The crystal structure of a cyclic glycolipid reveals a carbohydrate-carbohydrate interaction interface

Diversiform and Transformable Glyco-Nanostructures Constructed from Amphiphilic Supramolecular Metallocarbohydrates through Hierarchical Self-Assembly: The Balance between Metallacycles and Saccharides

During the past decade, self-assembly of saccharide-containing amphiphilic molecules toward bioinspired functional glycomaterials has attracted continuous attention due to their various applications in fundamental and practical areas. However, it still remains a great challenge to prepare hierarchical glycoassemblies with controllable and diversiform structures because of the complexity of saccharide structures and carbohydrate-carbohydrate interactions. Herein, through hierarchical self-assembly of modulated amphiphilic supramolecular metallocarbohydrates, we successfully prepared various well-defined glyco-nanostructures in aqueous solution, including vesicles, solid spheres, and opened vesicles depending on the molecular structures of metallocarbohydrates. More attractively, these glyco-nanostructures can further transform into other morphological structures in aqueous solutions such as worm-like micelles, tubules, and even tupanvirus-like vesicles (TVVs). It is worth mentioning that distinctive anisotropic structures including the opened vesicles (OVs) and TVVs were rarely reported in glycobased nano-objects. This intriguing diversity was mainly controlled by the subtle structural trade-off of the two major components of the amphiphiles, i.e., the saccharides and metallacycles. To further understand this precise structural control, molecular simulations provided deep physical insights on the morphology evolution and balancing of the contributions from saccharides and metallacycles. Moreover, the multivalency of glyco-nanostructures with different shapes and sizes was demonstrated by agglutination with a diversity of sugar-binding protein receptors such as the plant lectins Concanavalin A (ConA). This modular synthesis strategy provides access to systematic tuning of molecular structure and self-assembled architecture, which undoubtedly will broaden our horizons on the controllable fabrication of biomimetic glycomaterials such as biological membranes and supramolecular lectin inhibitors.

Nitrogen-containing macrocycles having a carbohydrate scaffold

Nitrogen-containing macrocyclic compounds (amines, amides, and N-heterocyclic derivatives) are important targets in supramolecular chemistry. This chapter discusses the importance of aza-macrocycles in general and, in particular, those receptors containing sugar unit(s). The combination of a carbohydrate scaffold bearing nitrogen-containing functional groups in macrocyclic molecules opens a convenient route to chiral receptors having potentially useful properties. The carbohydrate-based macrocycles discussed are classified into several general groups: (1) aza-crown ethers containing a carbohydrate subunit, (2) cyclic homooligomers from amino sugars, (3) sugar-based cryptands, (4) cyclic peptides containing amino sugar units (including C2- and C3-symmetrical macrocyclic glycopeptides), (5) nitrogen- containing glycophanes, and (6) 1,2,3-triazoles containing synthetic cyclodextrin analogues. The general strategies employed, as well as specific ones leading to such complex derivatives, are surveyed. Applications of such carbohydrate receptors, pointing to their importance as hosts in supramolecular chemistry, are discussed.

The third dimension of reading the sugar code by lectins: design of glycoclusters with cyclic scaffolds as tools with the aim to define correlations between spatial presentation and activity

Coding of biological information is not confined to nucleic acids and proteins. Endowed with the highest level of structural versatility among biomolecules, the glycan chains of cellular glycoconjugates are well-suited to generate molecular messages/signals in a minimum of space. The sequence and shape of oligosaccharides as well as spatial aspects of multivalent presentation are assumed to underlie the natural specificity/selectivity that cellular glycans have for endogenous lectins. In order to eventually unravel structure-activity profiles cyclic scaffolds have been used as platforms to produce glycoclusters and afford valuable tools. Using adhesion/growth-regulatory galectins and the pan-galectin ligand lactose as a model, emerging insights into the potential of cyclodextrins, cyclic peptides, calixarenes and glycophanes for this purpose are presented herein. The systematic testing of lectin panels with spatially defined ligand presentations can be considered as a biomimetic means to help clarify the mechanisms, which lead to the exquisite accuracy at which endogenous lectins select their physiological counterreceptors from the complexity of the cellular glycome.

