Selective anion binding by a cofacial binuclear zinc complex of a Schiff-base pyrrole macrocycle

The synthesis of the new cofacial binuclear zinc complex [Zn(2)(L)] of a Schiff-base pyrrole macrocycle is reported. It was discovered that the binuclear microenvironment between the two metals of [Zn(2)(L)] is suited for the encapsulation of anions, leading to the formation of [K(THF)(6)][Zn(2)(μ-Cl)(L)]·2THF and [Bu(n)(4)N][Zn(2)(μ-OH)(L)] which were characterized by X-ray crystallography. Unusually obtuse Zn-X-Zn angles (X = Cl: 150.54(9)° and OH: 157.4(3)°) illustrate the weak character of these interactions and the importance of the cleft preorganization to stabilize the host. In the absence of added anion, aggregation of [Zn(2)(L)] was inferred and investigated by successive dilutions and by the addition of coordinating solvents to [Zn(2)(L)] solutions using NMR spectroscopy as well as isothermal microcalorimetry (ITC). On anion addition, evidence for deaggregation of [Zn(2)(L)], combined with the formation of the 1:1 host-guest complex, was observed by NMR spectroscopy and ITC titrations. Furthermore, [Zn(2)(L)] binds to Cl(-) selectively in THF as deduced from the ITC analyses, while other halides induce only deaggregation. These conclusions were reinforced by density functional theory (DFT) calculations, which indicated that the binding energies of OH(-) and Cl(-) were significantly greater than for the other halides.

Anion-directed assembly of a three-dimensional metal-organic rotaxane framework

A three-dimensional extended, metal-organic rotaxane framework (MORF) that incorporates encircled "struts" has been synthesized through a one-pot self-assembly process involving a macrocyclic tetraimidazolium "molecular box", naphthalene dicaboxylate dianion, and Zn(II) cations. The present system represents progress towards controlling the features of three-dimensional metal-organic frameworks.

Overcoming biochemical pharmacologic mechanisms of platinum resistance with a texaphyrin-platinum conjugate

In our effort to investigate further texaphyrin conjugation as a means of increasing delivery and accumulation of known anticancer platinum agents in cancer cells, we have continued our studies on the mode of action of a texaphyrin-platinum conjugate, particularly in cisplatin-resistant tumor cells that are characterized by several mechanisms of resistance, including reduced drug accumulation. Our results provide support for the proposal that intracellular platinum and Pt-DNA adduct levels were significantly increased using our conjugate relative to corresponding Pt controls. Moreover, no differences were found in cellular accumulation and Pt-DNA adduct formation between Pt sensitive and Pt resistant ovarian cells. As a result, resistance to the conjugate was lower than cisplatin in resistant cells. Based on these results we conclude that texaphyrin conjugation provides a promising strategy for overcoming biochemical pharmacologic mechanisms of resistance.

Chemical functionalization of oligodeoxynucleotides with multiple boronic acids for the polyvalent binding of saccharides

A novel saccharide host containing four boronic acid recognition units on a single DNA duplex terminus was constructed. This construct allowed boronic acid sugar recognition in the context of double-stranded DNA to be established while highlighting the benefits of multivalency. Following the solid-phase synthesis of a bis-boronic acid tag, two end-functionalized oligonucleotides with complementary sequences were functionalized through amide ligation. By annealing the boronic acid-DNA conjugates, a tetra-boronic acid DNA duplex was assembled. The saccharide binding ability of this tetra-boronic acid host was revealed through cellulose paper chromatography in the absence and presence of various saccharides. While no appreciable saccharide binding was seen in the case of a bis-boronic DNA conjugate, the increased migration of the tetra-boronic acid host relative to the control sequences in the presence of selected monosaccharides underscored the importance of multivalent effects. We thus identified a requirement for multiple recognition sites in these conjugate systems and expect the results to facilitate future efforts toward applying synthetic recognition systems to the realm of macromolecules.

Anion responsive TTF-appended calix[4]arenes. Synthesis and study of two different conformers

Two new cone- and 1,3-alternate-calix[4]arenes (cone-1 and 1,3-alt-1), bearing four modified TTF (tetrathiafulvalene) substituents on the upper rim, have been synthesized. The binding ability of these two sets of conformers for various anions, including F(-), Cl(-), Br(-), I(-), PF6(-), ClO4(-), HSO4(-), CH3COO(-), H2PO4(-), and HP2O7(3-), was tested in organic media by monitoring the changes in their UV/vis and (1)H NMR spectra as a function of added anion, as well as via cyclovoltammetry (CV) (all anions studied as their respective TBA salts). On the basis of the present findings, we propose that incorporation of four TTF units within an overall calix[4]arene-based recognition framework produces a preorganized receptor system that displays a modest preference for the pyrophosphate (HP2O7(3-)) anion.

