On resin synthesis of sulfated oligosaccharides

Enzyme-Sialylation-Controlled Chemical Sulfation of Glycan Epitopes for Decoding the Binding of Siglec Ligands

Widely distributed in nature, sulfated glycan epitopes play important roles in diverse pathophysiological processes. However, due to their structural complexity, the preparation of glycan epitopes with structurally defined sulfation patterns is challenging, which significantly hampers the detailed elucidation of their biological functions at the molecular level. Here, we introduce a strategy for site-specific chemical sulfation of glycan epitopes, leveraging enzymatic sialylation and desialylation processes to precisely control the regio-specificity of sulfation of disaccharide or trisaccharide glycan backbones. Using this method, a sulfated glycan library covering the most common sialylated glycan epitopes was prepared in high yield and efficiency. By screening a microarray prepared with this glycan library, we systematically probed their binding specificity with human Siglecs (sialic acid-binding immunoglobulin-type lectins), many of which function as glyco-immune checkpoints to suppress immune system activation. Our investigation revealed that sulfation and sialylation patterns serve as important determinants of Siglec binding affinity and specificity. Thus, these findings offer new insights for the development of research tools and potential therapeutic agents targeting glyco-immune checkpoints by modulating the Siglec signaling pathway.

On resin synthesis of phosphoethanolamine cellulose

Phosphoethanoamine (pEtN) cellulose is a chemically modified cellulose present in some bacterial biofilms. To deepen our understanding of this biopolymer and its biological function, access to chemically defined pEtN-cellulose oligosaccharides is desirable. Herein, we report an on resin protocol for the fast synthesis of tailor-made pEtN-celluloses. The cellulose backbone is prepared by automated glycan assembly and then specifically functionalized with pEtN groups, allowing for access to a collection of ten pEtN-cellulose oligomers with different amount and pattern of pEtN.

Overview of the current procedures in synthesis of heparin saccharides

Natural heparin, a glycosaminoglycan consisting of repeating hexuronic acid and glucosamine linked by 1 → 4 glycosidic bonds, is the most widely used anticoagulant. To subvert the dependence on animal sourced heparin, alternative methods to produce heparin saccharides, i.e., either heterogenous sugar chains similar to natural heparin, or structurally defined oligosaccharides, are becoming hot subjects. Although the success by chemical synthesis of the pentasaccharide, fondaparinux, encourages to proceed through a chemical approach generating homogenous product, synthesizing larger oligos is still cumbersome and beyond reach so far. Alternatively, the chemoenzymatic pathway exhibited exquisite stereoselectivity of glycosylation and regioselectivity of modification, with the advantage to skip the tedious protection steps unavoidable in chemical synthesis. However, to a scale of drug production needed today is still not in sight. In comparison, a procedure of de novo biosynthesis in an organism could be an ultimate goal. The main purpose of this review is to summarize the current available/developing strategies and techniques, which is expected to provide a comprehensive picture for production of heparin saccharides to replenish or eventually to replace the animal derived products. In chemical and chemoenzymatic approaches, the methodologies are discussed according to the synthesis procedures: building block preparation, chain elongation, and backbone modification.

Automated Synthesis of Algal Fucoidan Oligosaccharides

Fucoidan, a sulfated polysaccharide found in algae, plays a central role in marine carbon sequestration and exhibits a wide array of bioactivities. However, the molecular diversity and structural complexity of fucoidan hinder precise structure-function studies. To address this, we present an automated method for generating well-defined linear and branched α-fucan oligosaccharides. Our syntheses include oligosaccharides with up to 20 cis-glycosidic linkages, diverse branching patterns, and 11 sulfate monoesters. In this study, we demonstrate the utility of these oligosaccharides by (i) characterizing two endo-acting fucoidan glycoside hydrolases (GH107), (ii) utilizing them as standards for NMR studies to confirm suggested structures of algal fucoidans, and (iii) developing a fucoidan microarray. This microarray enabled the screening of the molecular specificity of four monoclonal antibodies (mAb) targeting fucoidan. It was found that mAb BAM4 has cross-reactivity to β-glucans, while mAb BAM2 has reactivity to fucoidans with 4-O-sulfate esters. Knowledge of the mAb BAM2 epitope specificity provided evidence that a globally abundant marine diatom, Thalassiosira weissflogii, synthesizes a fucoidan with structural homology to those found in brown algae. Automated glycan assembly provides access to fucoidan oligosaccharides. These oligosaccharides provide the basis for molecular level investigations into fucoidan's roles in medicine and carbon sequestration.

