Synthesis of sugar arrays in microtiter plate

Rapid and Facile Microwave-Assisted Surface Chemistry for Functionalized Microarray Slides

We describe a rapid and facile method for surface functionalization and ligand patterning of glass slides based on microwave-assisted synthesis and a microarraying robot. Our optimized reaction enables surface modification 42-times faster than conventional techniques and includes a carboxylated self-assembled monolayer, polyethylene glycol linkers of varying length, and stable amide bonds to small molecule, peptide, or protein ligands to be screened for binding to living cells. We also describe customized slide racks that permit functionalization of 100 slides at a time to produce a cost-efficient, highly reproducible batch process. Ligand spots can be positioned on the glass slides precisely using a microarraying robot, and spot size adjusted for any desired application. Using this system, we demonstrate live cell binding to a variety of ligands and optimize PEG linker length. Taken together, the technology we describe should enable high-throughput screening of disease-specific ligands that bind to living cells.

α-Azido ketones. Part 7: synthesis of 1,4-disubstituted triazoles by the "click" reaction of various terminal acetylenes with phenacyl azides or α-azidobenzo(hetera)cyclanones

Copper(I)-catalyzed alkyne-azide 1,3-cycloaddition reaction (CuAAC, Sharpless-Meldal reaction) of various α-azido ketones such as substituted 2-azidoacetophenones, 2-azidobenzosuberone and 3-azido(thio)chromanones with terminal alkynes was studied. The reaction resulted in the formation of the expected 1,2,3-triazoles in moderate to good yields although the reactivity was somewhat lower than in the case of simple azides. Reaction of ethynylchromones as alkynes gave interesting dichromonoid systems bridged by a triazole unit.

Carbohydrate microarrays in 96-well polystyrene microtiter plates

A method for the preparation of carbohydrate microarrays inside 96-well polystyrene microtiter plates is described. The key step in this strategy represents the synthesis of carbohydrate-dextran conjugates by copper (I)-catalyzed [3 + 2] cycloaddition between alkyne carbohydrate derivative and a specially designed azido dextran polymer. The conjugates adsorb efficiently on polystyrene surface and can be printed inside 96-well plates using a non-contact piezoelectric microarrayer. Model interactions with a selection of lectins (concanavalin A, wheat germ agglutinin, Erythrina Cristagalli) display the efficiency of the immobilization method, its reproducibility and the specificity of biomolecular interactions occurring at the polystyrene-water interface.

Synthesis and characterization of maltose-based amphiphiles as supramolecular hydrogelators

Low molecular mass amphiphilic glycolipids have been prepared by linking a maltose polar head and a hydrophobic linear chain either by amidation or copper(I)-catalyzed azide-alkyne [3 + 2] cycloaddition. The liquid crystalline properties of these amphiphilic materials have been characterized. The influence of the chemical structure of these glycolipids on the gelation properties in water has also been studied. Glycolipids obtained by the click coupling of the two components give rise to stable hydrogels at room temperature. The fibrillar structure of supramolecular hydrogels obtained by the self-assembly of these gelators have been characterized by electron microscopy. Fibers showed some torsion, which could be related with a chiral supramolecular arrangement of amphiphiles, as confirmed by circular dichroism (CD). The sol-gel transition temperature was also determined by differential scanning calorimetry (DSC) and NMR.

Toward automated oligosaccharide synthesis

Carbohydrates have been shown to play important roles in biological processes. The pace of development in carbohydrate research is, however, relatively slow due to the problems associated with the complexity of carbohydrate structures and the lack of general synthetic methods and tools available for the study of this class of biomolecules. Recent advances in synthesis have demonstrated that many of these problems can be circumvented. In this Review, we describe the methods developed to tackle the problems of carbohydrate-mediated biological processes, with particular focus on the issue related to the development of the automated synthesis of oligosaccharides. Further applications of carbohydrate microarrays and vaccines to human diseases are also highlighted.

