Platform Synthetic Lectins for Divalent Carbohydrate Recognition in Water

Probing the Importance of Host Symmetry on Carbohydrate Recognition

The design of molecular cages with low symmetry could allow for more specific tuning of their properties and better mimic the unsymmetrical and complex environment of protein pockets. However, the added value of lowering symmetry of molecular receptors has been rarely demonstrated. Herein, C₃ - and C₁ -symmetrical cages, presenting the same recognition sites, have been synthesized and investigated as hosts for carbohydrate recognition. Structurally related derivatives of glucose, galactose and mannose were found to have greater affinity to the receptor with the lowest symmetry than to their C₃ -symmetrical analogue. According to the host cavity modelling, the C₁ symmetry receptor exhibits a wider opening than its C₃ -symmetrical counterpart, providing easier access and thus promoting guest proximity to binding sites. Moreover, our results show the high stereo- and substrate selectivity of the C₁ symmetry cage with respect to its C₃ counterpart in the recognition of sugars.

Biomimetic Tweezers for N-Glycans: Selective Recognition of the Core GlcNAc2 Disaccharide of the Sialylglycopeptide SGP

In recent years, glycomics have shown how pervasive the role of carbohydrates in biological systems is and how chemical tools are essential to investigate glycan function and modulate carbohydrate-mediated processes. Biomimetic receptors for carbohydrates can carry out this task but, although significant affinities and selectivities toward simple saccharides have been achieved, targeting complex glycoconjugates remains a goal yet unattained. In this work we report the unprecedented recognition of a complex biantennary sialylglycopeptide (SGP) by a tweezers-shaped biomimetic receptor, which selectively binds to the core GlcNAc₂ disaccharide of the N-glycan with an affinity of 170 μM. Because of the simple structure and the remarkable binding ability, this biomimetic receptor can represent a versatile tool for glycoscience, opening the way to useful applications.

Regiochemical Effects on the Carbohydrate Binding and Selectivity of Flexible Synthetic Carbohydrate Receptors with Indole and Quinoline Heterocyclic Groups

Synthetic carbohydrate receptors (SCRs) that bind cell-surface carbohydrates could be used for disease detection, drug-delivery, and therapeutics, or for the site-selective modification of complex carbohydrates but their potential has not been realized because of remaining challenges associated with binding affinity and substrate selectivity. We have reported recently a series of flexible SCRs based upon a biaryl core with four pendant heterocyclic groups that bind glycans selectively through noncovalent interactions. Here we continue to explore the role of heterocycles on substrate selectivity by expanding our library to include a series of indole and quinoline heterocycles that vary in their regiochemistry of attachment to the biaryl core. The binding of these SCRs to a series of biologically-relevant carbohydrates was studied by 1H NMR titrations in CD2Cl2 and density-functional theory calculations. We find SCR030, SCR034 and SCR037 are selective, SCR031, SCR032, and SCR039 are strong binders, and SCR033, SCR035, SCR036, and SCR038 are promiscuous and bind weakly. Computational analysis reveals the importance of C-H⋯π and H-bonding interactions in defining the binding properties of these new receptors. By combining these data with those obtained from our previous studies on this class of flexible SCRs, we develop a series of design rules that account for the binding of all SCRs of this class and anticipate the binding of future, not-yet imagined tetrapodal SCRs.

