Advances in anion transport and supramolecular medicinal chemistry.

Advances in anion transport by synthetic supramolecular systems are discussed in this article. Developments in the design of discrete molecular carriers for anions and supramolecular anion channels are reviewed followed by an overview of the use of these systems in biological systems as putative treatments for diseases such as cystic fibrosis and cancer.

Playing supramolecular dominoes with light: building and breaking a photoreversible G-quadruplex made from guanosine, boric acid and an azobenzene.

Addition of azobenzene-derivative 1 in its E configuration to an aqueous solution containing various guanosine borate esters induces a helical G-quartet based self-organization, stabilized by intercalation of the dye. The process is driven, in a domino fashion, by the initial host-guest interaction between the dye and a specific guanosine borate diester, whose structure can be thus assigned. This inclusion complex templates the formation of G-quartets. The quartets, in turn, pile up to form a supramolecular G-quadruplex structure, in which other G species present in solution are progressively included. The G-quadruplex can be reversibly broken and reformed by photoisomerization of the dye. This hierarchical and photosensitive self-assembly is unprecedented for simple guanosine derivatives.

Drawing with Iron on a Gel Containing a Supramolecular Siderophore.

Guanosine-5'-hydroxamic acid (3) forms hydrogels when mixed with guanosine (1) and KCl. The 5'-hydroxamic acid (HA) unit is pH-responsive and also chelates Fe3+ . When gels are prepared under basic conditions, the 5'-HA groups are deprotonated and the anionic hydrogel binds cationic thiazole orange (TO), signaled by enhanced fluorescence. The HA nucleoside 3, when immobilized in the G-quartet gel, acts as a supramolecular siderophore to form red complexes with Fe3+ . We patterned the hydrogel's surface with FeCl3 , by hand and by using a 3D printer. Patterns form instantly, are visible by eye, and can be erased using vitamin C. This hydrogel, combining self-assembled G-quartet and siderophore-Fe3+ motifs, is strong, can be molded into different shapes, and is stable on the bench or under salt water.

G4-quartet hydrogels from 5'-hydrazino-guanosine for the non-covalent and covalent remediation of contaminants from water.

The creation of supramolecular hydrogels from relatively simple building blocks demonstrates the power of molecular self-assembly to make functional materials. G4-quartet hydrogels are appealing for a number of applications, including the environmental remediation of pollutants in water. We find that the guanosine analog, 5'-deoxy-5'-hydrazinoguanosine (HG 2) self-assembles into a self-standing hydrogel in the presence of stoichiometric amounts (0.25 equiv.) of KCl. The higher water solubility of HG 2 (14.5 mM), compared to that of the parent compound G 1 (2.1 mM), likely contributes to its enhanced gelation. The structural basis for this HG 2·KCl hydrogel, confirmed by PXRD, IR and CD, is the G4·K+ quartet, which forms extended 1D ion-channel assemblies that entangle to give a stable and long-lived hydrogel. We also find that adding KCl to a saturated solution of HG 2 triggers the generation of colloidal G4·K+ assemblies in situ that selectively and efficiently binds the anionic dye naphthol blue black (NBB) over a cationic dye. In addition to this non-covalent electrostatic binding of anions, the nucleophilic 5'-hydrazino group in the HG 2·KCl hydrogel HG 2 enables the efficient absorption of propionaldehyde from both the gas phase and from water solution via the formation of covalent hydrazone linkages with the gel matrix.

Anion transport and supramolecular medicinal chemistry.

New approaches to the transmembrane transport of anions are discussed in this review. Advances in the design of small molecule anion carriers are reviewed in addition to advances in the design of synthetic anion channels. The application of anion transporters to the potential future treatment of disease is discussed in the context of recent findings on the selectivity of anion transporters.

Templating and Catalyzing [2+2] Photocycloaddition in Solution Using a Dynamic G-Quadruplex.

We describe a templating/covalent capture strategy that enables photochemical formation of 8 cyclobutanes in one noncovalent assembly. This process was characterized by experiment and quantum mechanical/molecular mechanics (ONIOM) calculations. Thus, KI and 16 units of 5'-cinnamate guanosine form a G-quadruplex where C=C π bonds in neighboring G4 -quartets are separated by 3.3 Å, enabling [2+2] photocycloaddition in solution. This reaction is high-yielding (>90 %), regio- and diastereoselective. Since all components are in dynamic equilibrium this photocycloaddition is catalytic in K+ .

