Metal Cation Binding Research Articles

Recognition-Controlled Membrane Translocation for Signal Transduction across Lipid Bilayers.

Membrane signaling proteins transduce information across lipid bilayer membranes in response to extra-cellular binding of chemical messengers. The design of chemical systems that initiate transmembrane signal transduction through molecular binding events is a critical step toward preparing responsive synthetic vesicles. Here we report a vesicle-based signaling system controlled by a metal cation binding event. Competition between binding of copper ions to a membrane-embedded synthetic transducer and to an extra-vesicle messenger (EDTA) is used to control translocation of the transducer across the lipid bilayer. The translocation process is coupled to activation of a catalyst that turns over encapsulated substrates on the inside of the vesicle to generate an amplified fluorescence output signal. External EDTA and copper ions can be used to reversibly switch catalysis inside the vesicles on and off in a controlled manner.

Direct investigation on energy bandgap of Si added ZnSnO system for stability enhancement by X-ray photoelectron spectroscopy

Amorphous silicon-zinc-tin-oxide (a-SZTO) has been fabricated by radio frequency (RF) magnetron sputtering at room temperature. Direct investigation on the change of the band gap of a-SZTO depending on Si contents was performed simply by combining the results of x-ray photoelectron spectroscopy (XPS), Kelvin probe microscopy, and UV-VIS optical spectrometer. The chemical properties, such as metal cation binding energy and oxygen vacancies related O 1s peak were measured by using XPS. We report on the electrical performance and the thermal stability of SZTO TFTs related with energy band gap analysis depending on Si contents. The activation energy and the density of states were calculated by applying temperature stress (TS). As Si contents increased, we have observed EC-EF was increased. The relative position of the EC shifts far from the EF because the shallow and/or deep states, such as ionized oxygen vacancies were decreased. It was confirmed that sample 3 with high Si content had the best stability (ΔVTH of 0.21 V) on thermal stress. This investigation on the variation of energy band gap and the stability of a-SZTO has represented possible step towards not only the control of band gap but also controllable achievement of electrical property and stability using optimized Si contents.

Cooperative Binding of Metal Cations to a Spiropyran‐Conjugated Calix[4]arene

AbstractCooperative binding occurs frequently in biological systems, but is less exploited in synthetic receptor molecules. We synthesized a bis(spiropyran)‐conjugated calix[4]arene (bSPTC) through a 1,2,3‐triazole linker and found cooperativity in bindings of a series of divalent metal cations (Ca2+, Mg2+, Cd2+, Co2+, Fe2+, Hg2+, Ni2+, and Zn2+). UV‐vis absorption spectroscopy and principle component analysis were used to differentiate the recognized metal cations. The binding stoichiometry was determined by various continuation methods, which indicated that bSPTC has two binding ratios, 1:1 for Zn2+ and 1:2 for all other metal cations. Importantly, positive cooperativity was suggested for the 1:2 stoichiometry of bSPTC to Mg2+ ions from the sigmoidal saturation curve and Hill equation analysis. Density functional theory (DFT) calculations imply that the first binding of Mg2+ to one spiropyran destabilizes the second spiropyran, which facilitates binding of a second Mg2+.

An Ion-Responsive Pincer-Crown Ether Catalyst System for Rapid and Switchable Olefin Isomerization.

Rapid, selective, and highly controllable iridium-catalyzed allylbenzene isomerization is described, enabled by tunable hemilability based on alkali metal cation binding with a macrocyclic "pincer-crown ether" ligand. An inactive chloride-ligated complex can be activated by halide abstraction with sodium salts, with the resulting catalyst [κ5 -(15c5 NCOPiPr )Ir(H)]+ exhibiting modest activity. Addition of Li+ provides a further boost in activity, with up to 1000-fold rate enhancement. Ethers and chloride salts dampen or turn off reactivity, leading to three distinct catalyst states with activity spanning several orders of magnitude. Mechanistic studies suggest that the large rate enhancement and high degree of tunability stem from control over substrate binding.