Design of a Novel Electrically Conducting Biocompatible Polymer with Degradable Linkages for Biomedical Applications

Upon the discovery of the first conducting polymer in 1976, the replacement of metal-based semiconductors in many applications with conducting polymers was inevitable. Perhaps more surprisingly these materials showed promise in many biomedical applications. The finding that electrical fields and charges modulate shape, size, and proliferation of a number of cell types including nerve and bone, has prompted biomedical engineers to focus on the development of conducting polymer scaffolds for tissue regeneration and neural probes. A number of polymers have been studied for these applications, including polypyrrole (PPy), polythiophene, and polyaniline. One serious limitation to the use of conducting polymers in biological systems is their inability to degrade. Therefore, it has been proposed to design and synthesize a biocompatible, biodegradable, semiconducting polymer, which could broaden the scope of applications for conducting polymers. For instance, these unique polymers could potentially be implemented in temporary tissue and neural scaffolds, drug delivery, short-term electrodes, and tethers between nanotubes. Therefore, the goal of this investigation is to synthesize a polymer that is degradable, while maintaining conductivity. Initially, the incorporation of pyrrole oligomers into a biodegradable polymer was attempted; however, to overcome difficulties associated with the instability of these oligopyrroles, a novel polymer design has been proposed which incorporates thiophene oligomers. Oligothiophenes (OTs) are apt to be more conductive than oligopyrroles due to a smaller band gap between the valence and conductance bands. Additionally, OTs share many of the advantageous properties of oligopyrroles, including cell compatibility, but are more stable. The proposed novel co-polymer consists of quaterthiophene oligomers tethered together by aliphatic ester linkages, which would potentially render the co-polymer conductive by means of inter and intra chain oligo overlap and degradable via cleavage of the ester bond by esterases in the body. The synthesis of the novel copolymer, poly-dialcohol dimethyl quaterthiophene-co-adipic acid (PAMQAA) has been completed successfully and corroborated by NMR and MALDI. Initial characterization indicates PAMQAA has the following properties: Mw = 11,240-64,080 g/mol, PI = 1.2-1.4, degradation temperature = ∼160°C. Currently PAMQAA is being characterized to assess its Tg, Tm, conductivity, degradability, and biocompatability. Polymer modifications could allow further optimization of properties, such as the rate of degradation and conductivity, to target better the application of interest.

Texaphyrins: tumor localizing redox active expanded porphyrins

Texaphyrins, a class of tumor selective expanded porphyrins capable of coordinating large metals, have been found to act as redox mediators within biological systems. This review summarizes studies involving their experimental use in cancer chemotherapy. Mechanistic insights involving their presumed mode of action are also described, as well as certain structure activity relationships. Finally, newer texaphyrin-based applications associated with targeted drug delivery are presented.

Boronic acid porphyrin receptor for ginsenoside sensing

Ginsenoside detection was pursued through the design of a porphyrin receptor containing two boronic acid units. This receptor was found to undergo different degrees of fluorescence quenching with five ginsenoside guests and an acylated derivative. The trends in the 1:1 binding constants, as well as ESI-HRMS analysis, support a binding mode in which the ginsenoside sugar units are bound to the boronic acid groups, while the steroid core and porphyrin ring participate in hydrophobic interactions.

A pyrrolyl-based triazolophane: a macrocyclic receptor with CH and NH donor groups that exhibits a preference for pyrophosphate anions

A pyrrolyl-based triazolophane, incorporating CH and NH donor groups, acts as a receptor for the pyrophosphate anion in chloroform solution. It shows selectivity for this trianion, followed by HSO(4)(-) > H(2)PO(4)(-) > Cl(-) > Br(-) (all as the corresponding tetrabutylammonium salts), with NH-anion interactions being more important than CH-anion interactions. In the solid state, the receptor binds the pyrophosphate anion in a clip-like slot via NH and CH hydrogen bonds.

Dinaphthoporphycenes

Naphthobipyrrole is a potentially useful building block for porphyrin and porphyrin analogue synthesis. Reported here is a simple, generalizable synthetic route to α-formylated, β-substituted naphthobipyrroles and their use in the preparation of binaphthoporphycenes. The resulting binaphthoporphycenes possess a planar geometry as determined via a single-crystal X-ray diffraction analysis; they also display absorption maxima that are bathochromically shifted compared to simple porphycenes.

