Synthesis of divalent α-D-mannopyranosylated clusters having enhanced binding affinities towards concanavalin A and pea lectins

A review of Vicieae lectins studies: End of the book or a story in the writing?

Vicieae tribe, Leguminosae family (Fabaceae), has been extensively studied. In particular, the study of lectins. The purification, physicochemical and structural characterizations of the various purified lectins and the analysis of their relevant biological activities are ongoing. In this review, several works already published about Vicieae lectins are addressed. Initially, we presented the purification protocols and the physicochemical aspects, such as specificity for carbohydrates, optimal activity in the face of variations in temperature and pH, as well metals-dependence. Following, structural characterization studies are highlighted and, finally, various biological activities already reported are summarized. Studies on lectins in almost all genera (Lathyrus, Lens, Pisum and Vicia) are considered, with the exception of Vavilovia which studies of lectins have not yet been reported. Like other leguminous lectins, Vicieae lectins present heterogeneous profiles of agglutination profiles for erythrocytes and other cells of the immune system, and glycoproteins. Most Vicieae lectins consist of two subunits, α and β, products of a single precursor protein derived from a single gene. The differences between the isoforms result from varying degrees of proteolytic processing. Along with the identification of these molecules and their characteristics, biological activities become very relevant and robust for both basic and applied research.

Effect of Dendrimer Generation and Aglyconic Linkers on the Binding Properties of Mannosylated Dendrimers Prepared by a Combined Convergent and Onion Peel Approach

An efficient study of carbohydrate-protein interactions was achieved using multivalent glycodendrimer library. Different dendrimers with varied peripheral sugar densities and linkers provided an arsenal of potential novel therapeutic agents that could be useful for better specific action and greater binding affinities against their cognate protein receptors. Highly effective click chemistry represents the basic method used for the synthesis of mannosylated dendrimers. To this end, we used propargylated scaffolds of varying sugar densities ranging from 2 to 18 for the attachment of azido mannopyranoside derivatives using copper catalyzed click cycloaddition. Mannopyranosides with short and pegylated aglycones were used to evaluate their effects on the kinetics of binding. The mannosylated dendrons were built using varied scaffolds toward the accelerated and combined "onion peel" strategy These carbohydrates have been designed to fight E. coli urinary infections, by inhibiting the formation of bacterial biofilms, thus neutralizing the adhesion of FimH type 1 lectin present at the tip of their fimbriae against the natural multiantennary oligomannosides of uroplakin 1a receptors expressed on uroepithelial tissues. Preliminary DLS studies of the mannosylated dendrimers to cross- link the leguminous lectin Con A used as a model showed their high potency as candidates to fight the E. coli adhesion and biofilm formation.

Electrochemical Impedance Spectroscopy Study of Concanavalin A Binding to Self-Assembled Monolayers of Mannosides on Gold Wire Electrodes

The interactions of the lectin Concanavalin A (Con A) with self-assembled monolayers (SAMs) of thiolated mono-, di-, and tri-mannosides were studied on the surface of gold wires using electrochemical impedance spectroscopy (EIS). The SAMs of mannosides were prepared either pure or along with thiolated triethylene glycol (TEG) at different molar ratios (1:1, 1:2, 1:4, 1:9, and 1:19) to better understand and optimize the interaction conditions. The charge-transfer resistance of the [Fe(CN)₆]^3-/^4- redox probe was compared before and after the interaction at different concentrations of Con A to determine the equilibrium dissociation constant (K_d) and limit of detection (LOD). Values of K_d were found in the nanomolar range showing multivalent interactions between mannosides and Con A, and LOD was found ranging from 4-13 nM depending on the type of mannoside SAM used. Analysis using the Hill equation suggests negative cooperativity in the binding behavior. Peanut agglutinin was used as a negative control, and cyclic voltammetry was used to further support the experiments. We have found that neither the pure nor the widely dispersed monolayers of mannosides provide the conditions for optimal binding of Con A. The binding of Con A to these SAMs is sensitive to the molar ratio of the mannoside used to prepare the SAM and to the structure of the mannoside. A simple cleaning method has also been shown to regenerate the used gold wire electrodes. The results from these experiments contribute to the development of simple, cheap, selective, and sensitive EIS-based bioassays, especially for lectin-carbohydrate interactions.