Glycan microarrays for decoding the glycome

In the last decade, glycan microarrays have revolutionized the analysis of the specificity of glycan-binding proteins (GBPs), providing information that simultaneously illuminates the biology mediated by them and decodes the informational content of the glycome. Numerous methods have emerged for arraying glycans in a "chip" format, and glycan libraries have been assembled that address the diversity of the human glycome. Such arrays have been successfully used for analysis of GBPs, which mediate mammalian biology, host-pathogen interactions, and immune recognition of glycans relevant to vaccine production and cancer antigens. This review covers the development of glycan microarrays and applications that have provided insights into the roles of mammalian and microbial GBPs.

Acid-base jointly promoted copper(I)-catalyzed azide-alkyne cycloaddition

In this novel acid-base jointly promoted CuAAC, the combination of CuI/DIPEA/HOAc was developed as a highly efficient catalytic system. The functions of DIPEA and HOAc have been assigned, and HOAc was recognized to accelerate the conversions of the C-Cu bond-containing intermediates and buffer the basicity of DIPEA. As a result, all drawbacks occurring in the popular catalytic system CuI/NR(3) were overcome easily.

Stereoselective synthesis of light-activatable perfluorophenylazide-conjugated carbohydrates for glycoarray fabrication and evaluation of structural effects on protein binding by SPR imaging

A series of light-activatable perfluorophenylazide (PFPA)-conjugated carbohydrate structures have been synthesized and applied to glycoarray fabrication. The glycoconjugates were structurally varied with respect to anomeric attachment, S-, and O-linked carbohydrates, respectively, as well as linker structure and length. Efficient stereoselective synthetic routes were developed, leading to the formation of the PFPA-conjugated structures in good yields over few steps. The use of glycosyl thiols as donors proved especially efficient and provided the final compounds in up to 70% total yield with high anomeric purities. PFPA-based photochemistry was subsequently used to generate carbohydrate arrays on a polymeric surface, and surface plasmon resonance imaging (SPRi) was applied for evaluation of carbohydrate-protein interactions using the plant lectin Concanavalin A (Con A) as a probe. The results indicate better performance and equal efficiency of S- and O-linked structures with intermediate linker length.

Functionalizing poly(cyclic olefins) using copper-catalyzed azide–alkyne “click” reactions

A robust method for functionalizing poly(cyclic olefin) surfaces comprised of Zeonor 1060R was developed. These surfaces were first oxidized using two separate methods, ozonolysis and oxygen plasma treatment, to increase their hydrophilic properties and produce carboxylic acids on the surface. The resulting acids enabled direct amide coupling with various amines containing azide or alkyne moieties onto the surface. These modified surfaces were further functionalized via copper-catalyzed azide–alkyne cycloadditions (CuAAC) on the surface and the method was optimized for this material. Surface modifications were characterized by water contact angle measurements, fluorescence microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Our data demonstrates a robust method for generating “clickable” Zeonor 1060R surfaces as demonstrated through attachment and subsequent detection of a triazole-linked fluorophore. The method developed enables the systematic modification of any poly(cyclic olefin) surface for functionalization using the click reaction.

Natural glycan microarrays

Glycan microarrays are emerging as increasingly used screening tools with a high potential for unraveling protein-carbohydrate interactions: probing hundreds or even thousands of glycans in parallel, they provide the researcher with a vast amount of data in a short time-frame, while using relatively small amounts of analytes. Natural glycan microarrays focus on the glycans' repertoire of natural sources, including both well-defined structures as well as still-unknown ones. This article compares different natural glycan microarray strategies. Glycan probes may comprise oligosaccharides from glycoproteins as well as glycolipids and polysaccharides. Oligosaccharides may be purified from scarce biological samples that are of particular relevance for the carbohydrate-binding protein to be studied. We give an overview of strategies for glycan isolation, derivatization, fractionation, immobilization and structural characterization. Detection methods such as fluorescence analysis and surface plasmon resonance are summarized. The importance of glycan density and multivalency is discussed. Furthermore, some applications of natural glycan microarrays for studying lectin and antibody binding are presented.