Co-existence of Distinct Supramolecular Assemblies in Solution and in the Solid State.

The formation of distinct supramolecular assemblies, including a metastable species, is revealed for a lipophilic guanosine (G) derivative in solution and in the solid state. Structurally different G-quartet-based assemblies are formed in chloroform depending on the nature of the cation, anion and the salt concentration, as characterized by circular dichroism and time course diffusion-ordered NMR spectroscopy data. Intriguingly, even the presence of potassium ions that stabilize G-quartets in chloroform was insufficient to exclusively retain such assemblies in the solid state, leading to the formation of mixed quartet and ribbon-like assemblies as revealed by fast magic-angle spinning (MAS) NMR spectroscopy. Distinct N-H⋅⋅⋅N and N-H⋅⋅⋅O intermolecular hydrogen bonding interactions drive quartet and ribbon-like self-assembly resulting in markedly different 2D 1 H solid-state NMR spectra, thus facilitating a direct identification of mixed assemblies. A dissolution NMR experiment confirmed that the quartet and ribbon interconversion is reversible-further demonstrating the changes that occur in the self-assembly process of a lipophilic nucleoside upon a solid-state to solution-state transition and vice versa. A systematic study for complexation with different cations (K+ , Sr2+ ) and anions (picrate, ethanoate and iodide) emphasizes that the existence of a stable solution or solid-state structure may not reflect the stability of the same supramolecular entity in another phase.

Supramolecular gels made from nucleobase, nucleoside and nucleotide analogs.

Supramolecular or molecular gels are attractive for various applications, including diagnostics, tissue scaffolding and targeted drug release. Gelators derived from natural products are of particular interest for biomedical purposes, as they are generally biocompatible and stimuli-responsive. The building blocks of nucleic acids (i.e. nucleobases, nucleosides, and nucleotides) are desirable candidates for supramolecular gelation as they readily engage in reversible, noncovalent interactions. In this review, we describe a number of organo- and hydrogels formed through the assembly of nucleosides, nucleotides, and their derivatives. While natural nucleosides and nucleotides generally require derivatization to induce gelation, guanosine and its corresponding nucleotides are well known gelators. This unique gelating ability is due to propensity of the guanine nucleobase to self-associate into stable higher-order assemblies, such as G-ribbons, G4-quartets, and G-quadruplexes.

A Molecular Chaperone for G4-Quartet Hydrogels.

Thioflavin T (ThT) functions as a molecular chaperone for gelation of water by guanosine and lithium borate. Substoichiometric ThT (1 mol % relative to hydrogelator) results in faster hydrogelation as monitored by (1)H NMR and visual comparison. Vial-inversion tests and rheology show that ThT increases the stiffness of the Li(+) guanosine-borate (GB) hydrogel. In addition, the dye promotes relatively rapid and complete repair of a Li(+) GB hydrogel destroyed by shearing. We used rheology to show that other planar aromatics, some cationic and one neutral dye (methylene violet), also stiffened the Li(+) GB hydrogel. Data from powder X-ray diffraction, UV, and circular dichroism spectroscopy and ThT fluorescence indicate that G4 quartets are formed by the Li(+) GB system. We observed a species in solution by (1)H NMR that was intermediate in size between monomeric gelator and NMR-invisible hydrogel. The concentration of this intermediate decreased much faster when ThT was present in solution, again showing that the dye can accelerate hydrogel formation. We propose that ThT functions as a molecular chaperone by end stacking on terminal G4-quartets and promoting the assembly of these smaller fragments into longer G4-based structures that can then provide more cross-linking sites needed for hydrogelation.

G4-quartet·M(+) borate hydrogels.