An Ion‐Responsive Pincer‐Crown Ether Catalyst System for Rapid and Switchable Olefin Isomerization

AbstractRapid, selective, and highly controllable iridium‐catalyzed allylbenzene isomerization is described, enabled by tunable hemilability based on alkali metal cation binding with a macrocyclic “pincer‐crown ether” ligand. An inactive chloride‐ligated complex can be activated by halide abstraction with sodium salts, with the resulting catalyst [κ5‐(15c5NCOPiPr)Ir(H)]+ exhibiting modest activity. Addition of Li+ provides a further boost in activity, with up to 1000‐fold rate enhancement. Ethers and chloride salts dampen or turn off reactivity, leading to three distinct catalyst states with activity spanning several orders of magnitude. Mechanistic studies suggest that the large rate enhancement and high degree of tunability stem from control over substrate binding.

Does specific parameterization of WHAM improve the prediction of copper competitive binding and toxicity on plant roots?

We aimed at assessing whether the binding and rhizotoxicity of metal cations such as copper that exhibit high affinity for plant roots could be adequately predicted using the Windermere Humic Aqueous Model (WHAM) default parameterization. Accordingly, we first compared the ability of the default parameterization of WHAM and a specific parameterization for terrestrial higher plants (WHAM-THP) to model the competitive binding of copper on wheat (Triticum aestivum L.) and tomato (Solanum lycopersicum L.) roots. Secondly, in an external dataset, we evaluated the ability of WHAM-THP to predict the copper concentration and toxicity to pea (Pisum sativum L.) roots relative to WHAM. WHAM-THP estimates generated a slightly better fit for the competitive binding of copper on wheat and tomato roots (log10 of the root-mean-square error, RMSE = 0.15) than WHAM estimates (RMSE = 0.24). WHAM-THP estimates slightly better fitted the copper concentration in pea roots (RMSE ≤ 0.49) than WHAM estimates (RMSE ≤ 0.67) at low copper exposure and pH ≤ 5. However, WHAM-THP did not at all improve the prediction of copper toxicity to pea roots (RMSE = 13% as also for WHAM). We thus conclude that, although the default parameterization of WHAM does not neatly predict the binding of metal cations on roots, it could however be used with confidence in predictive ecotoxicology for terrestrial higher plants without any specific parameterization.

Binding of Monovalent and Multivalent Metal Cations to Polyethylene Oxide in Methanol Probed by Electrophoretic and Diffusion NMR.

Complex formation in methanol between monodisperse polyethylene oxide (PEO) and a large set of cations was studied by measuring the effective charge acquired by PEO upon complexation. Quantitative data were obtained at a low ionic strength of 2 mM (for some salts, also between 0.5 and 6 mM) by a combination of diffusion nuclear magnetic resonance (NMR) and electrophoretic NMR experiments. For strongly complexing cations, the magnitude of the acquired effective charge was on the order of 1 cation per 100 monomer units. For monovalent cations, the relative strength of binding varies as Na+ < K+ ≈ Rb+ ≈ Cs+, whereas Li+ exhibited no significant binding. All polyvalent cations bind very weakly, except for Ba2+ that exhibited strong binding. Anions do not bind, as is shown by the lack of response to the chemical nature of anionic species (perchlorate, iodide, or acetate). Diffusion experiments directly show that the acetate anion with monovalent cations does not associate with PEO. Considering all cations, we find that the observed binding does not follow any Hofmeister order. Instead, binding occurs below a critical surface charge density, which indicates that the degree of complexation is defined by the solvation shell. A large solvation shell prevents the binding of most multivalent ions.

Metal cation binding to acetylenic tetrathiafulvalene–pyridine conjugates: affinity tuned by preorganization and cavity size

A series of three structurally related mono-, bidentate and macrocyclic TTF-pyridine hosts were prepared and titrated with several metal cations including Ag+ and Pb2+ and studied using NMR- and UV–vis spectroscopy and cyclic voltammetry. For Ag+, we found an eightfold increase in binding affinity between the bidentate and macrocyclic host and conversely, for Pb2+, a 100-fold drop. Density functional theory (DFT) calculations support the increased binding affinity for Ag+ is due to an N⋯N distance for the uncomplexed macrocycle very much suited for binding of Ag+ but being too small for Pb2+. The bidentate host, on the other hand, is of a suitable size for Pb2+.

