Synthesis of multitopic neoglycopeptides displaying recognition and detection motifs

Multivalent glycocyclopeptides: conjugation methods and biological applications

Click chemistry was extensively used to decorate synthetic multivalent scaffolds with glycans to mimic the cell surface glycocalyx and to develop applications in glycosciences. Conjugation methods such as oxime ligation, copper(I)-catalyzed alkyne-azide cycloaddition, thiol-ene coupling, squaramide coupling or Lansbury aspartylation proved particularly suitable to achieve this purpose. This review summarizes the synthetic strategies that can be used either in a stepwise manner or in an orthogonal one-pot approach, to conjugate multiple copies of identical or different glycans to cyclopeptide scaffolds (namely multivalent glycocyclopeptides) having different size, valency, geometry and molecular composition. The second part of this review will describe the potential of these structures to interact with various carbohydrate binding proteins or to stimulate immunity against tumor cells.

Aminooxylated Carbohydrates: Synthesis and Applications

Among other classes of biomolecules, carbohydrates and glycoconjugates are widely involved in numerous biological functions. In addition to addressing the related synthetic challenges, glycochemists have invested intense efforts in providing access to structures that can be used to study, activate, or inhibit these biological processes. Over the past few decades, aminooxylated carbohydrates have been found to be key building blocks for achieving these goals. This review provides the first in-depth overview covering several aspects related to the syntheses and applications of aminooxylated carbohydrates. After a brief introduction to oxime bonds and their relative stabilities compared to related C═N functions, synthetic aspects of oxime ligation and methodologies for introducing the aminooxy functionality onto both glycofuranosyls and glycopyranosyls are described. The subsequent section focuses on biological applications involving aminooxylated carbohydrates as components for the construcion of diverse architectures. Mimetics of natural structures represent useful tools for better understanding the features that drive carbohydrate-receptor interaction, their biological output and they also represent interesting structures with improved stability and tunable properties. In the next section, multivalent structures such as glycoclusters and glycodendrimers obtained through oxime ligation are described in terms of synthetic design and their biological applications such as immunomodulators. The second-to-last section discusses miscellaneous applications of oxime-based glycoconjugates, such as enantioselective catalysis and glycosylated oligonucleotides, and conclusions and perspectives are provided in the last section.

Fluorescent Human EP3 Receptor Antagonists

Exchange of the lipophilc part of ortho-substituted cinnamic acid lead structures with different small molecule fluorophoric moieties via a dimethylene spacer resulted in hEP3R ligands with affinities in the nanomolar concentration range. Synthesized compounds emit fluorescence in the blue, green, and red range of light and have been tested concerning their potential as a pharmacological tool. hEP3Rs were visualized by confocal laser scanning microscopy on HT-29 cells, on murine kidney tissues, and on human brain tissues and functionally were characterized as antagonists on human platelets. Inhibition of PGE2 and collagen-induced platelet aggregation was measured after preincubation with novel hEP3R ligands. The pyryllium-labeled ligand 8 has been shown as one of the most promising structures, displaying a useful fluorescence and highly affine hEP3R antagonists.

α and βl-Fucopyranosyl oxyamines: key intermediates for the preparation of fucose-containing glycoconjugates by oxime ligation

We report herein the synthesis of new alpha and beta aminooxylated L-fucopyranosyl derivatives for the preparation of glycoclusters through oxime ligation. The glycosylation reaction between activated triacetylated L-fucopyranosyl fluoride and N-hydroxyphthalimide was carried out in the presence of boron trifluoride-diethyl etherate and the stereochemical outcome of glycosylation was compared in dichloromethane, acetonitrile or tetrahydrofuran. Interestingly, an unexpected alpha and beta anomer ratio was obtained in spite of the presence of an acetate participating group at the carbon 2, particularly the 1,2-cis glycosylation was largely favoured in acetonitrile. The resulting alpha and beta N-oxyphthalimido fucopyranosyl derivatives were finally deprotected with methylhydrazine to obtain the corresponding free aminooxylated fucopyranosyls. The structure of single-crystal alpha anomer 12 was analysed by X-ray diffraction.

Novel template-assembled oligosaccharide clusters as epitope mimics for HIV-neutralizing antibody 2G12. Design, synthesis, and antibody binding study