The ability to modulate the physical properties of a supramolecular hydrogel may be beneficial for biomaterial and biomedical applications. We find that guanosine (G 1), when combined with 0.5 equiv of potassium borate, forms a strong, self-supporting hydrogel with elastic moduli >10 kPa. The countercation in the borate salt (MB(OH)4) significantly alters the physical properties of the hydrogel. The gelator combination of G 1 and KB(OH)4 formed the strongest hydrogel, while the weakest system was obtained with LiB(OH)4, as judged by (1)H NMR and rheology. Data from powder XRD, (1)H double-quantum solid-state magic-angle spinning (MAS) NMR and small-angle neutron scattering (SANS) were consistent with a structural model that involves formation of borate dimers and G4·K(+) quartets by G 1 and KB(OH)4. Stacking of these G4·M(+) quartets into G4-nanowires gives a hydrogel. We found that the M(+) cation helps stabilize the anionic guanosine-borate (GB) diesters, as well as the G4-quartets. Supplementing the standard gelator mixture of G 1 and 0.5 equiv of KB(OH)4 with additional KCl or KNO3 increased the strength of the hydrogel. We found that thioflavin T fluoresces in the presence of G4·M(+) precursor structures. This fluorescence response for thioflavin T was the greatest for the K(+) GB system, presumably due to the enhanced interaction of the dye with the more stable G4·K(+) quartets. The fluorescence of thioflavin T increased as a function of gelator concentration with an increase that correlated with the system's gel point, as measured by solution viscosity.

A G4·K⁺ hydrogel stabilized by an anion.

Supramolecular hydrogels derived from natural products have promising applications in diagnostics, drug delivery, and tissue engineering. We studied the formation of a long-lived hydrogel made by mixing guanosine (G, 1) with 0.5 equiv of KB(OH)4. This ratio of borate anion to ligand is crucial for gelation as it links two molecules of 1, which facilitates cation-templated assembly of G4·K(+) quartets. The guanosine-borate (GB) hydrogel, which was characterized by cryogenic transmission electron microscopy and circular dichroism and (11)B magic-angle-spinning NMR spectroscopy, is stable in water that contains physiologically relevant concentrations of K(+). Furthermore, non-covalent interactions, such as electrostatics, π-stacking, and hydrogen bonding, enable the incorporation of a cationic dye and nucleosides into the GB hydrogel.

Influence of B-ring modifications on proton affinity, transmembrane anion transport and anti-cancer properties of synthetic prodigiosenes.

Prodigiosin is the parent compound of the tripyrrolic natural products known as the prodigiosenes. Some of these natural products and their synthetic analogs show anti-cancer, immunosuppressive and antimicrobial actions, amongst other biological activities. One mechanism put forth to explain their biological activity is that since prodigiosenes are typically protonated at physiological pH they can alter intracellular pH via HCl co-transport (or Cl(-)/OH(-) exchange) across cell membranes. In this study we synthesized a series of prodigiosene analogs with different -O-aryl substituents attached to the B-ring of the tripyrrolic skeleton. NMR studies showed that these analogs can exist as a mixture of two stable α and ß conformers in acidic solution, and that both conformers can bind anions in solution. We found that the electronic nature of the O-aryl substituent on the B-ring influences the rate at which these prodigiosenes catalyze transmembrane anion transport, i.e. the prodigiosenes with the higher pKa had greater Cl(-)/NO3(-) exchange rates. Four of the synthetic prodigiosenes were tested for their in vitro anti-cancer activities in the NCI60 human tumour panel. Despite their promising in vitro anti-cancer activity (GI50 values ranging from 18 to 74 nM), there was no evidence that this activity is influenced by the extent of protonation of these synthetic prodigiosenes.

Synthetic prodigiosenes and the influence of C-ring substitution on DNA cleavage, transmembrane chloride transport and basicity.

Analogues of the tripyrrolic natural product prodigiosin bearing an additional methyl and a carbonyl group at the C-ring were synthesised and evaluated. In vitro anticancer activity screening (NCI) and the study of modes of action (copper-mediated cleavage of double-stranded DNA and transmembrane transport of chloride anions) showed that the presence of the methyl group is not detrimental to activity. Furthermore, although the presence of an ester conjugated to the prodigiosene C-ring seems to decrease both pK(a) and chloride transport efficiency compared to the natural product, these analogues still exhibit a high rate of chloride transport. All analogues exhibit good in vitro anticancer activity and reduced toxicity compared to the natural product: compare an acute systemic toxicity of 100 mg kg(-1) in mice vs. 4 mg kg(-1) for prodigiosin, pointing towards a larger therapeutic window than for the natural product.

