Presence Of Crown Ether Research Articles

Complex formation and extraction of three-charged metal cations in the presence of crown ethers and trichloracetacic acid

Complex formation and extraction of three-charged metal cations in the presence of crown ethers and trichloracetacic acid

Functional DNA Structures and Their Biomedical Applications

Since the discovery of the double-helix structure in 1953, nucleic acids have been developed from natural genetic codes into functional building blocks in a wide range of biotechnology and material...

Facile Combined Experimental and Computational Study: g-C3N4@PDMS-Assisted Knoevenagel Condensation Reaction under Phase Transfer Conditions

A new recyclable g-C3N4@PDMS heterogeneous catalyst has been developed as an efficient catalyst with an appreciable reactivity toward Knoevenagel condensation in the presence of crown ether (PTC). ...

DNA-Responsive SiO2 Nanoparticles, Metal-Organic Frameworks, and Microcapsules for Controlled Drug Release.

Recent advances addressing the development of stimuli-responsive nucleic acid (DNA)-functionalized micro/nanocarriers for the controlled release of drugs are presented. The DNA associated with the drug-loaded carriers acts as capping units that lock the drugs in the carriers. In the presence of appropriate triggers, the capping units are unlocked, resulting in the release of the drugs. Three types of DNA-modified carriers are discussed, including mesoporous SiO2 nanoparticles (MP SiO2 NPs), metal-organic framework nanoparticles (NMOFs) and micro/nanocapsules. The triggers to unlock the DNA gating units include pH, the dissociation of K+-stabilized G-quadruplexes in the presence of crown ethers, the catalytic dissociation of the capping units by enzymes or DNAzymes, the dissociation of capping units by the formation of aptamer-ligand complexes (particularly ligands acting as biomarkers for different diseases), and the use of light for the photochemical unlocking of the DNA gates. Different issues related to the targeting of the different drug-loaded carriers to cancer cells, the switchable ON/OFF release of the drug loads, and the selective cytotoxicity of the drug-loaded carriers toward cancer cells are discussed. Finally, the future perspectives of the stimuli-responsive DNA-based, drug-loaded micro/nanocarriers for future nanomedicine and, in particular, the development of autonomous sense-and-treat systems are addressed.

Partitioning of Cs+ and Na+ ions by dibenzo-18-crown-6 ionophore in biphasic aqueous systems of octanol and ionic liquid

Abstract Molecular dynamics (MD) simulations were carried out to obtain molecular level insights on the behavior of Cs+/Na+ ions at the water–ionic liquid and water–octanol interface in the presence of dibenzo-18-crown-6 (DB18C6) ionophore with an aim to compare an ionic liquid (IL) to a octanol as receiving organic solvent phase. It was observed that the rate of phase separation for the octanol system was rapid as compared to the IL system. The free crown ethers (CE) were found to be highly solvated by the IL phase. A dual cationic exchange mechanism was observed at the [BMIM]/water interface. The [BMIM]+ cation was found to exchange with both the metal ions in aqueous phase as well as with the metal ion aided by the ionophore. The self-diffusion coefficient of the 1:2 complex (0.07×10−9 m2/s) at the octanol/water interface were found to be smaller than that of 1:1 complex (0.37 and 0.14×10−9 m2/s). It was observed that the surface tension of ILs decreased in the presence of complexes and free CE, whereas the surface tension of water was found to increase in presence of salts (Cs+NO3 − and Na+NO3 −). The experimentally determined value of D Cs was found to be quite high in IL phase (1.595) compared to the octanol phase (0.139) in presence of CE. The kinetics of Cs+ was found to be very fast having rate with values of k 1 ^ $\widehat {{k_1}}$ =1.79×10−12 s−1 and k 2 ^ $\widehat {{k_2}}$ =0.205×10−12 s−1 in IL and water phase, respectively. The present results may help us in understanding the role of diluents in the assisted metal ion extraction but also in the future design of diluents and ionophore.

Lipase-catalyzed synthesis of sucrose monoester: Increased productivity by combining enzyme pretreatment and non-aqueous biphasic medium

Sucrose monocaprate was synthesized by carrying out a lipase-catalyzed transesterification in a non-aqueous biphasic medium. Vinyl caprate was mechanically dispersed into a solution of sucrose in DMSO. The use of DMSO allowed increasing sucrose concentration up to 0.7M (in DMSO). The denaturing effect of DMSO on lipase was avoided by pretreatment of lipase by pH adjustment in the presence of crown ether. This pretreatment maintained a significant catalytic activity which led to 0.2M sucrose monoester within 1h at 50°C, which represented higher productivity than already reported. Detailed structural characterization revealed that only monoester was recovered and the 2-O-acylated sucrose monocaprate was the major isomer in the final product.