Environmentally responsive threading, dethreading, and fixation of anion-induced pseudorotaxanes

The tetracationic macrocycle cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene) hexafluorophosphate (1(4+)·4PF(6)(-)) acts as a large, flexible "molecular box" that supports the formation of environmentally responsive anion-induced pseudorotaxanes, as well as other extended structures, including metal-linked supramolecular polyrotaxanes. Specifically, the combination of the tetracation 1(4+) and bis-carboxylate guests derived from 4,4'-biphenyldicarboxylic acid and 2,6-naphthalenedicarboxylic acid results in the formation of pseudorotaxanes that respond to changes in environmental stimuli, including pH and temperature. The resulting structures can be "locked into place" via the addition of a metal-linker in the form of Ag(I); this gives rise to an ordered metal-linked polyrotaxane. The interpenetrated constructs described in this article were characterized in solution and in the solid state by one- and two-dimensional ((1)H and NOESY) NMR spectroscopy, as well as by mass spectrometry (ESI-MS) and single-crystal X-ray diffraction methods.

N-Tosylpyrrolidine calix[4]pyrrole: synthesis and ion binding studies

The synthesis and preliminary solution phase ion binding properties of the N-tosylpyrrolidine calix[4]pyrrole 2 are reported. This β-octaalkyl-substituted calix[4]pyrrole, the first to be prepared via a direct condensation reaction, was obtained by reacting the 3,4-alkyl-functionalized pyrrole 8 with acetone in the presence of an acid catalyst. On the basis of (1)H NMR spectroscopic analyses and isothermal titration calorimetry, it was concluded that, compared with the parent, β-unsubstituted calix[4]pyrrole (1), compound 2 possesses significantly enhanced binding ability for halide anions in chloroform. Furthermore, 2 proved capable of solubilizing in chloroform solution the otherwise insoluble salts, CsF and CsCl. These effects are ascribed to the interactions between the four tosyl groups present in 2 and the counter cations of the halide anion salts.

Stopped-flow kinetic analysis of the interaction of cyclo[8]pyrrole with anions

The on and off rates corresponding to the binding of two test anions (acetate, AcO(-), and dihydrogen phosphate, H(2)PO(4)(-), studied as their tetrabutylammonium salts) to diprotonated cyclo[8]pyrrole have been determined in CH(3)CN using stopped-flow analyses carried out at various temperatures. For dihydrogen phosphate, this afforded the activation enthalpies and entropies associated with both off and on processes. The different dynamic behavior seen for these test anions underscores the utility of kinetic analyses as a possible new tool for the advanced characterization of anion receptors.

Naphthobipyrrole: versatile synthesis and electropolymerization

A facile synthetic route to a new polycyclic pyrrole derivative, 3,8-diethyl-1,10-dihydro-benzo[e]pyrrolo[3,2-g]indole (1), is reported. This annulated bipyrrole acts as a monomer for electropolymerization and forms an electrochromic conducting polymer (poly1) when electrooxidized at low potentials (0.4 V vs Ag/Ag(+)) in acetonitrile. The presence of alkyl substituents at the 3 and 8 carbons (β-pyrrolic positions) induces regioselective 2,5'-coupling of the pyrrole repeat units and gives rise to the more uniform polymeric product, poly1. Poly1 exhibits globular morphology, as judged from SEM pictures. Its spectroelectrochemical features can be attributed to the formation of four possible states: neutral, polaron, bipolaron, and transverse bipolaron. The relatively low switching potentials (-0.6 to +0.9 V vs Ag/Ag(+) in MeCN) displayed by poly1 leads us to suggest that 1 has a role to play as a polymerizable moiety for the development of multicolor electrochromic materials.

Tetrathiafulvalene diindolylquinoxaline: a dual signaling anion receptor with phosphate selectivity

Incorporation of tetrathiafulvalene into the backbone of a known neutral phosphate receptor, diindolylquinoxaline, yields a dual optical-electrochemical chemosensor for dihydrogen phosphate that functions in dichloromethane. This system shows selectivity for dihydrogen phosphate over other small anions and can be used to detect the presence of this analyte via fluorescence quenching or cyclic voltammetry.

Bismuth- and lead-texaphyrin complexes: towards potential α-core emitters for radiotherapy

Lead(II)-texaphyrins and the first discrete binuclear μ-oxo bismuth(III)-texaphyrin are reported. The latter was characterized via single crystal X-ray diffraction analysis. Cell proliferation assays using the A2780 ovarian cancer cell line were used to determine the cytotoxicity of the complexes.

Ion pair receptors

Compared with simple ion receptors, which are able to bind either a cation or an anion, ion pair receptors bearing both a cation and an anion recognition site offer the promise of binding ion pairs or pairs of ions strongly as the result of direct or indirect cooperative interactions between co-bound ions. This critical review focuses on the recent progress in the design of ion pair receptors and summarizes the various binding modes that have been used to accommodate ion pairs (110 references).