Novel porphyrin-cryptand cyclic systems: receptors for saccharide recognition in water

Manifesting Subtle Differences of Neutral Hydrophilic Guest Isomers in a Molecular Container by Phase Transfer

Achieving strong host-guest interactions between synthetic hosts and hydrophilic guests in solution is challenging because solvation effects overwhelm other effects. To resolve this issue, we transferred complexes of cucurbit[7]uril (CB[7]) and monosaccharides to the gas phase and report here their intrinsic host-guest chemistry in the absence of solvation effects. It was observed that effective host-guest interactions in the gas phase mediated by ammonium cations allow the differentiation of the monosaccharide isomers in complex with CB[7] upon vibrational excitation. The potential of the unique observation was extended to a quantitative supramolecular analytical method for the monosaccharide guests. The combination of host-guest chemistry and phase transfer presented in this study is an effective approach to overcome current limitations in supramolecular chemistry.

Binding or aggregation? Hazards of interpretation in studies of molecular recognition by porphyrins in water

Reports have suggested that polar porphyrins such as tetraphenylporphine tetrasulfonate (TPPS) can serve as carbohydrate receptors in water. Here we find that TPPS shows changes in UV-visible absorption when treated with glucose, but that these are best explained by altered aggregation states and not by formation of a closely-bound complex.

Sugar recognition: designing artificial receptors for applications in biological diagnostics and imaging

At the cellular level, numerous processes ranging from protein folding to disease development are mediated by a sugar-based molecular information system that is much less well known than its DNA- or protein-based counterparts. The subtle structural diversity of such sugar tags nevertheless offers an excellent, if challenging, opportunity to design receptors for the selective recognition of biorelevant sugars. Over the past 40 years, growing interest in the field of sugar recognition has led to the development of several promising artificial receptors, which could soon find widespread use in medical diagnostics and cell imaging.

Cucurbit[7]uril: a high-affinity host for encapsulation of amino saccharides and supramolecular stabilization of their α-anomers in water

Cucurbit[7]uril (CB[7]), an uncharged and water-soluble macrocyclic host, binds protonated amino saccharides (D-glucosamine, D-galactosamine, D-mannosamine and 6-amino-6-deoxy-D-glucose) with excellent affinity (Ka =10(3) to 10(4)  M(-1) ). The host-guest complexation was confirmed by NMR spectroscopy, isothermal titration calorimetry (ITC), and MALDI-TOF mass spectral analyses. NMR analyses revealed that the amino saccharides, except D-mannosamine, are bound as α-anomers within the CB[7] cavity. ITC analyses reveal that CB[7] has excellent affinity for binding amino saccharides in water. The maximum affinity was observed for D-galactosamine hydrochloride (Ka =1.6×10(4)  M(-1) ). Such a strong affinity for any saccharide in water using a synthetic receptor is unprecedented, as is the supramolecular stabilization of an α-anomer by the host.

Selective binding of D2h-symmetrical, acetylene-linked pyridine/pyridone macrocycles to maltoside

A macrocyclic host molecule having pyridine-pyridone-pyridine modules for saccharide recognition was prepared by Cu(II)-mediated oxidative homocoupling of a tandem diethynyl precursor. In CH(2)Cl(2), the host molecule associated with dodecyl β-maltoside much more strongly (K(a) = 1.4 × 10(6) M(-1)) than with octyl monohexosides (K(a) = ca. 2 × 10(3) to 1 × 10(4) M(-1)), accompanied with induced CDs. An all-pyridine macrocyclic host was also studied, and its binding strength with saccharides was weaker than that for the pyridine-pyridone-pyridine host.

Carbohydrate recognition by boronolectins, small molecules, and lectins

Carbohydrates are known to mediate a large number of biological and pathological events. Small and macromolecules capable of carbohydrate recognition have great potentials as research tools, diagnostics, vectors for targeted delivery of therapeutic and imaging agents, and therapeutic agents. However, this potential is far from being realized. One key issue is the difficulty in the development of "binders" capable of specific recognition of carbohydrates of biological relevance. This review discusses systematically the general approaches that are available in developing carbohydrate sensors and "binders/receptors," and their applications. The focus is on discoveries during the last 5 years.

