Tris-bipyridine ruthenium complex-based glyco-clusters: amplified luminescence and enhanced lectin affinities

Metal complexes as "protein surface mimetics"

A key challenge in chemical biology is to identify small molecule regulators for every single protein. However, protein surfaces are notoriously difficult to recognise with synthetic molecules, often having large flat surfaces that are poorly matched to traditional small molecules. In the surface mimetic approach, a supramolecular scaffold is used to project recognition groups in such a manner as to make multivalent non-covalent contacts over a large area of protein surface. Metal based supramolecular scaffolds offer unique advantages over conventional organic molecules for protein binding, including greater stereochemical and geometrical diversity conferred through the metal centre and the potential for direct assessment of binding properties and even visualisation in cells without recourse to further functionalisation. This feature article will highlight the current state of the art in protein surface recognition using metal complexes as surface mimetics.

Multivalent Carbohydrate-Lectin Interactions: How Synthetic Chemistry Enables Insights into Nanometric Recognition

Glycan recognition by sugar receptors (lectins) is intimately involved in many aspects of cell physiology. However, the factors explaining the exquisite selectivity of their functional pairing are not yet fully understood. Studies toward this aim will also help appraise the potential for lectin-directed drug design. With the network of adhesion/growth-regulatory galectins as therapeutic targets, the strategy to recruit synthetic chemistry to systematically elucidate structure-activity relationships is outlined, from monovalent compounds to glyco-clusters and glycodendrimers to biomimetic surfaces. The versatility of the synthetic procedures enables to take examining structural and spatial parameters, alone and in combination, to its limits, for example with the aim to produce inhibitors for distinct galectin(s) that exhibit minimal reactivity to other members of this group. Shaping spatial architectures similar to glycoconjugate aggregates, microdomains or vesicles provides attractive tools to disclose the often still hidden significance of nanometric aspects of the different modes of lectin design (sequence divergence at the lectin site, differences of spatial type of lectin-site presentation). Of note, testing the effectors alone or in combination simulating (patho)physiological conditions, is sure to bring about new insights into the cooperation between lectins and the regulation of their activity.

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.

Synthesis and Structure of Novel Sugar-Substituted Bipyridine Complexes of Rhenium and 99m-Technetium

Novel ligands have been obtained from the reaction of 4,4'-dibromomethyl-2,2'-bipyridine with 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosylthiol, 2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosylthiol or 2,3,4,6-tetra-O-acetyl-alpha-D-thioacetylmannopyranoside in which the sugar residues are thioglycosidically linked to the bipyridine in the 4,4'-position. Cleavage of the acetyl groups affords hydrophilic symmetric ligands with free hydroxyl groups. Reaction of the new glycoconjugated ligands (L) with [Re(CO)(5)Cl] yields fluorescent complexes of general formula [Re(L)(CO)(3)Cl], which were characterised by mass spectrometry, elemental analysis and (1)H and (13)C NMR, IR, UV/Vis and fluorescence spectroscopy. These complexes exhibit excellent solubility and stability in organic solvents or water, depending on the residues of the sugar. One complex, namely tricarbonyl-4,4'-bis[(2,3,4,6-tetra-O-acetyl-beta-D-glycopyranosyl)thiomethyl]-2,2'-bipyridinerheniumtricarbonylo chloride, has been characterised by X-ray crystallography. A non-symmetric structure of the complexes could be assigned. Radiolabelling of the unprotected ligands with [(99m)Tc(H(2)O)(3)(CO)(3)](+) affords the corresponding water-soluble technetium complexes (in quantitative yields), which were characterised by their HPLC radiation traces. The formed complexes are stable for several hours in the presence of histidine but show partial ligand-exchange after one day.

Cooperative lectin recognition of periodical glycoclusters along DNA duplexes: alternate hybridization and full hybridization

We describe herein the construction of periodically, spatially controlled glycoclusters along DNA duplexes and their cooperative lectin recognition. Site-specifically alpha-mannosylated oligodeoxynucleotide 20-mer (Man-ODN20) was synthesized via the phosphoramidite solid-phase synthesis. Alternate hybridization of the Man-ODN20 with the half-sliding complementary ODN 20-mer (hscODN20) gave an alternately prolonged Man-cluster Man-ODN20/hscODN20. The binding of the Man-cluster to FITC-labeled ConA lectin showed sigmoidal fluorescence dependency on the concentration of Man-ODN, indicating that some mannose residues along the repeating DNA duplex were cooperatively bound to ConA (apparent affinity constant: K(af)=2.4 x 10(4)M(-1) and Hill coefficient: n=3.5). The duplex of Man-ODN20 with full complementary ODN 20-mer (fcODN20) was little bound to ConA. The binding behavior of Man-ODN20/hscODN20 is compared with that of the alternately prolonged Gal-cluster Gal-ODN20/hscODN20 previously reported. Duplexes 20-mer, 40-mer, and 60-mer presenting one, two, and three periodic galactoses were also prepared by full hybridization of 20-mer beta-galactosylated oligodeoxynucleotide (Gal-ODN20) with the periodically repeating full complementary 20-mer, 40-mer, and 60-mer ODNs. RCA(120) lectin was found to little bind the 20-mer and 40-mer duplexes and to bind weakly and non-cooperatively the 60-mer duplex (K(af)=1.1 x 10(4)M(-1)). The cooperative lectin recognition of these glycoclusters in relation with the degree of association (DA) of ODN and the numbers of glycosides along the DNA duplex is discussed.

Artificial metalloglycoclusters: compact saccharide shell to induce high lectin affinity as well as strong luminescence

Tris-bipyridine ferrous and ruthenium complexes carrying various saccharide appendages have been investigated to develop sensory systems for monitoring saccharide-binding phenomena. Ferrous O-glycoclusters having spacer moieties inserted between saccharide appendages and the complex core showed enhanced affinities to lectins, but ferrous N-glycoclusters, in which the saccharide-appendages are directly linked to the complex core via amide linkage, had low lectin-affinities. Molecular dynamics calculation indicated that the O-glycoclusters have flexible and densely packed saccharide clusters, in contrast to the octahedrally fixed saccharide arrays of N-glycoclusters. Flexibility of saccharide clusters is essential for their enhanced affinity, probably to induce conformational change to fit the recognition sites of lectins. According to these insights, ruthenium O-glycoclusters have been designed as luminescence biosensors. The ruthenium complexes carrying alpha-manno clusters exhibited excellent affinities (IC(min) = 9.0 x 10(-)(8) M) to concanavalin A (ConA). It is suggested from conformational analysis that densely packed mannoclusters can be fit properly to the recognition site of ConA. The binding was enthalpicaly driven (deltaH degrees = -21.8 kcal/mol). This binding behavior is quite similar to that of 1-3/1-6 trimannoside to ConA. They have strongly amplified luminescence (Phi(em) = 0.15), and their luminescence intensities were changed (approximately 40%) upon binding to the specific lectins. The ruthenium glycoclusters can be a suitable sensory system for saccharide-binding phenomena.

Ruthenium complexes carrying a disialo complex-type oligosaccharide: enzymatic synthesis and its application to a luminescent probe to detect influenza viruses

Tris-bipyridine ruthenium-complexes carrying a disialo complex-type oligosaccharide were prepared via a one-pot transglycosylation using endo-glycosidase (Endo M); they bind to type-A influenza viruses with excellent affinity (IC50 = 8.4 microM), and their luminescence intensity is strongly depressed by virus-binding.