Calixarene and Resorcarene Based Receptors: From Structural and Thermodynamic Studies to the Synthesis of a New Mercury(II) Selective Material

Materials used in current technological approaches for the removal of mercury lack selectivity. Given that this is one of the main features of supramolecular chemistry, receptors based on calix[4]arene and calix[4]resorcarene containing functional groups able to interact selectively with polluting ions while discriminating against biologically essential ones were designed. Thus two receptors, a partially functionalized calix[4]arene derivative, namely, 5,11,17,23-tetra-tert-butyl [25-27-bis(diethyl thiophosphate amino)dihydroxy] calix[4]arene (1) and a fully functionalized calix[4]resorcarene, 4,6,10,12,16,18,22,24-diethyl thiophosphate calix[4]resorcarene (2) are introduced. Mercury(II) was the identified target due to the environmental and health problems associated with its presence in water Thus following the synthesis and characterization of 1 and 2 in solution ((1)HNMR) and in the solid state (X-ray crystallography) the sequence of experimental events leading to cation complexation studies in acetonitrile and methanol ((1)H NMR, conductance, potentiometric, and calorimetric measurements) with the aim of assessing their behavior as mercury selective receptors are described. The cation selectivity pattern observed in acetonitrile follows the sequence Hg(II) > Cu(II) > Ag(I). In methanol 1 is also selective for Hg(II) relative to Ag(I) but no interaction takes place between this receptor and Cu(II) in this solvent. Based on previous results and experimental facts shown in this paper, it is concluded that the complexation observed with Cu(II) in acetonitrile occurs through the acetonitrile-receptor adduct rather than through the free ligand. Receptor 2 has an enhanced capacity for uptaking Hg(II) but forms metalate complexes with Cu(II). These studies in solution guided the inmobilization of receptor 1 into a silica support to produce a new and recyclable material for the removal of Hg(II) from water. An assessment on its capacity to extract this cation from water relative to Cu(II) and Ag (I) shows that the cation selectivity pattern of the inmobilized receptor is the same as that observed for the free receptor in methanol. These findings demonstrate that fundamental studies play a critical role in the selection of the receptor to be attached to silicates as well as in the reaction medium used for the synthesis of the new decontaminating agent.

Inhibition of myeloperoxidase- and neutrophil-mediated oxidant production by tetraethyl and tetramethyl nitroxides

The powerful oxidant HOCl (hypochlorous acid and its corresponding anion, −OCl) generated by the myeloperoxidase (MPO)–H2O2–Cl− system of activated leukocytes is strongly associated with multiple human inflammatory diseases; consequently there is considerable interest in inhibition of this enzyme. Nitroxides are established antioxidants of low toxicity that can attenuate oxidation in animal models, with this ascribed to superoxide dismutase or radical-scavenging activities. We have shown (M.D. Rees et al., Biochem. J. 421, 79–86, 2009) that nitroxides, including 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidin-1-yloxyl radical), are potent inhibitors of HOCl formation by isolated MPO and activated neutrophils, with IC50 values of ~1 and ~6 µM respectively. The utility of tetramethyl-substituted nitroxides is, however, limited by their rapid reduction by biological reductants. The corresponding tetraethyl-substituted nitroxides have, however, been reported to be less susceptible to reduction. In this study we show that the tetraethyl species were reduced less rapidly than the tetramethyl species by both human plasma (89–99% decreased rate of reduction) and activated human neutrophils (62–75% decreased rate). The tetraethyl-substituted nitroxides retained their ability to inhibit HOCl production by MPO and activated neutrophils with IC50 values in the low-micromolar range; in some cases inhibition was enhanced compared to tetramethyl substitution. Nitroxides with rigid structures (fused oxaspiro rings) were, however, inactive. Overall, these data indicate that tetraethyl-substituted nitroxides are potent inhibitors of oxidant formation by MPO, with longer plasma and cellular half-lives compared to the tetramethyl species, potentially allowing lower doses to be employed.