Progress in biomimetic carbohydrate recognition

The importance of carbohydrate recognition in biology, and the unusual challenges involved, have lead to great interest in mimicking saccharide-binding proteins such as lectins. In this review, we discuss the design of artificial carbohydrate receptors, focusing on those which work under natural (i.e. aqueous) conditions. The problem is intrinsically difficult because of the similarity between substrate (carbohydrate) and solvent (water) and, accordingly, progress has been slow. However, recent developments suggest that solutions can be found. In particular, the "temple" family of carbohydrate receptors show good affinities and excellent selectivities for certain all-equatorial substrates. One example is selective for O-linked beta-N-acetylglucosamine (GlcNAc, as in the O-GlcNAc protein modification), while another is specific for beta-cellobiosyl and closely related disaccharides. Both show roughly millimolar affinities, matching the strength of some lectin-carbohydrate interactions.

Binding mechanisms in supramolecular complexes

Supramolecular chemistry has expanded dramatically in recent years both in terms of potential applications and in its relevance to analogous biological systems. The formation and function of supramolecular complexes occur through a multiplicity of often difficult to differentiate noncovalent forces. The aim of this Review is to describe the crucial interaction mechanisms in context, and thus classify the entire subject. In most cases, organic host-guest complexes have been selected as examples, but biologically relevant problems are also considered. An understanding and quantification of intermolecular interactions is of importance both for the rational planning of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents.

Oxime-Based Receptors for Mono- and Disaccharides

Representatives of a new series of acyclic oxime-based receptors were prepared and their binding properties toward neutral sugar molecules studied. 1H NMR and fluorescence titrations revealed that receptors 2a and 2b, incorporating suitable positioned amine and oxime moieties, are able to form strong 1:1 complexes (Ka1 approximately 10(5) M-1) with dodecyl alpha- and beta-maltoside in chloroform solutions. Furthermore, the binding studies with beta-glucopyranoside indicated the formation of complexes with 1:1 and 1:2 receptor-monosaccharide binding stoichiometry (with overall binding constant beta2 approximately 10(5) M-2). Both hydrogen bonding and interactions of the sugar CH's with the phenyl rings of the receptor contribute to the stabilization of the receptor-sugar complexes. Molecular modeling calculations, synthesis, and binding studies are described.

Anion-Binding Properties of the Tripyrrolemethane Group: A Combined Experimental and Theoretical Study

Tripyrrolemethane- and bistripyrrolemethane-containing systems were recently reported to be efficient and selective hosts for anions. Nevertheless, the basic intrinsic properties of tripyrrolemethane as a ligand for anions have not yet been explored. Here we report the study of the anion-binding properties of the tripyrrolemethane group. We applied a combined experimental and theoretical approach to determine the affinity of the tripyrrolemethane system for different anions in the gas phase, in solution and in the crystalline state. In the crystal, the tripyrrolemethane group forms a number of different complexes with the bromide ion, some involving the participation of more than one ligand species. Despite the very similar basicity of fluoride and dihydrogen phosphate, the tripyrrolemethane ligand exhibits a clear preference for the fluoride anion in solution, which indicates an anion-binding system and not merely deprotonation. Although the affinity of the tripyrrolemethane ligand for other ions was negligible in solution, gas-phase studies show that complexation with larger halide ions is favoured over complexation with fluoride.

Solid phase synthesis of a prototype of a new class of biomimetic receptors for anionic carbohydrates

A solid phase synthesis for a new biomimetic receptor was developed. First binding studies show that binds anionic carbohydrates such as glucose-1-phosphate and cAMP with association constants in the lower millimolar range in 20% buffered water in DMSO using both polar and apolar interactions.

