conformationally well-defined. Assignment of the conformation of the B-A linkage (alpha-(1-->3)) was problematic due to TOCSY relay, but could be solved by NOESY and T-ROESY techniques. A conformational model for the polysaccharide is proposed.

Convergent synthesis of an elusive hexasaccharide corresponding to the cell-wall polysaccharide of the beta-hemolytic Streptococcus group A

A convergent synthesis of a hexasaccharide corresponding to the cell-wall polysaccharide of the beta-hemolytic Streptococcus Group A is described. The strategy relies on the preparation of a key linear trisaccharide unit beta-D-GlcpNAc-(1-->3)-alpha-L-Rhap-(1-->2)-alpha-L-Rhap which has previously resisted our efforts. The trisaccharide functions both as a glycosyl acceptor and donor to give an elusive hexasaccharide. This fully functionalized unit can serve, in turn, as a glycosyl acceptor or donor for the synthesis of higher-order structures. Deprotection gives a hitherto unknown hexasaccharide for use as a hapten in immunochemical studies. The characterization of all compounds by high-resolution 1H and 13C NMR spectroscopy is also described.