Synthesis of five nona-β-(1→6)-d-glucosamines with various patterns of N-acetylation corresponding to the fragments of exopolysaccharide of Staphylococcus aureus

Synthesis and application of bacterial exopolysaccharides

Exopolysaccharides are produced and excreted by bacteria in the generation of biofilms to provide a protective environment. These polysaccharides are generally generated as heterogeneous polymers of varying length, featuring diverse substitution patterns. To obtain well-defined fragments of these polysaccharides, organic synthesis often is the method of choice, as it allows for full control over chain length and the installation of a pre-determined substitution pattern. This review presents several recent syntheses of exopolysaccharide fragments of Pseudomonas aeruginosa and Staphylococcus aureus and illustrates how these have been used to study biosynthesis enzymes and generate synthetic glycoconjugate vaccines.

β-Glycosylations with 2-Deoxy-2-(2,4-dinitrobenzenesulfonyl)-amino-glucosyl/galactosyl Selenoglycosides: Assembly of Partially N-Acetylated β-(1 → 6)-Oligoglucosaminosides

An efficient protocol has been established for β-glycosylations with 2-deoxy-2-(2,4-dinitrobenzenesulfonyl)amino (2dDNsNH)-glucopyranosyl/galactopyranosyl selenoglycosides using PhSeCl/AgOTf as an activating system. The reaction features highly β-selective glycosylation with a wide range of alcohol acceptors that are either sterically hindered or poorly nucleophilic. Thioglycoside- and selenoglycoside-based alcohols prove to be viable nucleophiles, opening up new opportunities for one-pot construction of oligosaccharides. The power of this approach is highlighted by the efficient assembly of tri-, hexa-, and nonasaccharides composed of β-(1 → 6)-glucosaminosyl residues based on one-pot preparation of a triglucosaminosyl thioglycoside with DNs, phthaloyl, and 2,2,2-trichloroethoxycarbonyl as the protecting groups of amino groups. These glycans are potential antigens for developing glycoconjugate vaccines against microbial infections.

Protecting group principles suited to late stage functionalization and global deprotection in oligosaccharide synthesis

Chemical synthesis is a powerful tool to access homogeneous complex glycans, which relies on protecting group (PG) chemistry. However, the overall efficiency of chemical glycan assembly is still low when compared to oligonucleotide or oligopeptide synthesis. There have been many contributions giving rise to collective improvement in carbohydrate synthesis that includes PG manipulation and stereoselective glycoside formation and some of this chemistry has been transferred to the solid phase or adapted for programmable one pot synthesis approaches. However, after all glycoside bond formation reactions are completed, the global deprotection (GD) required to give the desired target OS can be challenging. Difficulties observed in the removal of permanent PGs to release the desired glycans can be due to the number and diversity of PGs present in the protected OSs, nature and structural complexity of glycans, etc. Here, we have reviewed the difficulties associated with the removal of PGs from densely protected OSs to obtain their free glycans. In particularly, this review focuses on the challenges associated with hydrogenolysis of benzyl groups, saponification of esters and functional group interconversion such as oxidation/reduction that are commonly performed in GD stage. More generally, problems observed in the removal of permanent PGs is reviewed herein, including benzyl, acyl (levulinoyl, acetyl), N-trichloroacetyl, N-2,2,2-trichloroethoxycarbonyl, N-phthaloyl etc. from a number of fully protected OSs to release the free sugar, that have been previously reported in the literature.

Synthesis of defined mono-de-N-acetylated β-(1→6)-N-acetyl-d-glucosamine oligosaccharides to characterize PgaB hydrolase activity

Many clinically-relevant biofilm-forming bacterial strains produce partially de-N-acetylated poly-β-(1→6)-N-acetyl-d-glucosamine (dPNAG) as an exopolysaccharide. In Gram-negative bacteria, the periplasmic protein PgaB is responsible for partial de-N-acetylation of PNAG prior to its export to the extracellular space. In addition to de-N-acetylase activity found in the N-terminal domain, PgaB contains a C-terminal hydrolase domain that can disrupt dPNAG-dependent biofilms and hydrolyzes dPNAG but not fully acetylated PNAG. The role of this C-terminal domain in biofilm formation has yet to be determined in vivo. Further characterization of the enzyme's hydrolase activity has been hampered by a lack of specific dPNAG oligosaccharides. Here, we report the synthesis of a defined mono de-N-acetylated dPNAG penta- and hepta-saccharide. Using mass spectrometry analysis and a fluorescence-based thin-layer chromatography (TLC) assay, we found that our defined dPNAG oligosaccharides are hydrolase substrates. In addition to the expected cleavage site, two residues to the reducing side of the de-N-acetylated residue, minor cleavage products on the non-reducing side of the de-N-acetylation site were observed. These findings provide quantitative data to support how PNAG is processed in Gram-negative bacteria.

Differential Recognition of Deacetylated PNAG Oligosaccharides by a Biofilm Degrading Glycosidase

Exopolysaccharides consisting of partially de-N-acetylated poly-β-d-(1→6)-N-acetyl-glucosamine (dPNAG) are key structural components of the biofilm extracellular polymeric substance of both Gram-positive and Gram-negative human pathogens. De-N-acetylation is required for the proper assembly and function of dPNAG in biofilm development suggesting that different patterns of deacetylation may be preferentially recognized by proteins that interact with dPNAG, such as Dispersin B (DspB). The enzymatic degradation of dPNAG by the Aggregatibacter actinomycetemcomitans native β-hexosaminidase enzyme DspB plays a role in biofilm dispersal. To test the role of substrate de-N-acetylation on substrate recognition by DspB, we applied an efficient preactivation-based one-pot glycosylation approach to prepare a panel of dPNAG trisaccharide analogs with defined acetylation patterns. These analogs served as effective DspB substrates, and the rate of hydrolysis was dependent on the specific substrate de-N-acetylation pattern, with glucosamine (GlcN) located +2 from the site of cleavage being preferentially hydrolyzed. The product distributions support a primarily exoglycosidic cleavage activity following a substrate assisted cleavage mechanism, with the exception of substrates containing a nonreducing GlcN that were cleaved endo leading to the exclusive formation of a nonreducing disaccharide product. These observations provide critical insight into the substrate specificity of dPNAG specific glycosidase that can help guide their design as biocatalysts.