Recent advances in the synthesis of extensive libraries of heparan sulfate oligosaccharides for structure-activity relationship studies

Heparan sulfate (HS) is a linear, sulfated and highly negatively-charged polysaccharide that plays important roles in many biological events. As a member of the glycosaminoglycan (GAG) family, HS is commonly found on mammalian cell surfaces and within the extracellular matrix. The structural complexities of natural HS polysaccharides have hampered the comprehension of their biological functions and structure-activity relationships (SARs). Although the sulfation patterns and backbone structures of HS can be major determinants of their biological activities, obtaining significant amounts of pure HS from natural sources for comprehensive SAR studies is challenging. Chemical and enzyme-based synthesis can aid in the production of structurally well-defined HS oligosaccharides. In this review, we discuss recent innovations enabling the syntheses of large libraries of HS and how these libraries can provide insights into the structural preferences of various HS binding proteins.

Synthesis and Detection of BODIPY-, Biotin-, and 19 F- Labeled Single-Entity Dendritic Heparan Sulfate Mimetics

Heparin and heparan sulfate (HS) are naturally occurring mammalian glycosaminoglycans, and their synthetic and semi-synthetic mimetics have attracted significant interest as potential therapeutics. However, understanding the mechanism of action by which HS, heparin, and HS mimetics have a biological effect is difficult due to their highly charged nature, broad protein interactomes, and variable structures. To address this, a library of novel single-entity dendritic mimetics conjugated to BODIPY, Fluorine-19 (19 F), and biotin was synthesized for imaging and localization studies. The novel dendritic scaffold allowed for the conjugation of labeling moieties without reducing the number of sulfated capping groups, thereby better mimicking the multivalent nature of HS-protein interactions. The 19 F labeled mimetics were assessed in phantom studies and were detected at concentrations as low as 5 mM. Flow cytometric studies using a fluorescently labeled mimetic showed that the compound associated with immune cells from tumors more readily than splenic counterparts and was directed to endosomal-lysosomal compartments within immune cells and cancer cells. Furthermore, the fluorescently labeled mimetic entered the central nervous system and was detectable in brain-infiltrating immune cells 24 hours after treatment. Here, we report the enabling methodology for rapidly preparing various labeled HS mimetics and molecular probes with diverse potential therapeutic applications.

Synthesis of a heparan sulfate tetrasaccharide using automated glycan assembly

Herein we utilise automated glycan assembly to complete solid-phase synthesis of defined heparan sulfate oligosaccharides, employing challenging D-glucuronate disaccharide donors. Using an orthogonally protected D-GlcN-α-D-GlcA donor, milligram-scale synthesis of a heparan sulfate tetrasaccharide is completed in 18% yield over five steps. Furthermore, orthogonal protecting groups enabled regiospecific on-resin 6-O-sulfation. This methodology provides an important benchmark for the rapid assembly of biologically relevant heparan sulfate sequences.

Towards the semi-synthesis of phosphorylated mimics of glycosaminoglycans: Screening of methods for the regioselective phosphorylation of chondroitin

Glycosaminoglycan (GAG) mimics carrying phosphate rather than sulfate anionic groups have been poorly investigated, in spite of their interesting perspectives. While some GAG-mimicking phosphorylated polymers have been reported, to the best of our knowledge no phosphorylated polysaccharides having the same backbone of natural sulfated GAGs have been accessed yet. To fill this gap, in this work two standard phosphorylation protocols and two recently reported procedures have been screened on a set of polysaccharide species composed by microbial sourced chondroitin and three partially protected, semi-synthetic derivatives thereof. A detailed structural characterization by 1H, 13C and 31P NMR spectroscopy revealed the higher versatility of the innovative, biomimetic reaction employing monopotassium salt of phosphoenolpyruvate (PEPK) with respect to standard phosphorylating agents (phosphoric acid or phosphorus oxychloride). Indeed, PEP-K and H3PO4 gave similar results in the regioselective phosphorylation of the primary hydroxyls of unprotected chondroitin, while only the former reacted on partially protected chondroitin derivatives in a controlled, regioselective fashion, affording chondroitin phosphate (CP) polysaccharides with different derivatization patterns. The reported results represent the first, key steps towards the systematic semi-synthesis of phosphorylated GAGs as a new class of GAG mimics and to the evaluation of their biological activities in comparison with native sulfated GAGs.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Linker, loading, and reaction scale influence automated glycan assembly

Automated glycan assembly (AGA) affords collections of well-defined glycans in a short amount of time. We systematically analyzed how parameters connected to the solid support affect the AGA outcome for three different glycan sequences. We showed that, while loading and reaction scale did not significantly influence the AGA outcome, the chemical nature of the linker dramatically altered the isolated yields. We identified that the major determinants of AGA yields are cleavage from the solid support and post-AGA purification steps.

Glycosaminoglycans: What Remains To Be Deciphered?