Correlating the Activity of Rhodium(I)-Phosphite-Lariat Ether Styrene Hydroformylation Catalysts with Alkali Metal Cation Binding through NMR Spectroscopic Titration Methods

Alkali metal salts can affect both the activities and regioselectivities of alkene hydroformylation catalysts containing polyether-functionalized phosphorus-donor ligands; however, it is unclear whether these effects arise from direct alkali metal cation binding to the active catalysts. To gain more insight into these effects, a series of phosphite-lariat ether ligands derived from the alkali metal cation binding agents 2-hydroxymethyl-12-crown-4 and 2-hydroxymethyl-15-crown-5 have been prepared. Rhodium(I) complexes of these ligands have been evaluated as styrene hydroformylation catalysts in the absence and presence of a variety of alkali metal salts. The activities of catalysts containing phosphites derived from 2,2′-biphenol or 1,1′-binaphthol increased significantly (up to 92%) in the presence of alkali metal cations that are “moderately oversized” for archetypal binding to the crown cavity. When this criterion are not met, a decrease in the catalytic activity is observed upon addition of an alkali m...

ChemInform Abstract: Controlled Folding, Motional, and Constitutional Dynamic Processes of Polyheterocyclic Molecular Strands

AbstractReview: [84 refs.

Protein-protein interaction network analysis of cirrhosis liver disease.

Evaluation of biological characteristics of 13 identified proteins of patients with cirrhotic liver disease is the main aim of this research. In clinical usage, liver biopsy remains the gold standard for diagnosis of hepatic fibrosis. Evaluation and confirmation of liver fibrosis stages and severity of chronic diseases require a precise and noninvasive biomarkers. Since the early detection of cirrhosis is a clinical problem, achieving a sensitive, specific and predictive novel method based on biomarkers is an important task. Essential analysis, such as gene ontology (GO) enrichment and protein-protein interactions (PPI) was undergone EXPASy, STRING Database and DAVID Bioinformatics Resources query. Based on GO analysis, most of proteins are located in the endoplasmic reticulum lumen, intracellular organelle lumen, membrane-enclosed lumen, and extracellular region. The relevant molecular functions are actin binding, metal ion binding, cation binding and ion binding. Cell adhesion, biological adhesion, cellular amino acid derivative, metabolic process and homeostatic process are the related processes. Protein-protein interaction network analysis introduced five proteins (fibroblast growth factor receptor 4, tropomyosin 4, tropomyosin 2 (beta), lectin, Lectin galactoside-binding soluble 3 binding protein and apolipoprotein A-I) as hub and bottleneck proteins. Our result indicates that regulation of lipid metabolism and cell survival are important biological processes involved in cirrhosis disease. More investigation of above mentioned proteins will provide a better understanding of cirrhosis disease.

Controlled Folding, Motional, and Constitutional Dynamic Processes of Polyheterocyclic Molecular Strands.

General design principles have been developed for the control of the structural features of polyheterocyclic strands and their effector-modulated shape changes. Induced defined molecular motions permit designed enforcement of helical as well as linear molecular shapes. The ability of such molecular strands to bind metal cations allows the generation of coiling/uncoiling processes between helically folded and extended linear states. Large molecular motions are produced on coordination of metal ions, which may be made reversible by competition with an ancillary complexing agent and fueled by sequential acid/base neutralization energy. The introduction of hydrazone units into the strands confers upon them constitutional dynamics, whereby interconversion between different strand compositions is achieved through component exchange. These features have relevance for nanomechanical devices. We present a morphological and functional analysis of such systems developed in our laboratories.

Modeling the CH Stretch Vibrational Spectroscopy of M(+)[Cyclohexane] (M = Li, Na, and K) Ions.

