Bifunctional mannoside-glucosinolate glycoconjugates as enzymatically triggered isothiocyanates and FimH ligands.

Glucosinolates are sulfur-containing secondary metabolites found in plants of the Brassicale order. They are precursors of isothiocyanate species, resulting from C-S hydrolysis catalysed by the thioglucohydrolase myrosinase. We describe the synthesis of bifunctional glucosinolate-mannoside glycoconjugates combining both the structural features of a substrate of myrosinase and a ligand of the lectin FimH. We show that these glycoconjugates serve as enzyme substrates and that myrosinase can indeed hydrolyze the glucosinolate moiety with affinities (KM, Vmax) comparable to the natural substrates glucomoringin and sinigrin. This enzymatic hydrolysis of the thioglycosidic bond led to the efficient formation of an isothiocyanate which was assessed by the formation of the corresponding dithiocarbamate derivatives. Finally, we show that our synthetic bifunctional glycoconjugates also serve as FimH ligands where the glucosinolate moiety does not hamper the interaction with the lectin. Our findings set the stage for an original bioconjugation tool, allowing for myrosinase-triggered specific labelling of lectins using glucosinolate glycoconjugates as non-toxic, water soluble isothiocyanate precursors.

Switching first contact: photocontrol of E. coli adhesion to human cells.

We have shown previously that carbohydrate-specific bacterial adhesion to a non-physiological surface can be photocontrolled by reversible E/Z isomerisation using azobenzene-functionalised sugars. Here, this approach is applied to the surface of human cells. We show not only that bacterial adhesion to the azobenzene glycoside-modified cell surface is higher in the E than in the Z state, but add data about the specific modulation of the effect.

Trehalose-based octopus glycosides for the synthesis of carbohydrate-centered PAMAM dendrimers and thiourea-bridged glycoclusters.

[structure: see text] The nonreducing disaccharide trehalose was modified into an octa-amino-functionalized core molecule to serve in the synthesis of carbohydrate-centered PAMAM glycodendrimers and thiourea-bridged glycoclusters.

A modular approach for the synthesis of oligosaccharide mimetics.

To allow modular syntheses of oligosaccharide mimetics, the potentially trifunctional glycoside 7 was synthesized and used as a scaffold for the successive attachment of further monosaccharide derivatives to lead to the di-, tri-, and tetrasaccharide mimetics 11, 13, and 16. This synthetic strategy can also be used to prepare oligovalent neoglycoconjugates, e.g., 18, which contains nine mannosyl units. The applied concept implies numerous options for the synthesis of a wide array of structural variations, biolabeling, or solid-phase synthesis as well as combinatorial approaches.

Binding inhibition of type 1 fimbriae to human granulocytes: a flow cytometric inhibition assay using trivalent cluster mannosides.

The binding of type 1 fimbriae from Escherichia coli to vital human neutrophilic granulocytes was inhibited by synthetic trivalent cluster mannosides. Binding of type 1 fimbriae was measured in a flow cytometric assay. Based on the molarity of mannosyl residues, the clusters exceed the inhibitory potency of methyl alpha-D-mannoside by a factor of more than 1,000 and the inhibitory potency of p-nitrophenyl alpha-D-mannoside by a factor of more than 10. The inhibition studies indicate that the trivalent cluster mannosides are very potent inhibitors of the binding of type 1 fimbriae to human neutrophilic granulocytes. Based on their defined structure, cluster mannosides are well suited for characterizing the molecular interactions of mannose-sensitive fimbriae with their cell membrane receptors.

Complex encounters at the macrophage-mycobacterium interface: studies on the role of the mannose receptor and CD14 in experimental infection models with Mycobacterium avium.

The initial interactions between mycobacterial cell wall components and receptor structures on the surface of macrophages may be critical in determining the outcome of infection. They may trigger the ingestion and digestion of microorganisms, but they may also promote the intracellular persistence and growth of mycobacteria. Using Mycobacterium avium as a model system, three approaches of different complexities were used to analyse some structural features and some functional consequences of M. avium interacting with the macrophage mannose receptor or CD14, a pattern recognition receptor. Binding specificities of a recombinant, truncated extracellular portion of the mannose receptor were assayed in a novel ELISA-formatted system using viable M. avium cells as ligands. Infection with M. avium strains differing in their virulence were performed in murine bone marrow-derived macrophages and in mice with a targeted deletion of the CD14 gene. These parallel and converging approaches not only help define the molecular basis for understanding early events in the pathogenesis of mycobacterial infections, but are also necessary to ultimately determine the relevance of in vitro findings in the context of actual manifestations of disease in vivo.

Synthesis of carbohydrate-centered oligosaccharide mimetics equipped with a functionalized tether.

Synthetic glycoclusters have gained substantial attention as mimetics of multivalent glycoconjugates. For their proposed glycobiological applications, it is advantageous to incorporate a functionalized tether into the clusters, which allows coupling to solid supports and other molecules such as reporter groups or even bioactive molecules. We herein report the use of carbohydrates as oligofunctional scaffolds for the synthesis of tethered cluster mannosides. Glycocluster 11 was prepared following two different pathways, starting either from glucose or the nonreducing disaccharide trehalose. The oligo alcohols 5 and 14 served as acceptors in the subsequent oligo-mannosylation reaction, in which three main problems were overcome: (i) incomplete glycosylation, (ii) cleavage of the core-glycoside, and (iii) ortho ester formation. Optimum conditions for the glycosylation were identified utilizing an advanced MALDI-TOF protocol.