Photo-click immobilization on quartz crystal microbalance sensors for selective carbohydrate-protein interaction analyses

A photoclick method based on azide photoligation and Cu-catalyzed azide-alkyne cycloaddition has been evaluated for the immobilization of carbohydrates to polymeric materials. The biomolecular recognition properties of the materials have been investigated with regard to applicable polymeric substrates and selectivity of protein binding. The method was used to functionalize a range of polymeric surfaces (polystyrene, polyacrylamide, poly(ethylene glycol), poly(2-ethyl-2-oxazoline), and polypropene) with various carbohydrate structures (based on α-D-mannose, β-D-galactose, and N-acetyl-β-D-glucosamine). The functionalized surfaces were evaluated in real-time studies of protein-carbohydrate interactions using a quartz crystal microbalance flow-through system with a series of different carbohydrate-binding proteins (lectins). The method proved to be robust and versatile, resulting in a range of efficient sensors showing high and predictable protein selectivities.

Ionic liquids in oligosaccharide synthesis: towards mucin-type glycan probes

The present article provides an overview on mucins and their role in biological processes, while aiming to familiarize readers with the current tools available for the synthesis of structurally defined mucin-type glycan probes including the advantages and potential applications of using ionic liquids in the synthesis of this important class of oligosaccharides. Furthermore, we also highlight recent developments in glycoarray technology that can enable high-sensitivity and high-throughput analysis of this important class of protein-carbohydrate interactions.

Selectivity among two lectins: probing the effect of topology, multivalency and flexibility of "clicked" multivalent glycoclusters

The design of multivalent glycoconjugates has been developed over the past decades to obtain high-affinity ligands for lectin receptors. While multivalency frequently increases the affinity of a ligand for its lectin through the so-called "glycoside cluster effect", the binding profiles towards different lectins have been much less investigated. We have designed a series of multivalent galactosylated glycoconjugates and studied their binding properties towards two lectins, from plant and bacterial origins, to determine their potential selectivity. The synthesis was achieved through copper(I)-catalysed azide-alkyne cycloaddition (CuAAC) under microwave activation between propargylated multivalent scaffolds and an azido-functionalised carbohydrate derivative. The interactions of two galactose-binding lectins from Pseudomonas aeruginosa (PA-IL) and Erythrina cristagalli (ECA) with the synthesized glycoclusters were studied by hemagglutination inhibition assays (HIA), surface plasmon resonance (SPR) and isothermal titration microcalorimetry (ITC). The results obtained illustrate the influence of the scaffold's geometry on the affinity towards the lectin and also on the relative potency in comparison with a monovalent galactoside reference probe.

Discovery of a quorum sensing modulator pharmacophore by 3D small-molecule microarray screening

The screening of large arrays of drug-like small-molecules was traditionally a time consuming and resource intensive task. New methodology developed within our laboratories provides an attractive low cost, 3D microarray-assisted screening platform that could be used to rapidly assay thousands of compounds. As a proof-of-principle the platform was exploited to screen a number of quorum sensing analogs. Quorum sensing is used by bacterium to initiate and spread infection; in this context its modulation may have significant clinical value. 3D microarray slides were probed with fluorescently labeled ligand-binding domains of the LuxR homolog CarR from Erwinia carotovora subsp. carotovora. The 3D microarray platform was used to discover the biologically active chloro-pyridine pharmacophore, which was validated using a fluorometric ligand binding assay and ITC. Analogs containing the chloro-pyridine pharmacophore were found to be potent inhibitors of N-acyl-homoserine-lactone (AHL) mediated quorum sensing phenotypes in Serratia (IC(50) = ∼5 μM) and Pseudomonas aeruginosa (IC(50) = 10-20 μM).