Multivalent glycoconjugate syntheses and applications using aromatic scaffolds

Glycan-protein interactions are of utmost importance in several biological phenomena. Although the variety of carbohydrate residues in mammalian cells is limited to less than a dozen different sugars, their spatial topographical presentation in what is now associated as the "glycocodes" provides the fundamental keys for specific and high affinity "lock-in" recognition events associated with a wide range of pathologies. Toward deciphering our understanding of these glycocodes, chemists have developed new creative tools that included dendrimer chemistry in order to provide monodisperse multivalent glycoconjugates. This review provides a survey of the numerous aromatic architectures generated for the multivalent presentation of relevant carbohydrates using covalent attachment or supramolecular self-assemblies. The basic concepts toward their controlled syntheses will be described using modern synthetic procedures with a particular emphasis on powerful organometallic methodologies. The large variety of dendritic aromatic scaffolds, together with a brief survey of their unique biophysical and biological properties will be critically reviewed. The distinctiveness of the resulting multivalent glycoarchitectures, encompassing glycoclusters, glycodendrimers and molecularly defined self-assemblies, in forming well organized cross-linked lattices with multivalent carbohydrate binding proteins (lectins) together with their photophysical, medical, and imaging properties will also be briefly highlighted. The topic will be presented in increasing order of aromatic backbone complexities and will end with fullerenes together with self-assembled nanostructures, thus complementing the various scaffolds described in this special thematic issue dedicated to multivalent glycoscience.

Design and creativity in synthesis of multivalent neoglycoconjugates

From the authors' opinion, this chapter constitutes a modest extension of the seminal and inspiring contribution of Stowell and Lee on neoglycoconjugates published in this series [C. P. Stowell and Y. C. Lee, Adv. Carbohydr. Chem. Biochem., 37 (1980) 225-281]. The outstanding progresses achieved since then in the field of the "glycoside cluster effect" has witnessed considerable creativity in the design and synthetic strategies toward a vast array of novel carbohydrate structures and reflects the dynamic activity in the field even since the recent chapter by the Nicotra group in this series [F. Nicotra, L. Cipolla, F. Peri, B. La Ferla, and C. Radaelli, Adv. Carbohydr. Chem. Biochem., 61 (2007) 353-398]. Beyond the more classical neoglycoproteins and glycopolymers (not covered in this work) a wide range of unprecedented and often artistically beautiful multivalent and monodisperse nanostructures, termed glycodendrimers for the first time in 1993, has been created. This chapter briefly surveys the concept of multivalency involved in carbohydrate-protein interactions. The topic is also discussed in regard to recent steps undertaken in glycobiology toward identification of lead candidates using microarrays and modern analytical tools. A systematic description of glycocluster and glycodendrimer synthesis follows, starting from the simplest architectures and ending in the most complex ones. Presentation of multivalent glycostructures of intermediate size and comprising, calix[n]arene, porphyrin, cyclodextrin, peptide, and carbohydrate scaffolds, has also been intercalated to better appreciate the growing synthetic complexity involved. A subsection describing novel all-carbon-based glycoconjugates such as fullerenes and carbon nanotubes is inserted, followed by a promising strategy involving dendrons self-assembling around metal chelates. The chapter then ends with those glycodendrimers that have been prepared using commercially available dendrimers possessing varied functionalities, or systematically synthesized using either divergent or convergent strategies.

