Synthesis and molecular recognition of carbohydrate-centered multivalent glycoclusters by a plant lectin RCA120

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

The design and synthesis of an α-Gal trisaccharide epitope that provides a highly specific anti-Gal immune response

Carbohydrate antigens displaying Galα(1,3)Gal epitopes are recognized by naturally occurring antibodies in humans. These anti-Gal antibodies comprise up to 1% of serum IgG and have been viewed as detrimental as they are responsible for hyperacute organ rejections. In order to model this condition, α(1,3)galactosyltransferase-knockout mice are inoculated against the Galα(1,3)Gal epitope. In our study, two α-Gal trisaccharide epitopes composed of either Galα(1,3)Galβ(1,4)GlcNAc or Galα(1,3)Galβ(1,4)Glc linked to a squaric acid ester moiety were examined for their ability to elicit immune responses in KO mice. Both target epitopes were synthesized using a two-component enzymatic system using modified disaccharide substrates containing a linker moiety for coupling. While both glycoconjugate vaccines induced the required high anti-Gal IgG antibody titers, it was found that this response had exquisite specificity for the Galα(1,3)Galβ(1,4)GlcNAc hapten used, with little cross reactivity with the Galα(1,3)Galβ(1,4)Glc hapten. Our findings indicate that while homogenous glycoconjugate vaccines provide high IgG titers, the carrier and adjuvanting factors can deviate the specificity to an antigenic determinant outside the purview of interest.

Controlling the selective and directional migration of hepatocytes by a complementary density gradient of glycosylated hyperbranched polymers and poly(ethylene glycol) molecules

Repair and regeneration of defected tissues and organs depends strongly on the directional migration of targeted cells, for example, the enhancement of directional migration of hepatocytes could be helpful in liver regeneration and transplantation. Herein a complementary gradient of galactose-modified hyperbranched polymers (LA-HPMA) and poly(ethylene glycol) (PEG) molecules was designed and prepared on a same substrate. Characterizations of X-ray photoelectron spectrometry and quartz crystal microbalance with dissipation (QCM-d) demonstrated the unidirectional change in grafting density of LA-HPMA and PEG molecules, respectively. On the LA-HPMA/PEG complementary gradient surface, the human hepatoma (HepG2) cells showed preferential orientation and enhanced directional migration toward the region of lower PEG density and higher LA-HPMA density. By contrast, the mouse embryonic fibroblasts (NIH3T3) showed random migration irrelevant to the gradient. The success of the complementary gradient relies on the specific interaction between galactose and asialoglycoprotein receptor (ASGPR) expressed on HepG2 cells. STATEMENT OF SIGNIFICANCE.

Synthesis of carbohydrate-substituted isoxazoles and evaluation of their antitubercular activity

Eight new sugar-substituted isoxazoles were synthesized by a 1,3-dipolar cycloaddition reaction of aromatic nitrile oxides with carbohydrate-substituted alkynes. Products were screened for antimycobacterial activity against the Mycobacterium tuberculosis H37Rv strain. Four compounds, 5e–h, significantly inhibit growth of the bacterial strain with a minimum inhibitory concentration (MIC) of 3.125 μg/mL.

Carbohydrates as potentially versatile core subcarriers for multivalent immunogens

Synthetic multivalent glycoclusters that carry carbohydrate antigen epitopes have been recognized as promising candidates for the development of carbohydrate based vaccines. Here we describe a convergent strategy for the synthesis of conjugation-ready multivalent glycoclusters using sugars as versatile core subcarriers. d-Glucose and gentiobiose were converted into poly-alkyne functionalized cores which were then decorated with an azide bearing model ligand d-glucose using click chemistry, to form structurally well-defined tetra- and heptavalent glycoclusters. Each cluster was conjugated to a model protein bovine serum albumin (BSA) by squaric acid chemistry. Carbohydrate clusters can be prepared in a variety of sizes and spatial arrangements by altering the structure and configuration of the core, depending on the mono-, or oligosaccharides used for their assembly. It is suggested that the use of carbohydrate as core subcarriers provides an opportunity to tailor the size and topology of antigens and modify multivalent presentation of immunogens in a way to optimize cluster effect for stronger immunoreactivity.