A Water Soluble Pd2 L4 Cage for Selective Binding of Neu5Ac

The sialic acid N-acetylneuraminic acid (Neu5Ac) and its derivatives are involved in many biological processes including cell-cell recognition and infection by influenza. Molecules that can recognize Neu5Ac might thus be exploited to intervene in or monitor such events. A key obstacle in this development is the sparse availability of easily prepared molecules that bind to this carbohydrate in its natural solvent; water. Here, we report that the carbohydrate binding pocket of an organic soluble [Pd2 L4 ]4+ cage could be equipped with guanidinium-terminating dendrons to give the water soluble [Pd2 L4 ][NO3 ]16 cage 7. It was shown by means of NMR spectroscopy that 7 binds selectively to anionic monosaccharides and strongest to Neu5Ac with Ka =24 M-1 . The cage had low to no affinity for the thirteen neutral saccharides studied. Aided by molecular modeling, the selectivity for anionic carbohydrates such as Neu5Ac could be rationalized by the presence of charge assisted hydrogen bonds and/or the presence of a salt bridge with a guanidinium solubilizing arm of 7. Establishing that a simple coordination cage such as 7 can already selectively bind to Neu5Ac in water paves the way to improve the stability, affinity and/or selectivity properties of M2 L4 cages for carbohydrates and other small molecules.

Mimicking Biological Recognition: Lessons in Binding Hydrophilic Guests in Water

Selective molecular recognition of hydrophilic guests in water plays a fundamental role in a vast number of biological processes, but synthetic mimicry of biomolecular recognition in water still proves challenging both in terms of achieving comparable affinities and selectivities. This Review highlights strategies that have been developed in the field of supramolecular chemistry to selectively and non-covalently bind three classes of biologically relevant molecules: nucleotides, carbohydrates, and amino acids. As several groups have systematically modified receptors for a specific guest, an evolutionary perspective is also provided in some cases. Trends in the most effective binding forces for each class are described, providing insight into selectivity and potential directions for future work.

Molecular Interrogation to Crack the Case of O-GlcNAc

The O-linked β-N-acetylglucosamine (O-GlcNAc) modification, termed O-GlcNAcylation, is an essential and dynamic post-translational modification in cells. O-GlcNAc transferase (OGT) installs this modification on serine and threonine residues, whereas O-GlcNAcase (OGA) hydrolyzes it. O-GlcNAc modifications are found on thousands of intracellular proteins involved in diverse biological processes. Dysregulation of O-GlcNAcylation and O-GlcNAc cycling enzymes has been detected in many diseases, including cancer, diabetes, cardiovascular and neurodegenerative diseases. Here, recent advances in the development of molecular tools to investigate OGT and OGA functions and substrate recognition are discussed. New chemical approaches to study O-GlcNAc dynamics and its potential roles in the immune system are also highlighted. It is hoped that this minireview will encourage more research in these areas to advance the understanding of O-GlcNAc in biology and diseases.

A Potent Mimetic of the Siglec-8 Ligand 6'-Sulfo-Sialyl Lewisx

Siglecs are members of the immunoglobulin gene family containing sialic acid binding N-terminal domains. Among them, Siglec-8 is expressed on various cell types of the immune system such as eosinophils, mast cells and weakly on basophils. Cross-linking of Siglec-8 with monoclonal antibodies triggers apoptosis in eosinophils and inhibits degranulation of mast cells, making Siglec-8 a promising target for the treatment of eosinophil- and mast cell-associated diseases such as asthma. The tetrasaccharide 6'-sulfo-sialyl Lewis^x has been identified as a specific Siglec-8 ligand in glycan array screening. Here, we describe an extended study enlightening the pharmacophores of 6'-sulfo-sialyl Lewis^x and the successful development of a high-affinity mimetic. Retaining the neuraminic acid core, the introduction of a carbocyclic mimetic of the Gal moiety and a sulfonamide substituent in the 9-position gave a 20-fold improved binding affinity. Finally, the residence time, which usually is the Achilles tendon of carbohydrate/lectin interactions, could be improved.