The synthesis of a new class of template-assembled oligomannose clusters as the mimics of the epitope of the HIV-neutralizing antibody 2G12 is described. The novel oligomannose clusters were successfully assembled on a cyclic decapeptide template using the Cu(I)-catalyzed 1,3-dipolar cycloaddition of azides to alkynes by introducing four units of a synthetic D1 arm tetrasaccharide (Manalpha1,2Manalpha1,2Manalpha1,3Manalpha-) of high-mannose N-glycan on one face of the template and two T-helper epitope peptides on the other face of the template. Their binding to human antibody 2G12 was studied using surface plasmon resonance (SPR) technology. It was found that while the synthetic monomeric D1 arm oligosaccharide and its fluorinated derivative interacted with 2G12 only weakly, the corresponding template-assembled oligosaccharide clusters showed high affinity to antibody 2G12, indicating a clear clustering effect in 2G12 recognition. Interestingly, the fluorinated D1 arm cluster, in which the 6-OH of the terminal mannosyl residue was replaced with a fluorine atom, showed a distinct kinetic model in 2G12 binding as compared with the cluster of the natural D1 arm oligosaccharides. The oligosaccharide clusters with varied length of spacer demonstrated different affinity to 2G12, suggesting that an appropriate spatial orientation of the sugar chains in the cluster was crucial for high affinity binding to the antibody 2G12. It was also found that the introduction of two T-helper epitopes onto the template did not affect the structural integrity of the oligomannose cluster. The novel synthetic glycoconjugates represent a new type of immunogen that may be able to raise carbohydrate-specific neutralizing antibodies against HIV-1.

Probing glycomics

The study of protein-carbohydrate interactions is one central theme of glycomics research. The challenges encountered when investigating these interactions have resulted in an approach that studies saccharides through the enzymes that process them. Proteins and their function are often probed by manipulating the genes that encode them. Efforts in proteoglycomics exploring protein-binding properties and the enzymatic modification of carbohydrates have intensified, and synthetic tools, including activity- and affinity-based probes, have enhanced our understanding of the roles of carbohydrates in biology.

Orthogonally Protected Cyclo-β-tetrapeptides as Solid-Supported Scaffolds for the Synthesis of Glycoclusters

Two novel peptide scaffolds, viz. cyclo[(N(alpha)-Alloc)Dpr-beta-Ala-(N(alpha)-Fmoc)Dpr-beta-Ala] (1) and cyclo[(N(alpha)-Alloc)Dpr-alpha-azido-beta-aminopropanoyl-(N(alpha)-Fmoc)Dpr-beta-Ala] (2), composed of orthogonally protected 2,3-diaminopropanoyl (Dpr) and beta-alanyl residues, have been described. Fmoc chemistry on a backbone amide linker derivatized resin has been used for the chain assembly. Selective removal of the 4-methyltrityl (Mtt) and 1-methyl-1-phenylethyl protections (PhiPr) exposes the beta-amino and carboxyl terminus, respectively, and on-resin cyclization then gives the desired orthogonally protected cyclo-beta-tetrapeptides (1 and 2). The alpha-amino groups, bearing the Fmoc and Alloc protections and the azide mask, allow stepwise orthogonal derivatization of these solid-supported cyclo-beta-tetrapeptide cores (1 and 2). This has been demonstrated by attachments of various sugar units [viz., acetyl- or toluoyl-protected carboxymethyl alpha-d-glycopyranosides (13-15) and methyl 6-O-(4-nitrophenoxycarbonyl)-alpha-d-glycopyranosides (22-24)] to obtain diverse di- and trivalent glycoclusters (33-42). Acidolytic release (TFA) from the support, followed by conventional NaOMe-catalyzed transesterification (33-40) or hydrazine-induced acyl substitution in DMF (41 and 42), gives the fully deprotected clusters (43-52) as final products.

On-bead synthesis and binding assay of chemoselectively template-assembled multivalent neoglycopeptides

The investigation of recognition events between carbohydrates and proteins, especially the control of how spatial factors and binding avidity are correlated in, remains a great interest for glycomics. Therefore, the development of efficient methods for the rapid evaluation of new ligands such as multivalent glycoconjugates is essential for diverse diagnostic or therapeutic applications. In this paper we describe the synthesis of chemoselectively-assembled multivalent neoglycopeptides and the subsequent recognition assay on a solid support. Aminooxylated carbohydrates (betaLac-ONH(2) 4, alphaGalNAc-ONH(2) 9 and alphaMan-ONH(2) 13) have been prepared as carbohydrate-based recognition elements and assembled as clusters onto a cyclopeptidic scaffold by an oxime-based strategy in solid phase. Further binding tests between lectins and beads of resin derivatized with neoglycopeptides displaying clustered lactoses, N-acetylgalactoses and mannoses (18-20) have shown specific recognition and enhanced affinity through multivalent interactions, suggesting that the local density of carbohydrate-based ligands at the bead surface is crucial to improve the interaction of proteins of weak binding affinity. This solid phase strategy involving both molecular assembly and biological screening provides a rapid and efficient tool for various applications in glycomics.

Efficient synthesis of C-terminal modified peptide ketones for chemical ligations

Synthesis of a C-terminal modified peptide with an alpha-amido methylketone was efficiently carried out using a backbone-amide-type linker loading with a monofunctionalized diamine, provided that no base such as piperidine or diisopropylethylamine or a reducing agent such as triisopopylsilane was used for the synthetic pathway. The ketoxime-forming chemoselective ligation between a methylketone and an aminooxy was quantitative in 5h at pH 2.