Glycoproteins functionalized natural and synthetic polymers for prospective biomedical applications: A review

Glycoproteins have multidimensional properties such as biodegradability, biocompatibility, non-toxicity, antimicrobial and adsorption properties; therefore, they have wide range of applications. They are blended with different polymers such as chitosan, carboxymethyl cellulose (CMC), polyvinyl pyrrolidone (PVP), polycaprolactone (PCL), heparin, polystyrene fluorescent nanoparticles (PS-NPs) and carboxyl pullulan (PC) to improve their properties like thermal stability, mechanical properties, resistance to pH, chemical stability and toughness. Considering the versatile charateristics of glycoprotein based polymers, this review sheds light on synthesis and characterization of blends and composites of glycoproteins, with natural and synthetic polymers and their potential applications in biomedical field such as drug delivery system, insulin delivery, antimicrobial wound dressing uses, targeting of cancer cells, development of anticancer vaccines, development of new biopolymers, glycoproteome research, food product and detection of dengue glycoproteins. All the technical scientific issues have been addressed; highlighting the recent advancement.

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

Convergent synthetic methodology for the construction of self-adjuvanting lipopeptide vaccines using a novel carbohydrate scaffold

A novel convergent synthetic strategy for the construction of multicomponent self-adjuvanting lipopeptide vaccines was developed. A tetraalkyne-functionalized glucose derivative and lipidated Fmoc-lysine were prepared by novel efficient and convenient syntheses. The carbohydrate building block was coupled to the self-adjuvanting lipidic moiety (three lipidated Fmoc-lysines) on solid support. Four copies of a group A streptococcal B cell epitope (J8) were then conjugated to the glyco-lipopeptide using a copper-catalyzed cycloaddition reaction. The approach was elaborated by the preparation of a second vaccine candidate which incorporated an additional promiscuous T-helper epitope.

Multichromophoric sugar for fluorescence photoswitching

A multichromophoric glucopyranoside 2 bearing three dicyanomethylenepyran (DCM) fluorophores and one diarylethene (DAE) photochrome has been prepared by Cu(I)-catalyzed alkyne–azide cycloaddition reaction. The fluorescence of 2 was switched off upon UV irradiation, in proportion with the open to closed form (OF to CF) conversion extent of the DAE moiety. A nearly 100% Förster-type resonance energy transfer (FRET) from all three DCM moieties to a single DAE (in its CF) moiety was achieved. Upon visible irradiation, the initial fluorescence intensity was recovered. The observed photoswiching is reversible, with excellent photo resistance.

“Onion peel” dendrimers: a straightforward synthetic approach towards highly diversified architectures

We report herein a novel “onion peel strategy” for the divergent construction of glycodendrimers using different building blocks at each layer of the dendritic growth.

Carbohydrate-PNA and aptamer-PNA conjugates for the spatial screening of lectins and lectin assemblies

Nucleic acid architectures offer intriguing opportunities for the interrogation of structural properties of protein receptors. In this study, we performed a DNA-programmed spatial screening to characterize two functionally distinct receptor systems: 1) structurally well-defined Ricinus communis agglutinin (RCA(120)), and 2) rather ill-defined assemblies of L-selectin on nanoparticles and leukocytes. A robust synthesis route that allowed the attachment both of carbohydrate ligands-such as N-acetyllactosamine (LacNAc), sialyl-Lewis-X (sLe(X)), and mannose-and of a DNA aptamer to PNAs was developed. A systematically assembled series of different PNA-DNA complexes served as multivalent scaffolds to control the spatial alignments of appended lectin ligands. The spatial screening of the binding sites of RCA(120) was in agreement with the crystal structure analysis. The study revealed that two appropriately presented LacNAc ligands suffice to provide unprecedented RCA(120) affinity (K(D) = 4 μM). In addition, a potential secondary binding site was identified. Less dramatic binding enhancements were obtained when the more flexible L-selectin assemblies were probed. This study involved the bivalent display both of the weak-affinity sLe(X) ligand and of a high-affinity DNA aptamer. Bivalent presentation led to rather modest (sixfold or less) enhancements of binding when the self-assemblies were targeted against L-selectin on gold nanoparticles. Spatial screening of L-selectin on the surfaces of leukocytes showed higher affinity enhancements (25-fold). This and the distance-activity relationships indicated that leukocytes permit dense clustering of L-selectin.