A kinetic study of the effect of ultrasound power on the sonohydrolysis of tetraethyl orthosilicate

A kinetic study of the ultrasound-stimulated and acid-catalyzed sonohydrolysis of tetraethyl orthosilicate (TEOS) in solventless TEOS-water heterogeneous mixtures was carried out by means of a calorimetric method as a function of the ultrasound power. The hydrolysis reaction starts in acidulated heterogeneous water-TEOS mixtures after an induction period under ultrasonic stimulation. The ultrasound power seems to play a role on the dynamical coupling of the system originating a continuum upward shifting of the base line during the induction period of sonication. The rate in which the base line is upward shifted diminishes with the power. The best coupling between the ultrasound and the reactant heterogeneous mixtures for this experimental setup was found to occur at 50 W, for which the gelation time was found to be a minimum. The kinetics of the heterogeneous TEOS sonohydrolysis was studied on the basis of a dissolution and reaction modeling. The heterogeneous reaction pathway as deduced from the kinetic study was drawn in a ternary diagram as a function of the ultrasound power. (C) 2006 Elsevier B.V. All rights reserved.

The synthesis of novel isoindoline nitroxides bearing water-solubilising functionality

A range of novel tetramethyl- and tetraethylisoindolinenitroxides, possessing aryl-linked carboxylic acids, amines, alcohols and phosphonic acids were prepared. Notably, the chemistry established for the aromatic dibromination of the tetramethylisoindolines was not easily transferred to the corresponding tetraethylisoindoline system. Instead, various tetraethylisoindoline analogues were accessed by the oxidation of methyl groups attached to the aromatic ring to give the carboxylic acids. The increased steric bulk of the tetraethyl structures should limit bio-reduction and these compounds may have potential as antioxidants.

The synthesis of novel nitroxides and their application as small-molecule antioxidants and profluorescent probes for oxidative stress

The detection and potential treatment of oxidative stress in biological systems has been explored using isoindoline-based nitroxide radicals. A novel tetraethyl-fluorescein nitroxide was synthesised for its use as a profluorescent probe for redox processes in biological systems. This tetraethyl system, as well as a tetramethyl-fluorescein nitroxide, were shown to be sensitive and selective probes for superoxide in vitro. The redox environment of cellular systems was also explored using the tetramethylfluorescein species based on its reduction to the hydroxylamine. Flow cytometry was employed to assess the extent of nitroxide reduction, reflecting the overall cellular redox environment. Treatment of normal fibroblasts with rotenone and 2-deoxyglucose resulted in an oxidising cellular environment as shown by the lack of reduction of the fluorescein-nitroxide system. Assessment of the tetraethyl-fluorescein nitroxide system in the same way demonstrated its enhanced resistance to reduction and offers the potential to detect and image biologically relevant reactive oxygen species directly. Importantly, these profluorescent nitroxide compounds were shown to be more effective than the more widely used and commercially available probes for reactive oxygen species such as 2’,7’-dichlorodihydrofluorescein diacetate. Fluorescence imaging of the tetramethyl-fluorescein nitroxide and a number of other rhodamine-nitroxide derivatives was undertaken, revealing the differential cellular localisation of these systems and thus their potential for the detection of redox changes in specific cellular compartments. As well as developing novel methods for the detection of oxidative stress, a number of novel isoindoline nitroxides were synthesised for their potential application as small-molecule antioxidants. These compounds incorporated known pharmacophores into the isoindoline-nitroxide structure in an attempt to increase their efficacy in biological systems. A primary and a secondary amine nitroxide were synthesised which incorporated the phenethylamine backbone of the sympathomimetic amine class of drugs. Initial assessment of the novel primary amine derivative indicated a protective effect comparable to that of 5-carboxy-1,1,3,3- tetramethylisoindolin-2-yloxyl. Methoxy-substituted nitroxides were also synthesised as potential antioxidants for their structural similarity to some amphetamine type stimulants. A copper-catalysed methodology provided access to both the mono- and di-substituted methoxy-nitroxides. Deprotection of the ethers in these compounds using boron tribromide successfully produced a phenolnitroxide, however the catechol moiety in the disubstituted derivative appeared to undergo reaction with the nitroxide to produce quinone-like degradation products. A novel fluoran-nitroxide was also synthesised from the methoxy-substituted nitroxide, providing a pH-sensitive spin probe. An amino-acid precursor containing a nitroxide moiety was also synthesised for its application as a dual-action antioxidant. N-Acetyl protection of the nitroxide radical was necessary prior to the Erlenmeyer reaction with N-acetyl glycine. Hydrolysis and reduction of the azlactone intermediate produced a novel amino acid precursor with significant potential as an effective antioxidant.