Carboxylate-Based Receptors for the Recognition of Carbohydrates in Organic and Aqueous Media

Acyclic receptors containing neutral and ionic hydrogen-bonding sites, such as amino-pyridine and carboxylate groups, were prepared and their binding properties toward neutral sugar molecules were studied. The binding studies with disodium and bis(tetramethylammonium) salts containing the dianion 11 have revealed that this type of receptor molecule is able to recognize the selected sugars in both organic and aqueous media. The carboxylate/pyridine-based receptor 11 exhibits in chloroform at least a 100-fold higher affinity for glucopyranosides than the previously described triarmed pyridine-based receptor 1, incorporating only neutral hydrogen-bonding sites. A substantial drop in the association constants is expectedly observed for an ester analogue of 11, compound 9. The dicarboxylate 11 is able to form complexes in water with methyl beta-D-glucopyranoside and D-cellobiose, with a preference for the disaccharide. The studies show the importance of charge-reinforced hydrogen bonds in the recognition of carbohydrates.

Electrochemically Triggered Open and Closed Pacman Bis-metalloporphyrins

A new type of very compact Pacman bis-porphyrin scaffold has been obtained in which a calix[4]arene spacer provides both cofacial preorganization and flexibility. Changes in the distance between the two tetrapyrrolic macrocycles can be triggered by electrochemistry, where closed and open conformers can be interconverted in a handclapping motion.

Recognition of Anionic Carbohydrates by an Artificial Receptor in Water

A tris-cationic artificial receptor 1 efficiently binds anionic carbohydrates even in aqueous solution as shown by NMR and UV titration experiments. Complex formation involves both ion pair formation and H-bonds to the sugar, explaining the preference for saccharides compared to simple anions and the observed selectivities among different sugars. [structure: see text]

A supramolecular approach to protein labeling. A novel fluorescent bioassay for concanavalin a activity

[reaction: see text] A new method for the bioassay of concanavalin A is based on the interaction of saccharide-substituted oligopyrrolic macrocycles with lectins. A general sensing mechanism involves the interaction of aggregated and primarily nonfluorescent labels with the target protein, label deaggregation, site-specific binding, and fluorescence signaling. Addition of saccharides to the fluorescent lectin-macrocycle complex leads to the release of the label and partial quenching of fluorescence due to reaggregation. Specificity of the protein-probe binding is discussed.

Anion receptors: a new class of amide/quaternized amine macrocycles and the chelate effect

A new class of tetraamide macrocyclic receptors for anions with two quaternized amine functionalities exhibited higher affinities for anions compared with the corresponding neutral amides. In two crystal structures of halide complexes of the prototypes with phenyl and pyridine spacers, the anions are held by hydrogen bonding with the amide hydrogens. The pyridine analogues display higher affinities in general than the phenyl systems, a phenomenon which is attributed to the anion version of the chelate effect.

Steroid–porphyrin conjugate for saccharide sensing in protic media

A new saccharide receptor in protic media has been designed and synthesized. The receptor combines advantages of steroids, which are responsible for saccharide binding, and of the porphyrin moiety acting as a signalling component of the molecule due to changes in UV-vis electronic spectra. The synthesis is based on condensation of steroid aldehyde with pyrrole to form the porphyrin unit with four protected steroid moieties. After deprotection, meso-substituted porphyrin contains 12-hydroxy groups on the steroidal part. The receptor is soluble in aqueous solutions and exhibits high complexation affinity towards saccharides. Because the receptor extensively aggregates in water, most of the experiments were performed in 50% aqueous 2-propanol where aggregation is significantly eliminated. Binding is evidenced by spectral changes in the Soret region of the receptor in UV-vis absorption spectra allowing the evaluation of the binding constants. Additional confirmation of binding is obtained using 1H NMR, Raman and IR spectroscopies and the surface plasmon resonance technique. The receptor exhibits higher selectivity for oligosaccharides over monosaccharide. The results point to the importance of a combination of multiple binding via H-bonding and hydrophobic interactions.