Selective Recognition of Alkyl Pyranosides in Protic and Aprotic Solvents

Molecular architecture and therapeutic potential of lectin mimics

Lectins are proteins of non-immune origin that bind specific carbohydrates without chemical modification. Coupled with the emerging biological and pathological significance of carbohydrates, lectins have become extensively used as research tools in glycobiology. However, lectin-based drug development has been impeded by high manufacturing costs, low chemical stability, and the potential risk of initiating an unfavorable immune response. As alternatives to lectins, non-protein small molecules having carbohydrate-binding properties (lectin mimics) are currently attracting a great deal of attention because of their ease of preparation and chemical modification. Lectin mimics of synthetic origin are divided roughly into two groups, boronic acid-dependent and boronic acid-independent lectin mimics. This article outlines their representative architectures and carbohydrate-binding properties, and discusses their therapeutic potential by reviewing recent attempts to develop antiviral and antimicrobial agents using their architectures. We also focus on the naturally occurring lectin mimics, pradimicins and benanomicins. They are the only class of non-protein natural products having a C-type lectin-like ability to recognize d-mannopyranosides in the presence of Ca^2 + ions. Their molecular basis of carbohydrate recognition and therapeutic potential are also discussed.

Peptide functionalized polydiacetylene liposomes act as a fluorescent turn-on sensor for bacterial lipopolysaccharide

Mixed polydiacetylene (PDA) liposomes functionalized on their surface with a fluorescent pentalysine peptide derivative and histidine in a ratio of 1:9 can identify bacterial lipopolysaccharide (LPS). Upon photopolymerization of the self-assembled liposomes the initial fluorescence of the peptide-diacetylene amphiphiles is quenched. Interaction with LPS in aqueous solution or on the surface of E. coli DH5α restores the fluorescence. This increase in fluorescence is selective for LPS relative to other negatively charged analytes including nucleotides and ctDNA. This simple turn-on fluorescent sensor allows detecting LPS even at low micromolar concentrations.

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.

Oligosaccharide sensing with chromophore-modified curdlan in aqueous media

A newly synthesized chromophore-modified curdlan functions as a saccharide chemosensor in aqueous solution, enabling us to discriminate tetrasaccharide acarbose from 24 mono-, di-, tri-, and tetrasaccharides.

A fluorescein-containing, small-molecule, water-soluble receptor for cytosine free bases

In this study, we synthesized small-molecule, water-soluble, fluorescein-containing ureido compounds 6 and 8 as target receptors for cytosine free bases and then investigated the binding of cytosine free bases with the receptors using (15)N NMR spectroscopy and partially labeled cytosine-2,4-(13)C-1,3,4-(15)N-cytosine. Binding with the receptor 6a (the disodium form of 6) caused the chemical shift of the nitrogen atom of the amino group of cytosine to move downfield; binding of the receptor 8a (the disodium form of 8), which is possessing no corresponding aryl nitrogen atom, had no effect on this signal. Fluorescence spectroscopy revealed that binding of cytosine and its derivatives led to quenching of the fluorescence of receptor 6a; in contrast, the quenching of receptor 8a was only slightly affected by cytosine. Because the fluorescence of 6a was not quenched by either deoxycytidine or uracil, it appears that this receptor is a specific for cytosine among the DNA bases. We used the fluorescence of 6a to measure the apparent binding constants for various cytosine derivatives, including the anticancer prodrug 5-fluorocytosine. Receptor 6a is the first small-molecule, water-soluble fluorescent receptor for the specific binding of cytosine free bases in aqueous solution.

Recognition properties of receptors consisting of imidazole and indole recognition units towards carbohydrates

Compounds 4 and 5, including both 4(5)-substituted imidazole or 3-substituted indole units as the entities used in nature, and 2-aminopyridine group as a heterocyclic analogue of the asparagine/glutamine primary amide side chain, were prepared and their binding properties towards carbohydrates were studied. The design of these receptors was inspired by the binding motifs observed in the crystal structures of protein-carbohydrate complexes. ¹H NMR spectroscopic titrations in competitive and non-competitive media as well as binding studies in two-phase systems, such as dissolution of solid carbohydrates in apolar media, revealed both highly effective recognition of neutral carbohydrates and interesting binding preferences of these acyclic compounds. Compared to the previously described acyclic receptors, compounds 4 and 5 showed significantly increased binding affinity towards β-galactoside. Both receptors display high β- vs. α-anomer binding preferences in the recognition of glycosides. It has been shown that both hydrogen bonding and interactions of the carbohydrate CH units with the aromatic rings of the receptors contribute to the stabilization of the receptor-carbohydrate complexes. The molecular modeling calculations, synthesis and binding properties of 4 and 5 towards selected carbohydrates are described and compared with those of the previously described receptors.

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.

Molecular recognition of carbohydrates by acyclic receptors employing noncovalent interactions

Artificial carbohydrate receptors using noncovalent interactions for sugar binding provide valuable model systems to study the underlying principles of carbohydrate-based molecular recognition processes. In addition, well-designed artificial receptors may serve as a basis for the development of saccharide sensors or therapeutics that intervene in biologically important carbohydrate recognitions. Several different strategies have been employed for the design of such synthetic systems. The main focus of this tutorial review is on the carbohydrate binding capabilities of receptors possessing an acyclic structure and employing noncovalent interactions for sugar binding. The acyclic scaffold provides simplicity in the synthetic plan for many modifications of the receptor structure, supplying a base for systematic studies toward recognition motifs for carbohydrates. The review covers both some earlier examples and newer developments in this field.

A synthetic lectin for O-linked beta-N-acetylglucosamine

Changing employment: Receptor 1 binds beta-N-acetylglucosaminyl (beta-GlcNAc) up to 100 times more strongly than it does glucose. This synthetic lectin shows affinities similar to wheat germ agglutinin (WGA), a natural lectin used to bind GlcNAc. Remarkably, 1 is more selective than WGA. It favors especially the glycoside unit in glycopeptide 2, a model of the serine-O-GlcNAc posttranslational protein modification.