Synthesis and evaluation of resveratrol-based nitroxides as potential treatments for hypertension

The synthesis and evaluation of novel resveratrol-based nitroxides have been explored for the potential treatment of hypertension. New methodology for the direct aryl iodination of isoindoline and isoindoline nitroxide using periodic acid and potassium iodide in concentrated sulphuric acid was developed. Diiodinated tetramethyl and tetraethyl isoindolines and a tetramethyl isoindoline nitroxide were prepared in excellent yields (70 – 82%). A diiodinated tetraethyl isoindoline nitroxide was generated from the corresponding nitroxide in modest yield (37%) alongside iodinated nitrones. The mono-iodinated species were also generated in modest yields (34 – 48%). Incorporation of the nitroxide unit into the structure of resveratrol was achieved using palladium-catalysed Heck coupling. Use of the previously prepared iodo products 5-iodo-1,1,3,3-tetramethylisoindolin-2-yloyl 18 and 5,6-diiodo-1,1,3,3-tetramethylisoindolin-2-yloyl 22 gave resveratrol nitroxides 12 and 13 in yields of 50% (optimized) and 1.6% respectively. Preliminary evaluation of the resveratrol analogue 12 as a treatment for hypertension was undertaken in the DOCA-salt rat model. A reduction in systolic blood pressure as well as alleviation of ventricular hypertrophy was observed. A larger study involving the DOCA salt rats is currently in progress.

Direct iodination of isoindolines and isoindoline nitroxides as precursors to functionalized nitroxides

A new method for the direct aryl iodination of isoindolines and isoindoline nitroxides which utilizes periodic acid and potassium iodide in sulfuric acid is presented. Di-iodo functionalized tetramethyl and tetraethyl isoindolines and a di-iodo tetramethyl isoindoline nitroxide were prepared in high yield (70-82%). The analogous mono-iodo species were afforded in modest yield (34-48%). Iodinated nitrones were also obtained from a tetraethyl isoindoline nitroxide.

Synthesis of mesoporous TiO2/SiO2 hybrid films as an efficient photocatalyst by polymeric micelle assembly

Thermally stable mesoporous TiO2/SiO2 hybrid films with pore size of 50 nm have been synthesized by adopting the polymeric micelle-assembly method. A triblock copolymer, poly(styrene-b-2-vinyl pyridine-b-ethylene oxide), which serves as a template for the mesopores, was utilized to form polymeric micelles. The effective interaction of titanium tetraisopropoxide (TTIP) and tetraethyl orthosilicate (TEOS) with the polymeric micelles enabled us to fabricate stable mesoporous films. By changing the molar ratio of TEOS and TTIP, several mesoporous TiO2/SiO2 hybrid films with different compositions can be synthesized. The presence of amorphous SiO2 phase effectively retards the growth of anatase TiO2 crystal in the pore walls and retains the original mesoporous structure, even at higher temperature (650 °C). These TiO2/SiO2 hybrid films are of very high quality, without any cracks or voids. The addition of SiO2 phase to mesoporous TiO2 films not only adsorbs more organic dyes, but also significantly enhances the photocatalytic activity compared to mesoporous pure TiO2 film without SiO2 phase.

Maturation of a transient outward potassium current in mouse fetal hypothalamic neurons in culture

The whole-cell voltage clamp technique was used to record potassium currents in mouse fetal hypothalamic neurons developing in culture medium from days 1 to 17. The neurons were derived from fetuses of IOPS/OF1 mice on the 14th day of gestation. The mature neurons (>six days in culture) showed both a transient potassium current and a non-inactivating delayed rectifier potassium current. These were identified pharmacologically by using the potassium channel blockers tetraethyl ammonium chloride and 4-aminopyridine, and on the basis of their kinetics and voltage sensitivities. The delayed rectifier potassium current had a threshold of −20 mV, a slow time-course of activation, and was sustained during the voltage pulse. The 4-aminopyridine-sensitive current was transient, and was activated from a holding potential more negative (−80 mV) than that required for evoking the delayed rectifier potassium current (−40 mV). The delayed rectifier potassium current was detectable from day 1 onwards, while the transient potassium current showed a distinct developmental trend. The time-constant of inactivation became faster with age in culture. The half steady-state inactivation potential showed a shift towards less negative membrane potentials with age, and the relationship was best described by a logarithmic regression equation.The developmental trend of the transient potassium current may relate functionally to the progressive morphological changes, and the appearance of synaptic connections during ontogenesis.