Mannosylated G(0) Dendrimers with Nanomolar Affinities toEscherichia coli FimH

Pentaerythritol and bis-pentaerythritol scaffolds were used for the preparation of first generation glycodendrimers bearing aryl alpha-D-mannopyranoside residues assembled using single-step Sonogashira and click chemistry. The carbohydrate precursors were built with either para-iodophenyl, propargyl, or 2-azidoethyl aglycones whereas the pentaerythritol moieties were built with terminal azide or propargyl groups, respectively. Cross-linking abilities of this series of glycodendrimers were first evaluated with the lectin from Canavalia ensiformis (Concanavalin A). Surface plasmon resonance measurements showed these two families of mannosylated clusters as the best ligands known to date toward Escherichia coli K12 FimH with subnanomolar affinities. Tetramer 4 had a K(d) of 0.45 nM. These clusters were 1000 times more potent than mannose for their capacity to inhibit the binding of E. coli to erythrocytes in vitro.

Solid-phase synthesis for the identification of high-affinity bivalent lectin ligands

The development of carbohydrate-based therapeutics has been frustrated by the low affinities that characterize protein-carbohydrate complexation. Because of the oligomeric nature of most lectins, the use of multivalency may offer a successful strategy for the creation of high-affinity ligands. The solid-phase evaluation of libraries of peptide-linked multivalent ligands facilitates rapid examination of a large fraction of linker structure space. If such solid-phase assays are to replicate solution binding behavior, the potential for intermolecular bivalent binding on bead surfaces must be eliminated. Here we report the solid-phase synthesis and analysis of peptide-linked, spatially segregated mono- and bivalent ligands for the legume lectin concanavalin A. Bead shaving protocols were used for the creation of beads displaying spatially segregated binding sequences on the surface of Tentagel resins. The same ligands were also synthesized on PEGA resin to determine the effect of ligand presentation on solid-phase binding. While we set out to determine the lower limit of assay sensitivity, the unexpected observation that intermolecular bivalent ligand binding is enhanced for bivalent ligands relative to monovalent ligands allowed direct observation of the level of surface blocking required to prevent intermolecular bivalent ligand binding. For a protein with binding sites separated by 65 A, approximately 99.9% of Tentagel(1) surface sites and 99.99% of the total sites on a PEGA bead must be blocked to prevent intermolecular bivalent binding. We also report agglutination and calorimetric solution-phase binding studies of mono- and bivalent peptide-linked ligands.

A general synthesis of structurally diverse building blocks for preparing analogues of C-linked antifreeze glycoproteins

A synthetic methodology to afford unusual glycoconjugate building blocks useful for the solid-phase synthesis of C-linked antifreeze glycoprotein (AFGP) analogues is described. Such compounds are urgently required in order to elucidate the molecular mechanism by which AFGPs function. All reactions are general in nature and accommodate structural variation in the carbohydrate moiety, polypeptide backbone, and amino acid side chain.

Design and synthesis of novel glycopolythiophene assemblies for colorimetric detection of influenza virus and E. coli

A novel family of glycopolythiophenes containing sialic acid or mannose ligands were prepared and evaluated for their ability to bind lectins, virus, and bacteria. For the set of glycopolythiophenes studied, the spacer-length between the polymer backbone and the ligand was varied to optimize binding interactions. The glycopolymers were blue-shifted (absorbance of ca. 400 nm) relative to the corresponding homo-polythiophenes (absorbance ca. 440 nm), suggesting a twisted conformation for the glycopolymers. The altered conformation is likely due to electrostatic or H-bonding interactions between the polymer chains, arising from the carbohydrate ligand. Further conformational changes in the polythiophene backbone were detected by the binding of specific receptors; lectins (wheat germ agglutinin, concanavalin A), Influenza virus, and Escherichia coli. The binding interactions result in an unusual red-shift in the visible absorption of the polymer backbone, suggesting a lengthening of the effective conjugated length upon interaction of the ligand with its congnate receptor. These conjugated glycopolymeric systems offer a potentially new platform for the detection of molecular binding interactions.