Towards echinomycin mimetics by grafting quinoxaline residues on glycophane scaffolds

Echinomycin is a natural depsipeptide, which is a bisintercalator, inserting quinoxaline units preferentially adjacent to CG base pairs of DNA. Herein the design and synthesis of echinomycin mimetics based on grafting of two quinoxaline residues onto a macrocyclic scaffold (glycophane) is addressed. Binding of the compounds to calf-thymus DNA was studied using UV-vis and steady state fluorescence spectroscopy, as well as thermal denaturation. An interesting observation was enhancement of fluorescence emission for the peptidomimetics on binding to DNA, which contrasted with observations for echinomycin. Molecular dynamics simulations were exploited to explore in more detail if bis-intercalation to DNA was possible for one of the glycophanes. Bis-intercalating echinomycin complexes with DNA were found to be stable during 20ns simulations at 298K. However, the MD simulations of a glycophane complexed with a DNA octamer displayed very different behaviour to echinomycin and its quinoxaline units were found to rapidly migrate out from the intercalation site. Release of bis-intercalation strain occurred with only one of the quinoxaline chromophores remaining intercalated throughout the simulation. The distance between the quinoxaline residues in the glycophane at the end of the MD simulation was 7.3-7.5Å, whereas in echinomycin, the distance between the residues was ∼11Å, suggesting that longer glycophane scaffolds would be required to generate bis-intercalating echinomycin mimetics.

Functional role of glycosphingolipids and gangliosides in control of cell adhesion, motility, and growth, through glycosynaptic microdomains

At cell surface microdomains, glycosyl epitopes, carried either by glycosphingolipids, N- or O-linked oligosaccharides, are recognized by carbohydrate-binding proteins or complementary carbohydrates. In both cases, the carbohydrate epitopes may be clustered with specific signal transducers, tetraspanins, adhesion receptors or growth factor receptors. Through this framework, carbohydrates can mediate cell signaling leading to changes in cellular phenotype. Microdomains involved in carbohydrate-dependent cell adhesion inducing cell activation, motility, and growth are termed "glycosynapse". In this review a historical synopsis of glycosphingolipids-enriched microdomains study leading to the concept of glycosynapse is presented. Examples of glycosynapse as signaling unit controlling the tumor cell phenotype are discussed in three contexts: (i) Cell-to-cell adhesion mediated by glycosphingolipids-to-glycosphingolipids interaction between interfacing glycosynaptic domains, through head-to-head (trans) carbohydrate-to-carbohydrate interaction. (ii) Functional role of GM3 complexed with tetraspanin CD9, and interaction of such complex with integrins, or with fibroblast growth factor receptor, to control tumor cell phenotype and its reversion to normal cell phenotype. (iii) Inhibition of integrin-dependent Met kinase activity by GM2/tetraspanin CD82 complex in glycosynaptic microdomain. Data present here suggest that the organizational status of glycosynapse strongly affects cellular phenotype influencing tumor cell malignancy.

Synthesis of Structurally Defined Scaffolds for Bivalent Ligand Display Based on Glucuronic Acid Anilides. The Degree of Tertiary Amide Isomerism and Folding Depends on the Configuration of a Glycosyl Azide

[structures: see text] Syntheses and structural analyses of bivalent carbohydrates based on anilides of glucuronic acid are described. Secondary anilides predominantly adopted the Z-anti structure; there is also evidence for population of the Z-syn isomer. Bivalent tertiary anilides displayed two signal sets in their NMR spectra, consistent with the presence of (i) a major isomer where both amides have E configurations (EE) and (ii) a minor isomer where one amide is E and the other Z (EZ). Qualitative NOE/ROE spectroscopic studies in solution support the proposal that the anti conformation is preferred for E amides. The crystal structure of one bivalent tertiary anilide showed E-anti and E-syn structural isomers; intramolecular carbohydrate-carbohydrate stacking was observed and mediated by carbonyl-pyranose, azide-azide, and pyranose-aromatic interactions. The EE to EZ isomer ratio, or the degree of folding, for tertiary amides, was greatest for a bivalent compound containing two alpha-glycosyl azide groups; this was enhanced in water, suggesting that hydrophobic interactions are partially but not wholly responsible. Computational methods predicted azide-aromatic (N...H-C interaction) and azide-azide interactions for folded isomers. The close contact of the azide and aromatic protons (N...H-C interaction) was observed upon examination of the close packing in the crystal structure of a related monomer. It is proposed that the alpha-azide group is more optimally aligned, compared to the beta-azide, to facilitate interaction and minimize the surface area of the hydrophobic groups exposed to water, and this leads to the increased folding. The alkylation of bivalent secondary anilides induces a switch from Z to E amide that alters the scaffold orientation. The synthesis of a bivalent mannoside, based on a secondary anilide scaffold, for investigation of mannose-binding receptor cross-linking and lattice formation is described.