Modulation of the Excited-State Dynamics of 2,2'-Bipyridine-3,3'-diol in Crown Ethers: A Possible Way To Control the Morphology of a Glycine Fibril through Fluorescence Lifetime Imaging Microscopy.

In this article, we have investigated the modulation of excited-state intramolecular double proton transfer (ESIDPT) dynamics of 2,2'-bipyridine-3,3'-diol (BP(OH)2) in two crown ethers (CEs), namely, 18-Crown-6 (18C6) and 15-Crown-5 (15C5). From steady-state UV-visible measurements, we have shown that there is no significant interaction between the dienol tautomeric form of BP(OH)2 and two CEs. However, in the presence of CEs, an additional emission band (∼415 nm) is generated along with the diketo tautomer band (∼465 nm). In time-resolved analysis, we have observed the generation of ∼260 ps rise component in the presence of 18C6. Therefore, by combining the results of steady-state and time-resolved emissions, we have proposed that the water-assisted ESIDPT route of BP(OH)2 generates a hydronium ion (H3O+) in the excited state. 18C6 binds nicely to this H3O+ ion. As a result, retarded ESIDPT dynamics is observed in 18C6. However, as 15C5 cannot bind H3O+ properly, no rise component is found. With the addition of potassium chloride (KCl), the contribution of the rise component decreases due to unavailability of free 18C6 cavity to capture the H3O+ ion generated in the excited state. Addition of calcium chloride (CaCl2) leads to complete removal of the rise component due to the inhibition of the water-assisted ESIDPT route. From wavelength-dependent behavior, we have observed that the rise component is present only at 465 nm in 18C6. We have also shown that the fibrillar morphology of glycine can be successfully probed through fluorescence lifetime imaging microscopy using BP(OH)2 as an imaging agent. Modulation of fibrillar morphology has been found in the presence of two CEs. The interaction of glycine fiber with CEs can be explained by lifetime distribution analysis.

Formation of highly conductive layers by crown ether molecules on the surface of a lithium anode at low temperatures

A sharp (250-fold) decrease in charge transfer resistance at the lithium anode/electrolyte interface in the presence of crown ethers in a range from 0 to –18 °C was found.

Unveiling the Nature of Supramolecular Crown Ether-C60 Interactions

Construction of artificial photoactive devices capable of mimicking photosynthesis by transforming light into chemical potential is one of the most sought objectives in the quest for new sources of energy.[1] For this, chemists have developed a wide variety of covalent and supramolecular structures based on donor-acceptor systems where photon energy is used to produce a photoinduced electron-transfer from the donor to the acceptor moiety.[2] As a result, in 2010 our group developed a supramolecular exTTF-B18C6 ether receptor for fullerenes which showed an extraordinary affinity (K a 5.01 × 106 M-1) towards [60]fullerene.[3]This was an outstanding result as previous attempts to complex fullerenes by only one unit of exTTF were unsuccessful. Thus, the presence of crown ethers as recognition patterns seemed to be of paramount importance. In this work, we have further studied this unexpected result and exploited the potentiality of crown ethers as recognition motifs for fullerenes. Therefore, a series of exTTF-(crown ether)2 receptors, designed to host C60, has been prepared. The size of the crown ether and the nature of the heteroatoms have been systematically changed to fine tune the association constants. Electrochemical measurements and transient absorption spectroscopy assisted in corroborating a charge transfer in the ground state and in the excited state leading to the formation of radical ion pairs featuring lifetimes in the range from 12 to 21 ps. To rationalize the nature of the exTTF-(crown ether)2•C60 stabilizing interactions, theoretical calculations have been carried out, prompting to a synergetic interplay of donor-acceptor, π-π, n-π and CH···π interactions, which is the basis for the affinity of our novel receptors towards C60.

Percolative Behavior Models Based on Artificial Neural Networks for Electrical Percolation of AOT Microemulsions in the Presence of Crown Ethers as Additives

Abstract A series of models, based on artificial neural networks, of the percolative behaviour of AOT microemulsions in the presence of crown ethers as additives have been developed. Input variables, related to the chemical structure of crown ethers and their packing with surfactant film, have been selected. As a result, a model has been chosen with a good forecast capability for percolation threshold of such microemulsions.