Toward Easy-to-Assemble, Large-Area Smart Windows: All-in-One Cross-Linked Electrochromic Material and Device

Conventional electrochromic devices with a sandwich structure consist of multiple interfaces, which enhance electron trapping on the interfaces. Furthermore, crack generation in the electrochromic layer is inevitable due to repeated ion insertion and extraction during the service process. These problems increase the fabrication complexity and lead to poor performance and stability, which are severely limiting and prime concerns for the future development of the electrochromism field. Here, a strategy of synthesizing an all-in-one self-healing electrochromic material, TAFPy-MA, is presented, which has been utilized for the fabrication of a high-reliability, large-scale, and easy-assembly smart electrochromic window. The all-in-one self-healing electrochromic material can undergo in situ redox reactions with Li⁺ ions to reduce resistance transfer and avoid interface obstacles, and the reversible Diels-Alder cross-linking network structure can heal the cracks to improve the reliability of the electrochromic layer. High ion diffusivity (1.13 × 10^-5 cm² s^-1), rapid color switching (3.9/3.7 s), high coloration efficiency (413 cm² C^-1), excellent stability (sustains 88.7% after 1000 cycles) and reliability (crack can be healed in 110 s), and large-scale smart windows (30 × 35 cm²) are achieved using the all-in-one electrochromic material, which exhibits fascinating and promising features for a wide range of applications in buildings, airplanes, etc.

Host-Guest Recognition and Fluorescence of a Tetraphenylethene-Based Octacationic Cage

We report the synthesis and characterization of a three-dimensional tetraphenylethene-based octacationic cage that shows host-guest recognition of polycyclic aromatic hydrocarbons (e.g. coronene) in organic media and water-soluble dyes (e.g. sulforhodamine 101) in aqueous media through CH⋅⋅⋅π, π-π, and/or electrostatic interactions. The cage⊃coronene exhibits a cuboid internal cavity with a size of approximately 17.2×11.0×6.96 ų and a "hamburger"-type host-guest complex, which is hierarchically stacked into 1D nanotubes and a 3D supramolecular framework. The free cage possesses a similar cavity in the crystalline state. Furthermore, a host-guest complex formed between the octacationic cage and sulforhodamine 101 had a higher absolute quantum yield (Φ_F =28.5 %), larger excitation-emission gap (Δλ_ex-em =211 nm), and longer emission lifetime (τ=7.0 ns) as compared to the guest (Φ_F =10.5 %; Δλ_ex-em =11 nm; τ=4.9 ns), and purer emission (Δλ_FWHM =38 nm) as compared to the host (Δλ_FWHM =111 nm).

π-Hole Interactions Involving Nitro Aromatic Ligands in Protein Structures

Studying noncanonical intermolecular interactions between a ligand and a protein constitutes an emerging research field. Identifying synthetically accessible molecular fragments that can engage in intermolecular interactions is a key objective in this area. Here, it is shown that so-called "π-hole interactions" are present between the nitro moiety in nitro aromatic ligands and lone pairs within protein structures (water and protein carbonyls and sulfurs). Ample structural evidence was found in a PDB analysis and computations reveal interaction energies of about -5 kcal mol-1 for ligand-protein π-hole interactions. Several examples are highlighted for which a π-hole interaction is implicated in the superior binding affinity or inhibition of a nitro aromatic ligand versus a similar non-nitro analogue. The discovery that π-hole interactions with nitro aromatics are significant within protein structures parallels the finding that halogen bonds are biologically relevant. This has implications for the interpretation of ligand-protein complexation phenomena, for example, involving the more than 50 approved drugs that contain a nitro aromatic moiety.

Transformation of a [4+6] Salicylbisimine Cage to Chemically Robust Amide Cages

In recent years, interest in shape-persistent organic cage compounds has steadily increased, not least because dynamic covalent bond formation enables such structures to be made in high to excellent yields. One often used type of dynamic bond formation is the generation of an imine bond from an aldehyde and an amine. Although the reversibility of the imine bond formation is advantageous for high yields, it is disadvantageous for the chemical stability of the compounds. Amide bonds are, in contrast to imine bonds much more robust. Shape-persistent amide cages have so far been made by irreversible amide bond formations in multiple steps, very often accompanied by low yields. Here, we present an approach to shape-persistent amide cages by exploiting a high-yielding reversible cage formation in the first step, and a Pinnick oxidation as a key step to access the amide cages in just three steps. These chemically robust amide cages can be further transformed by bromination or nitration to allow post-functionalization in high yields. The impact of the substituents on the gas sorption behavior was also investigated.