Neoglycolipid (NGL)-based oligosaccharide microarrays and highlights of their recent applications in studies of the molecular basis of pathogen–host interactions

Carbohydrate microarray technologies are new developments at the frontier of glycomics that are showing great promise as tools for high-throughput analysis of carbohydrate-mediated interactions and the elucidation of carbohydrate ligands involved not only in endogenous receptor systems, but also pathogen–host interactions. The main advantage of microarray analysis is that a broad range of glycan sequences can be immobilized on solid matrices as minute spots and simultaneously interrogated. Different methodologies have emerged for constructing carbohydrate microarrays. The NGL (neoglycolipid)-based oligosaccharide microarray platform is among the relatively few systems that are beyond proof-of-concept and have provided new biological information. In the present article, I dwell, in some detail, on the NGL-based microarray. Highlights are the recent applications of NGL-based microarrays that have contributed to knowledge on the molecular basis of pathogen–host interactions, namely the assignments of the carbohydrate-binding specificities of several key surface-adhesive proteins of Toxoplasma gondii and other apicomplexan parasites, and the elucidation of receptor-binding specificities of the pandemic influenza A (H1N1) 2009 (H1N1pdm) virus compared with seasonal H1N1 virus.

Discovery and design of carbohydrate-based therapeutics

IMPORTANCE OF THE FIELD

Till now, the importance of carbohydrates has been underscored, if compared with the two other major classes of biopolymers such as oligonucleotides and proteins. Recent advances in glycobiology and glycochemistry have imparted a strong interest in the study of this enormous family of biomolecules. Carbohydrates have been shown to be implicated in recognition processes, such as cell-cell adhesion, cell-extracellular matrix adhesion and cell-intruder recognition phenomena. In addition, carbohydrates are recognized as differentiation markers and as antigenic determinants. Due to their relevant biological role, carbohydrates are promising candidates for drug design and disease treatment. However, the growing number of human disorders known as congenital disorders of glycosylation that are being identified as resulting from abnormalities in glycan structures and protein glycosylation strongly indicates that a fast development of glycobiology, glycochemistry and glycomedicine is highly desirable.

AREAS COVERED IN THIS REVIEW

The topics give an overview of different approaches that have been used to date for the design of carbohydrate-based therapeutics; this includes the use of native synthetic carbohydrates, the use of carbohydrate mimics designed on the basis of their native counterpart, the use of carbohydrates as scaffolds and finally the design of glyco-fused therapeutics, one of the most recent approaches. The review covers mainly literature that has appeared since 2000, except for a few papers cited for historical reasons.

WHAT THE READER WILL GAIN

The reader will gain an overview of the current strategies applied to the design of carbohydrate-based therapeutics; in particular, the advantages/disadvantages of different approaches are highlighted. The topic is presented in a general, basic manner and will hopefully be a useful resource for all readers who are not familiar with it. In addition, in order to stress the potentialities of carbohydrates, several examples of carbohydrate-based marketed therapeutics are given.

TAKE HOME MESSAGE

Carbohydrates are a rich class of natural compounds, possessing an intriguing and still not fully understood biological role. This richness offers several strategies for the design of carbohydrate-based therapeutics.

Chemical approaches to glycobiology

Glycans are ubiquitous components of all organisms. Efforts to elucidate glycan function and to understand how they are assembled and disassembled can reap benefits in fields ranging from bioenergy to human medicine. Significant advances in our knowledge of glycan biosynthesis and function are emerging, and chemical biology approaches are accelerating the pace of discovery. Novel strategies for assembling oligosaccharides, glycoproteins, and other glycoconjugates are providing access to critical materials for interrogating glycan function. Chemoselective reactions that facilitate the synthesis of glycan-substituted imaging agents, arrays, and materials are yielding compounds to interrogate and perturb glycan function and dysfunction. To complement these advances, small molecules are being generated that inhibit key glycan-binding proteins or biosynthetic enzymes. These examples illustrate how chemical glycobiology is providing new insight into the functional roles of glycans and new opportunities to interfere with or exploit these roles.

Photo-click immobilization of carbohydrates on polymeric surfaces--a quick method to functionalize surfaces for biomolecular recognition studies

Methods to rapidly functionalize specific polymeric surfaces with alkynes, which can subsequently be linked to azide-containing carbohydrates, are presented. The methods comprise two main concepts: azide photoligation and Cu-catalyzed azide-alkyne cycloaddition. 2-Azidoethyl-functionalized alpha-D-mannopyranoside was synthesized and covalently attached to alkyne-functionalized polymeric surfaces using the techniques. The protein recognition properties of the carbohydrate-presenting surfaces were evaluated using quartz crystal microbalance biosensor instrumentation.