High-resolution 39K NMR spectroscopy of bio-organic solids.

We report the first implementation of the multiple-quantum magic-angle-spinning method to obtain high-resolution (39)K NMR spectra for bio-organic solids. The observed spectral resolution in the isotropic dimension is nearly at the sub-ppm level, which approaches the intrinsic resolution limit determined primarily by quadrupole relaxation. We show that high-resolution solid-state (39)K NMR spectroscopy can be used as a new means of probing K(+) ions in biomolecular systems.

Recent advances in the transmembrane transport of anions.

Anions cannot diffuse passively through biological membranes and membrane-bound proteins mainly govern the transmembrane movement of these charged species. The use of synthetic compounds that are able to facilitate the transmembrane transport of anions is a fascinating and burgeoning topic. The study of facilitated anion transport across lipid bilayers is an emerging field in supramolecular and bioorganic chemistry. In this critical review we describe the recent research progress in this area, focusing on literature published during the years 2007-2009. An overview of the assays that are used in the transmembrane transport of anions is also included (158 references).

Oxidation of 8-thioguanosine gives redox-responsive hydrogels and reveals intermediates in a desulfurization pathway.

A disulfide made by oxidation of 8-thioguanosine is a supergelator. The hydrogels are redox-responsive, as they disassemble upon either reduction or oxidation of the S-S bond. We also identified this disulfide, and 2 other compounds, as intermediates in oxidative desulfurization of 8-thioG to guanosine.

Catechols as membrane anion transporters.

We report that an amphiphilic bis-catechol (3) functions as a transmembrane anion transporter. Activity depends on the catechol's substitution and amphiphilicity. We also describe a liposomal assay that allows one to readily measure anion transport selectivity. This assay reveals that anion transport selectivity for this amphiphilic bis-catechol follows a Hofmeister sequence wherein anions that are easier to dehydrate are made more permeable to the membrane by 3.

Probing the Na+ binding site in a calix[4]arene-guanosine conjugate dimer by solid-state 23Na NMR and quantum chemical calculation.

Using solid-state (23)Na NMR and quantum chemical calculations we have found that the Na(+) ion bound to a calix[4]arene-guanosine conjugate dimer resides slightly above the G-quartet plane and simultaneously coordinates to a water molecule in a square-pyramidal (penta-coordination) geometry.

Self-Assembly of Metallo-Nucleoside Hydrogels for Injectable Materials That Promote Wound Closure.

Injectable hydrogels are increasingly being used as scaffolds for in situ tissue engineering and wound healing. Most of these injectable hydrogels are made from polymers, and there are fewer examples of such soft materials made via self-assembly of low-molecular weight gelators. We report the room-temperature synthesis of a functional hydrogel formed by mixing cytidine (C) with 0.5 equiv each of B(OH)3 and AgNO3. The structural basis for this supramolecular hydrogel (C-B-C·Ag+) involves orthogonal formation of cytidine borate diesters (C-B-C) and Ag+-stabilized C-C base pairs, namely, the C·Ag+·C dimer. The C-B-C·Ag+ hydrogels, which can have high water content (at least 99.6%), are stable (no degradation after 1 year in the light), stimuli-responsive, and self-supporting, with elastic moduli of up to 104 Pa. Incorporation of Ag+ ions into the gel matrix endows the C-B-C·Ag+ hydrogel with significant antibacterial capability. Importantly, the rapid switching between the sol and gel states for this supramolecular hydrogel, as a response to shear stress, enables 3D printing of a flexible medical patch made from the C-B-C·Ag+ hydrogel. The C-B-C·Ag+ hydrogel was used to promote the closure of burn wounds in a mouse model.

A G4·K+ hydrogel made from 5'-hydrazinoguanosine for remediation of α,ß-unsaturated carbonyls.

A G4·K+ hydrogel made from 5'-hydrazinoguanosine and KCl reacts with α,ß-unsaturated carbonyls of different electrophilicities (acrolein, methyl vinyl ketone and methyl acrylate) in water and the gas phase to form cyclic adducts. This aza-Michael addition/cyclization domino reaction by the 5'-hydrazino G4·K+ hydrogel has promise for environmental remediation of toxic α,ß-unsaturated carbonyls from water and the atmosphere.