Oligonucleotide carbohydrate-centered galactosyl cluster conjugates synthesized by click and phosphoramidite chemistries

Oligonucleotide glycoconjugates with a mannose or galactose core bearing four galactose residues introduced by phosphoramidite chemistry and copper catalyzed azide alkyne 1,3-dipolar cycloaddition (click chemistry) have been synthesized. A first click reaction allowed the introduction on a solid support of a mannose core on which four pentynyl linkers were introduced using a phosphoramidite derivative. After the elongation of the oligonucleotide, a second click reaction performed either on solid support or in solution allowed the introduction of four galactose azide derivatives. Repeating the phosphoramidite and click chemistries afforded an oligonucleotide glycoconjugate dendrimer bearing 16 galactoses on its periphery.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Glycosylated nordihydroguaiaretic acids as anti-cancer agents

Three perglycosylated nordihydroguaiaretic acids (NDGA) were synthesized through the Huiseng 1,3-dipolar cycloaddition reaction. These sugar-NDGA conjugates containing triazole-linkages possessed good solubility in water. NDGA-(triazol-galactose)(4) (12b) and NDGA-(triazol-glucose)(4) (12c) were found to act as inhibitors against human hepatocellular carcinoma Hep3B cells in culture.

Recent applications of the CuI-catalysed Huisgen azide–alkyne 1,3-dipolar cycloaddition reaction in carbohydrate chemistry

This article surveys recent applications of Cu(I)-catalysed 1,3-dipolar cycloaddition of azides and alkynes in carbohydrate chemistry, highlighting developments in the preparation of simple glycoside and oligosaccharide mimetics, glyco-macrocycles, glycopeptides, glyco-clusters and carbohydrate arrays.

Intervention with bacterial adhesion by multivalent carbohydrates

Bacterial adhesion is often a prelude to infection. In many cases, this process is governed by protein-carbohydrate interactions. Intervention at this early stage of infection is a conceptually highly attractive alternative to conventional antibiotics that are increasingly prone to resistance. The lack of high-affinity inhibitors of adhesion has proven to be a hurdle for further exploitation of this concept; however, new developments indicate a positive change. Structure-based design at the monovalent level and also evaluation of glycodendrimers and glycopolymers have yielded structures of high affinity. In addition to the development of inhibitors, topics of this review include available structural information of adhesion proteins, carbohydrate specificities of the various pathogens and their adhesion proteins. Other new developments aimed at affecting bacterial adhesion and the use of the adhesins for bacterial detection are also discussed.

Protein microarrays to study carbohydrate-recognition events

In order to expand areas in which protein microarrays can be used to solve important biological problems, we have investigated ways in which the technique can be employed for functional glycomics. Initially, our protein microarrays were used for the rapid identification of carbohydrate-binding proteins using trifunctional carbohydrate probes and fluorescent dye-labeled polysaccharides. Glycan probes were selectively bound to the corresponding lectins immobilized on the solid surface. In addition, these microarrays were also employed for profiling of carbohydrates on Jurkat T-cell surfaces. These cells adhered to ConA, RCA(120), SNA and WGA, indicating expression of alpha-Man, Gal, NeuNAcalpha2,6Gal and GlcNAc residues on their surfaces. Furthermore, we determined binding affinities between WGA and carbohydrates by measuring IC(50) values of GlcNAc that inhibited 50% of trivalent GlcNAc binding to WGA immobilized on the solid surface. All the experiments show that protein microarrays can be used to study carbohydrate-recognition events in the field of glycomics.

Carbohydrate triazoles and isoxazoles as inhibitors of galectins-1 and -3

Galactosides and lactosides bearing triazoles or isoxazoles, regiospecifically prepared by [1,3]-dipolar cycloadditions between alkynes, azides or nitrile oxides, provided specific galectin-1 and -3 inhibitors with potencies as low as 20 microM.