Alkali-Metal Ion Catalysis in Nucleophilic Substitution Reactions of 5-Nitro-8-quinolyl Picolinate with Alkali Metal Ethoxides: Effect of Modification of Nonleaving Group from Benzoyl to Picolinyl on Reactivity and Transition State Structure

A kinetic study on nucleophilic substitution reaction of 5-nitro-8-quinolyl picolinate (6) with alkali-metal ethoxides (EtOM; M = K, Na, and Li) in anhydrous ethanol is reported. The plot of <TEX>$k_{obsd}$</TEX> vs. [EtOM] curves upward in the absence of crown ethers but is linear with significantly decreased reactivity in the presence of crown ethers. Dissection of <TEX>$k_{obsd}$</TEX> into <TEX>$k_{EtO}$</TEX>- and <TEX>$k_{EtOM}$</TEX> (i.e., the second-order rate constants for the reactions with the dissociated <TEX>$EtO^-$</TEX> and ion-paired EtOM, respectively) has revealed that the ion-paired EtOM is significantly more reactive than the dissociated <TEX>$EtO^-$</TEX> (e.g., <TEX>$k_{EtOM}/k_{EtO^-}$</TEX> = 33.4-141). This indicates that the reaction of 6 is catalyzed by <TEX>$M^+$</TEX> ions in the order <TEX>$Na^+$</TEX> > <TEX>$Li^+$</TEX> > <TEX>$K^+$</TEX> and the catalytic effect disappears in the presence of a proper crown ether. Picolinate ester 6 is much more reactive and is more strongly catalyzed by <TEX>$M^+$</TEX> ions than 5-nitro-8-quinolyl benzoate (5). It has been concluded that <TEX>$M^+$</TEX> ions catalyze the reaction of 6 by increasing electrophilicity of the reaction center through a cyclic transition state, which is structurally not possible for the reaction of 5.

Enhanced Extraction and Separation of Zr(IV) and Hf(IV) from Acidic Chloride Solutions Using 4-Sebacoylbis(1-phenyl-3-methyl-5-pyrazolone) in Presence of Crown Ethers

Enhanced Extraction and Separation of Zr(IV) and Hf(IV) from Acidic Chloride Solutions Using 4-Sebacoylbis(1-phenyl-3-methyl-5-pyrazolone) in Presence of Crown Ethers

Terahertz time-domain and FTIR spectroscopic study of interaction of α-chymotrypsin and protonated tris with 18-crown-6

Chymotrypsin activity in nonaqueous solvents increases in the presence of crown-ethers, presumably, due to the interaction of crown-ether molecules with surface amino-groups of protein. FTIR and THz-TDS spectroscopic techniques are used to study the interaction of crown-ether with chymotrypsin and protonated tris(hydroxymethyl)aminomethane, which serves as a model of the amino-groups of protein. Spectra measured at different relative molar concentrations indicate the interaction of the components. The spectral changes and variations in the absolute values of the absorption coefficient are discussed. The similarity of the spectral changes is demonstrated for protein-crown–ether and tris-crown–ether samples.

Reductive Deprotonation and Dehydrogenation of Phenylhydrazine at a Nickel Center To Give a Nickel Diazenido Complex

The reaction of [L(tBu)Ni(OEt(2))] (where L(tBu) = [HC(C(t)BuNC(6)H(3)((i)Pr)(2))(2)](-)) with phenylhydrazine leads to the phenylhydrazido(1-) complex [L(tBu)Ni(η(2)-NPhNH(2))] (1) with concomitant formation of H(2). Treatment of 1 with potassium graphite in the presence of crown ether again leads to H(2) evolution and affords the heterobimetallic complex [L(tBu)Ni(μ-η(2):η(2)-NPhNH)]K(18-crown-6) containing the doubly deprotonated phenylhydrazido(2-) ligand. 1 can be converted into a phenyldiazenido complex [L(tBu)Ni(η(1)-NNPh)] in the course of a dehydrogenation reaction employing 1,2-diisopropylazo dicarboxylate (DIAD) as the oxidant.

Oxidative desulfurization of diesel fuels in the presence of crown ethers and transition metal peroxo complexes

The catalytic activity of crown ethers and their complexes with transition metal cations in diesel fuel desulfurization has been investigated. With dibenzo-18-crown-6 in combination with NbCl5, the total sulfur content of the fuel can be reduced by a factor of 2 or larger. The effects of the reaction temperature, nature of metal and crown ether, and sorbent acidity on the residual sulfur content are reported. Methods of separation of oxidized compounds, namely adsorption chromatography and extraction are compared in terms of desulfurization efficiency. The latter method is demonstrated to be preferable.