Synthesis of a pentasaccharide and neoglycoconjugates related to fungal α-(1→3)-glucan and their use in the generation of antibodies to trace Aspergillus fumigatus cell wall

3-Aminopropyl α-(1→3)-pentaglucoside, a fragment of α-(1→3)-glucan of the cell wall of Aspergillus fumigatus, has been synthesized in a blockwise approach. The application of mono- and disaccharide N-phenyltrifluoroacetimidates bearing a stereodirecting 6-O-benzoyl group was essential for stereoselective α-glucosylations. In the products, p-methoxyphenyl and levulinoyl groups served as orthogonal protecting groups for the anomeric position and 3-OH group, respectively. Their removal from shared blocks led to donors and acceptors that were used for the synthesis of pentasaccharides. Coupling of free α-(1→3)-pentaglucoside with biotin and bovine serum albumin (BSA) gave glycoconjugate tools for mycological studies. Immunization of mice with the BSA conjugate induced the generation of antibodies that recognize α-(1→3)-glucan on A. fumigatus cell wall and distinguish its morphotypes. This discovery represents a first step to the development of a diagnostic test system and a vaccine to detect and fight this life-threatening pathogen.

Synthesis and plant growth regulation activity of α-d-ManpNAc-(1→2)-[α-L-Rhap-(1→3)-]α-L-Rhap-(1→4)-β-d-GlupNAc-(1→3)-α-L-Rhap, the repeating unit of O-antigen of Rhizobium trifolii 4s

The synthesis of a pentasaccharide 2 containing acetamido-2-deoxy-d-glucose and acetamido-2-deoxy-d-mannose related to the cell wall polysaccharide of Rhizobium trifolii 4s has been achieved by a [2+3] approach from commercially available l-rhamnose, d-glucose, and d-glucosamine as the starting materials. The target molecule was equipped with a p-methoxylphenyl handle at the reducing terminus to allow for further glycoconjugate formation via selective cleavage of this group. The bioassay suggested that the synthetic pentasaccharide 2 can stimulate the growth of wheat coleoptile similarly to indole-3-acetic acid (IAA), and promote the wheat seedling development before winter by seed treatment at a concentration of 20mg/L.

Linear and cyclic oligo-β-(1→6)-D-glucosamines: Synthesis, conformations, and applications for design of a vaccine and oligodentate glycoconjugates

Poly-β-(1→6)-N-acetyl-D-glucosamine is an exopolysaccharide secreted by numerous pathogenic bacteria, includingStaphylococcus aureus,Escherichia coli,Yersinia pestis,Bordetella pertussis,Acinetobacter baumannii,Burkholderiaspp., and others. A convergent approach was developed for the synthesis of oligosaccharide fragments consisting of 5, 7, 9, and 11 glucosamine orN-acetylglucosamine units and for the preparation of five nona-β-(1→6)-D-glucosamines with variousN-acetylation patterns. Penta- and nona-β‑(1→6)-D-glucosamines conjugated to protein carriers through a specially developed sulfhydryl linker proved to be highly immunogenic in mice and rabbits and elicited antibodies that mediated opsonic killing of multiple strains ofS. aureus(including methicillin-resistantS. aureus, MRSA) andE. coli, and protected againstS. aureusskin abscesses and lethalE. coliandB. cenocepaciaperitonitis. These findings provide a basis for the construction of a unique semisynthetic vaccine against multiple bacterial targets. Conformational studies by means of special NMR experiments and computer modeling revealed that the oligo-β-(1→6)-D-glucosamine chain exists mostly in a helix-like conformation, where the terminal monosaccharides are arranged close to each other. Owing to this feature, oligoglucosamines consisting of 2 to 7 residues easily form products of cycloglycosylation. Cyclooligo-β-(1→6)-D-glucosamines represent a new family of functionalized cyclic oligosaccharides. Owing to their molecular architectonics, these compounds are convenient scaffolds for the design of conjugates with defined valency, symmetry, flexibility, and function.

Methods to identify enzymes that degrade the main extracellular polysaccharide component of Staphylococcus epidermidis biofilms

The production of extracellular poly-β-1,6-N-acetyl-d-glucosamine (PNAG) by Staphylococcus epidermidis is the principal determinant of biofilm formation on indwelling medical devices. Enzymes that degrade PNAG therefore provide an attractive strategy for biofilm removal and for the manufacture of biofilm-resistant coatings. Here we present methods that allow the identification of PNAG-degrading enzymes with the ability to detach biofilms. Our protocol includes the preparation of soluble PNAG from S. epidermidis cultures, the incubation of soluble PNAG with candidate enzymes and assays that detect the release of N-acetyl-d-glucosamine using high-pH anion-exchange chromatography (HPAEC) followed in parallel by pulsed amperometric detection (PAD) and online electrospray ionization mass spectrometry (ESI-MS). We validated our procedures using dispersin B, which is currently the only known PNAG-degrading enzyme.