Structure analysis of trivalent glycoclusters by post-source decay matrix-assisted laser desorption/ionization mass spectrometry.

Post-source decay (PSD) matrix-assisted laser desorption/ionization time-of-flight mass spectra were found to be useful for the structural elucidation of a series of tris [2-(glycosylthiourylene)ethyl]amines. The reported fragmentation behaviours of [M + H]+, [M + Na]+ and [M - H]- ions differ from each other significantly; however, they can be compared to tree pruning in every case. Whereas detailed structural information on unprotected glycoclusters is obtained from all PSD experiments, only the positive-ion mode can be used to gain relevant information about the acetylated glycoclusters.

GlcNAc-terminated glycodendrimers form defined precipitates with the soluble dimeric receptor of rat natural killer cells, sNKR-P1A.

Synthetic GlcNAc-terminated thiourea-bridged glycoclusters were found to be potent inhibitors of binding of the soluble dimeric receptor of rat natural killer cells, sNKR-P1A protein, to its high affinity ligand. Moreover, we have shown here that characteristic precipitation curves can be recorded upon mixing of the GlcNAc glycoclusters with sNKR-P1A. For the GlcNAc8 glycocluster the precipitation curve is biphasic, with high affinity and low affinity precipitates differing in their sensitivity towards GlcNAc-mediated inhibition of precipitation. Quantitative analyses of the precipitates indicate the occurrence of a single sugar binding site per sNKR-P1A subunit, and lead to a model of the most possible spatial arrangements of the glycocluster-receptor lattices. These results provide new tools for further studies on carbohydrate recognition by NKR-P1A.

Inhibition of the type 1 fimbriae-mediated adhesion of Escherichia coli to erythrocytes by multiantennary alpha-mannosyl clusters: the effect of multivalency.

Alpha-mannosyl glycoclusters and glycodendrimers were tested as multivalent inhibitors of the type 1 (mannose-specific) fimbriae of a recombinant E. coli HB101 strain. Inhibition of haemagglutination of guinea pig erythrocytes was determined on microtiter plates. The effect of multivalency is pronounced for up to three mannosyl residues in the molecule, whereas larger derivatives do not have an appreciable effect on binding to the fimbrial carbohydrate binding domain. The best glycoclusters tested reach the binding potency of the known potent inhibitor pNPMan (3). The results support the idea of a monovalent recognition site at the adhesive protein FimH, which might best accommodate molecules with the size of a trisaccharide or those which expose up to three alpha-mannosyl residues, such as the glycocluster 8. The results obtained with the thiourea-bridged alpha-mannosyl clusters, possessing defined sugar valencies, facilitate the development of high affinity inhibitors of the fimbrial lectin on type 1 fimbriae.

Synthetic approaches to 2-deoxyglycosyl phosphates.

By the use of the N-iodosuccinimide (NIS)-procedure, various glycals could be converted into 2-deoxy-2-iodoglycosyl phosphates. Treatment of glycals 1 and 7 with NIS and dibenzyl phosphate gave the corresponding alpha-1,2-trans-diaxial 2-deoxy-2-iodoglycosyl phosphates 2 and 8 as the main products. The beta-1,2-trans-diequatorial compounds 3 and 9 were isolated as by-products. Analogous reaction of glycals 4 and 10 gave the corresponding 2-deoxy-2-iodoglycosyl phosphates 5, 6, 11, and 12 as crude products, which were characterized by 1H NMR spectroscopy. Classical phosphorylation of 2-deoxy-glycosyl chlorides 14 and 16 with silver dibenzyl phosphate gave the corresponding dibenzyl 2-deoxy-alpha-glycosyl phosphates 15 and 17. Alternatively, glycosylation of tri-O-acetyl-D-glucal using dibenzyl phosphate and triphenylphosphine hydrobromide afforded 15 in lower yield. The application of S-(2-deoxyglycosyl) phosphorodithioates as glycosyl donors provided the most convenient way to dibenzyl 2-deoxyglycosyl phosphates. The alpha-glycosyl phosphates 15, 20, and 22 could be synthesized by reaction of the 2-deoxyglycosyl dithiophosphates 18, 19, and 21 with dibenzyl phosphate and activation by iodonium di-sym-collidine perchlorate. Similarly, the 2,6-dideoxyglycosyl dithiophosphates 23 and 25 gave the 2,6-dideoxy phosphates 24 and 26; however, the isolation of these labile compounds could not be effected.

Synthesis and properties of sulfated alkyl glycosides.

Alkyl glycosides were sulfated with sulfur trioxide-pyridine. Dodecyl alpha- and beta-D-glucopyranoside gave the corresponding 6-sulfates in 75 and 51% yields, respectively. Separation from polysulfated compounds was carried out by reversed-phase HPLC. Tetradecyl beta-maltopyranoside (16) gave a 88: 12 mixture of 6'- and 6-sulfates. The sulfated compounds were characterized by 1H-, 13C-, and 2-dimensional NMR spectroscopy. Surfactant and thermotropic liquid-crystalline properties of the sugar derivatives were examined. All of the glycosides show smectic phases (SA), and the clearing points rise by introduction of sulfate groups. Even glycosides having no unprotected hydroxy groups may show SA-phases when bearing sulfate groups. The mesomorphic properties cannot be explained by formation of distinct aggregates, but rather must be interpreted by an effective intramolecular contrast.