Synthesis of large-pore mesoporous nanocrystalline TiO2 microspheres

Thermally stable mesoporous nanocrystalline TiO2 with strongly pronounced spherical morphology was obtained in the presence of crown ether as structure-directing agent using sol–gel synthesis in combination with hydrothermal treatment. Introduction of small amount of surfactant additive to reaction mixture leads to increasing degree of crystallization and modification of texture of titanium oxide. Anatase is the only crystal phase formed at calcination temperatures up to 500°C. The samples were characterized by XRD, SEM, TEM, and nitrogen adsorption–desorption methods. Microspheres with diameters ranging from 1.8 to 2.3μm are assembled by aggregation of spherical particles (~45nm) that were actually formed by primary spherical anatase particles with average crystallite size 8.3nm. The specific surface area of nanocrystalline TiO2 is 120m2/g with a pore size of 11.3nm and pore volume of 0.34cm3/g.

THz and IR Spectroscopy of Molecular Systems That Simulate Function-Related Structural Changes of Proteins

The activity of enzymes in organic solvents substantially increases in the presence of crown ethers. Tris(hydroxymethyl)aminomethane (tris) is chosen as a model compound to simulate the interaction of surface amino groups of proteins with crown ether. The methods of FTIR and time-domain THz spectroscopy are used to study the interaction of tris with 18-crown-6. The THz spectra of the complexes are measured for the first time.

A Mild and Efficient Protocol for the Protection of 3-Hydroxychromones Under Phase-Transfer Catalysis

A mild and efficient protocol for the introduction of different protecting groups on 3-hydroxychromones (3-HCs) under phase-transfer catalysis conditions in toluene or dichloromethane/aqueous potassium hydroxide system in the presence of crown ether has been developed. The method is useful for the protection of base-sensitive chromone derivatives. Protected chromones are easier to handle and to purify, and therefore suitable for further chemical transformations. The protecting groups were cleaved cleanly using standard conditions.

Hydrogen peroxide oxidative desulfurization of model diesel fuel mixtures in the presence of crown ethers and transition metal peroxo complexes

The catalytic ability of crown ethers and their complexes with transition metal cations in the desulfurization reaction of mixtures that simulate diesel fuel has been investigated. It has been found that the use of monoaza-15-crown-5-ethers with an admixture of NbCl5 resulted in a fourfold decrease of the total sulfur content, thereby indicating partial oxidation of benzothiophenes and dibenzothiophenes. The complexation of azacrown ethers with NbCl5 has been studied by 1H NMR spectroscopy. A moderate efficiency of adsorption purification procedure using silica and alumina for both initial and oxidized model mixtures has been revealed.

Aggregation of Perfluoroctanoate Salts Studied by 19F NMR and DFT Calculations: Counterion Complexation, Poly(ethylene glycol) Addition, and Conformational Effects

The aggregation of perfluoroctanoate salts in H(2)O is studied by (19)F NMR on solutions of LiPFO, NaPFO, and CsPFO, without and with the addition of two poly(ethylene glycol) (PEG) oligomers of molecular weight 1500 and 3400 Da, respectively, and with the addition of suitable crown ethers. The (19)F chemical shift (cs) trends are monitored, at 25 °C, in a concentration range including the critical micellar concentration (cmc) or, in the presence of PEG, the critical aggregation concentration (cac). The cac values in the samples with PEG are lower than the cmc values of the corresponding samples without PEG; moreover, the (19)F cs trends above the cac and above the polymer saturation concentration reveal and help to explain some peculiarities of the aggregation process of PEG on PFO micelles, which, in the first step, seems to occur while the surfactant concentration in water is still increasing. Also in LiPFO/H(2)O or NaPFO/H(2)O solutions containing 12-crown-4 or 15-crown-5 ethers, suitable to complex Li(+) or Na(+) ions, respectively, the cmc decreases. On the other hand, the micellization process in the presence of crown ethers does not show other peculiarities. The prevailing conformations of the PFO chain are discussed on the basis of quantum-mechanical calculations. The theoretical chemical shifts were computed at the DFT level of theory, taking into account the effects of the environment by means of the IEF-PCM method. The helical structure is the most stable one, but anti conformations are easily accessible, in both the aqueous and fluorinated environment. The comparison between computed and experimental chemical shifts indicates that anti conformations are more important in the micelles than in water and in CsPFO micelles than in LiPFO or NaPFO ones.