Magnetic, electric, and dielectric properties of FeCo alloy nanoparticles dispersed in amorphous matrix

Nanocrystalline Fe53Co47 alloy was synthesized by a single-step transmetallation chemical method at room temperature. The Fe53Co47 alloy nanoparticles of 77 and 47 wt% were dispersed in silica matrix by the sol-gel process using tetraethyl orthosilcate. Structural studies reveal that the as-prepared alloy powders are in bcc phase and silica is in an amorphous state. The phase-transition temperature and Mossbauer spectra analysis of the Fe-Co alloy establishes the homogeneous alloy formation. A saturation magnetization of 218 emu/g was obtained for pure FeCo alloy at room temperature. Scanning electron microscopic analysis demonstrates the hollow-sphere morphology for FeCo alloy particles. Magnetic nanocomposite consisting of 47 wt% FeCo-silica shows enhanced thermal stability over the native FeCo alloy. Electrical and dielectric properties of 47 wt% FeCo-silica nanocomposites were investigated as a function of frequency and temperature. It was found that the dielectric constants and dielectric loss were stable throughout the measured temperature (310-373 K). Our results indicate that FeCo-silica nanocomposite is a promising candidate for high-frequency applications. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A Novel Method for the Uniform Incorporation of Grain-Boundary Layer Modifiers in Positive Temperature Coefficient of Resistivity Barium Titanate

The presently developed two-stage process involves diping the prefired porous disks of n-BaTiO3 in nonaqueous solutions containing Al-buty rate, Ti-isopropoxide, and tetraethyl silicate and subsequent sintering. This leads to uniform distribution of the grain-boundary layer (GBL) modifiers (Al2O3+ TiO2+ SiO2) and better control of the grain size as well as the positive temperature coefficient of resistivity characteristics. The technique is particularly suited for GBL modifiers in low concentrations (< 1%).

Solid-state NMR study of bioactive binary borosilicate glasses

A series of binary borosilicate glasses prepared by the sol-gel method are shown to be bioactive. Tetraethyl orthosilicate (TEOS) and trimethylborate (TMB) in acidic medium are used to prepare xB(2)O(3)center dot(1-x)SiO2 glass systems for x = 0.045-0.167. The formation of a layer of apatite-like mineral on the glass surface becomes apparent after soaking in simulated body fluid for 48 h. We have measured the B-11-B-11 homonuclear second moments of the borosilicate glasses and inferred that no macroscopic phase separation occurred in our glasses. The B-11 chemical shift data also show that the formation of clustered boroxol rings is negligible in our glass system. Although the bioactivity of our borosilicate glasses is less than that of CaO-SiO2 sol-gel glasses, these simple binary systems could be taken as reference glass systems for the search of new bioactive borosilicate glasses. (C) 2008 Elsevier Ltd. All rights reserved.

Development of functionalized terbium fluorescent nanoparticles for antibody labeling and time-resolved fluoroimmunoassay application

Silica-based functionalized terbium fluorescent nanoparticles were prepared, characterized and developed as a fluorescence probe for antibody labeling and time-resolved fluoroimmunoassay. The nanoparticles were prepared in a water-in-oil (W/O) microemulsion containing a strongly fluorescent Tb3+ chelate. N,N.N-1,N-1-12,6-bis(3'-aminomethyl-1'-pyrazolyl)phenylpyridine] tetrakis(acetate)-Tb3+ (BPTA-Tb3+), Triton X-100, octanol, and cyclohexane by controlling copolymerization of tetraethyl orthosilicate (TEOS) and 3-[2-(2- aminoethylamino)-ethylamino]propyl-trimethoxysilane (AEPS) with ammonia water. The characterizations by transmission electron microscopy and fluorometric quantum methods show that the nanoparticles are spherical and uniform in size, 45 +/- 3 nm in diameter, strongly fluorescent with fluorescence yield of 10% and a long fluorescence lifetime of 2.0 ms. The amino groups directly introduced to the nanoparticle's surface by using AEPS in the preparation made the surface modification and bioconjugation of the nanoparticles easier. The nanoparticle-labeled anti-human alpha-fetoprotein antibody was prepared and used for time-resolved fluoroimmunoassay of (x-fetoprotein (AFP) in human serum samples. The assay response is linear from 0.10 ng ml(-1) to about 100 ng ml(-1) with the detection limit of 0.10 ng ml(-1). The coefficient variations (CVs) of the method are less than 9.0%. and the recoveries are in the range of 84-98% for human serum sample measurements. (C) 2004 Elsevier B.V. All rights reserved.