Glycosaminoglycans (GAGs) are complex polysaccharides exhibiting a vast structural diversity and fulfilling various functions mediated by thousands of interactions in the extracellular matrix, at the cell surface, and within the cells where they have been detected in the nucleus. It is known that the chemical groups attached to GAGs and GAG conformations comprise "glycocodes" that are not yet fully deciphered. The molecular context also matters for GAG structures and functions, and the influence of the structure and functions of the proteoglycan core proteins on sulfated GAGs and vice versa warrants further investigation. The lack of dedicated bioinformatic tools for mining GAG data sets contributes to a partial characterization of the structural and functional landscape and interactions of GAGs. These pending issues will benefit from the development of new approaches reviewed here, namely (i) the synthesis of GAG oligosaccharides to build large and diverse GAG libraries, (ii) GAG analysis and sequencing by mass spectrometry (e.g., ion mobility-mass spectrometry), gas-phase infrared spectroscopy, recognition tunnelling nanopores, and molecular modeling to identify bioactive GAG sequences, biophysical methods to investigate binding interfaces, and to expand our knowledge and understanding of glycocodes governing GAG molecular recognition, and (iii) artificial intelligence for in-depth investigation of GAGomic data sets and their integration with proteomics.

Enabling Technologies in Carbohydrate Chemistry: Automated Glycan Assembly, Flow Chemistry and Data Science

The synthesis of defined oligosaccharides is a complex task. Several enabling technologies have been introduced in the last two decades to facilitate synthetic access to these valuable biomolecules. In this concept, we describe the technological solutions that have advanced glycochemistry using automated glycan assembly, flow chemistry and data science as examples. We highlight how the synergies between these different technologies can further advance the field, with progress toward the realization of a self-driving lab for glycan synthesis.

Structures and functions of algal glycans shape their capacity to sequester carbon in the ocean

Algae synthesise structurally complex glycans to build a protective barrier, the extracellular matrix. One function of matrix glycans is to slow down microorganisms that try to enzymatically enter living algae and degrade and convert their organic carbon back to carbon dioxide. We propose that matrix glycans lock up carbon in the ocean by controlling degradation of organic carbon by bacteria and other microbes not only while algae are alive, but also after death. Data revised in this review shows accumulation of algal glycans in the ocean underscoring the challenge bacteria and other microbes face to breach the glycan barrier with carbohydrate active enzymes. Briefly we also update on methods required to certify the uncertain magnitude and unknown molecular causes of glycan-controlled carbon sequestration in a changing ocean.

VaporSPOT: Parallel Synthesis of Oligosaccharides on Membranes

Automated chemical synthesis has revolutionized synthetic access to biopolymers in terms of simplicity and speed. While automated oligosaccharide synthesis has become faster and more versatile, the parallel synthesis of oligosaccharides is not yet possible. Here, a chemical vapor glycosylation strategy (VaporSPOT) is described that enables the simultaneous synthesis of oligosaccharides on a cellulose membrane solid support. Different linkers allow for flexible and straightforward cleavage, purification, and characterization of the target oligosaccharides. This method is the basis for the development of parallel automated glycan synthesis platforms.

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.

New chemical processes to streamline carbohydrate synthesis

Carbohydrates hold potential for the future of therapeutic development due to their important role in essential biological processes. However, it is still challenging to produce homogenous materials, especially for non-mammalian sugars that are considered rare. Recent developments in this field have focused on catalytic methods, including organometallic and organocatalytic approaches to regioselective functionalization. Many approaches to glycosylations also utilize catalysts, increasingly in combination with photoredox conditions, to achieve stereoselectivity. Additionally, there have been significant advancements in the automation of glycosylation to synthesize oligosaccharides in less time and with fewer manually conducted steps by the user.

Synthetic phosphoethanolamine-modified oligosaccharides reveal the importance of glycan length and substitution in biofilm-inspired assemblies

Bacterial biofilm matrices are nanocomposites of proteins and polysaccharides with remarkable mechanical properties. Efforts understanding and tuning the protein component have been extensive, whereas the polysaccharide part remained mostly overlooked. The discovery of phosphoethanolamine (pEtN) modified cellulose in E. coli biofilms revealed that polysaccharide functionalization alters the biofilm properties. To date, the pattern of pEtN cellulose and its mode of interactions with proteins remains elusive. Herein, we report a model system based on synthetic epitomes to explore the role of pEtN in biofilm-inspired assemblies. Nine pEtN-modified oligosaccharides were synthesized with full control over the length, degree and pattern of pEtN substitution. The oligomers were co-assembled with a representative peptide, triggering the formation of fibers in a length dependent manner. We discovered that the pEtN pattern modulates the adhesion of biofilm-inspired matrices, while the peptide component controls its stiffness. Unnatural oligosaccharides tune or disrupt the assembly morphology, revealing interesting targets for polysaccharide engineering to develop tunable bio-inspired materials.

Automated Solid Phase Assisted Synthesis of a Heparan Sulfate Disaccharide Library

Heparan sulfate (HS) regulates a wide range of biological events, including blood coagulation, cancer development, cell differentiation, and viral infections. It is generally recognized that structures of HS can critically...