The CH stretch vibrations of M(+)[cyclohexane][Ar] (M = Li, Na, and K) cluster ions were theoretically modeled. Results were compared to the corresponding infrared photodissociation spectra of Patwari and Lisy [ J. Chem. Phys A 2007 , 111 , 7585 ]. The experimental spectra feature a substantial spread in CH stretch vibration frequencies due to the alkali metal cation binding to select hydrogens of cyclohexane. This spread was observed to increase with decreasing metal ion size. Exploring the potential energy landscape revealed the presence of three conformers whose energy minima lie within ∼1 kcal of each other. It was determined that in all conformers the metal ion interacts with three hydrogen atoms; these hydrogen atoms can be either equatorial or axial. The corresponding spectra for these conformers were obtained with a theoretical model Hamiltonian [ J. Chem. Phys. 2013 , 138 , 064308 ] that consists of local mode CH stretches bilinearly coupled to each other and Fermi coupled to lower frequency modes. Frequencies and coupling parameters were obtained from electronic structure calculations that were subsequently scaled on the basis of previous studies. Theoretical spectra of a single low energy conformer were found to match well with the experimental spectra. The relative frequency shifts with changing metal ion size were accurately modeled with parameters generated by using ωB97X-D/6-311++(2d,p) calculations.

Binding Selectivity of Macrocycle Ionophores in Ionic Liquids versus Aqueous Solution and Solvent‐free Conditions

The understanding of supramolecular recognition in room-temperature ionic liquids (RTILs) is key to develop the full potential of these materials. In this work, we provide insights into the selectivity of the binding of alkali metal cations by standard cyclodextrin and calixarene macrocycles in RTILs. A direct laser desorption/ionization mass spectrometry approach is employed to determine the relative abundances of the inclusion complexes formed through competitive binding in RTIL solutions. The results are compared with the binding selectivities measured under solvent-free conditions and in water/methanol solutions. Cyclodextrins and calixarenes in which the peripheral OH groups are substituted by bulkier side groups preferentially bind to Cs(+) . Such specific ionophoric behavior is substantially enhanced by solvation effects in the RTIL. This finding is rationalized with the aid of quantum mechanical calculations, in terms of the conformational features and steric interactions that drive the solvation of the inclusion complexes by the bulky RTIL counterions.

Metal ion-responsive photonic colloidal crystalline micro-beads with electrochemically tunable photonic diffraction colours

This work reports a metal ion-responsive photonic colloidal crystalline (PCC) microspheric material and the convenient, reversible switching of its photonic diffraction properties by the redox reaction of copper in a simple electrochemical cell. The PCC micro-beads were fabricated from the orderly three-dimensional packing of core-shell nanoparticles with hydrogel coatings laden with anionic phosphate functionalities. Electrostatic binding of metal cations by the hydrogel coating lowered its internal osmotic pressure and caused shrinkage of the core-shell nanoparticles. This brought about a blue-shift of the photonic diffraction λmax of the micro-beads. The maximum shift, from 650 to 590nm (colour change from magenta to green) was found to be produced by Cu2+ at a concentration of 1mM. A simple electrochemical photonic device was constructed by sandwiching a suspension of the photonic micro-beads in 1mM Cu2+ solution between two transparent ITO-glass electrodes. Photonic diffraction colour of the micro-beads was reversibly switched via the electrochemical reduction/oxidation of Cu2+ inside the device. The colour change can be observed by naked eyes under ambient light. This concept of rapid electrochemical regulation of photonic diffraction colour of PCC micro-beads may be useful in reflective displays applications.

Effect of micronization on the physicochemical properties of insoluble dietary fiber from citrus (Citrus junos Sieb. ex Tanaka) pomace.

The aim of this work was to study the effect of micronization (mechanical and jet grindings) on the physicochemical properties of the insoluble dietary fiber from citrus pomace in comparison with ordinary grinding. The results showed that micronization treatment effectively pulverized the IDF-CP powders to micron scale and significantly increased the soluble dietary fiber content (p < 0.05). Compared with mechanical grinding, jet grinding was more effective in size reduction and resulted in IDF-CP powders with narrower particle size distributions. Micronized IDF-CP powders had smaller particle size, smoother surface, higher fluidity, cation-exchange capacity, and metal cation binding capacity values, but lower water holding capacity, oil holding capacity, and swelling capacity values. These functional properties were significantly dependent on surface area and particle size (D0.5). The present study suggested that micronization treatments could modify functional properties of IDF-CP powders, which promotes their use in food applications.

Effects of weak organic acids on the size distribution and size-dependent metal binding of humic substances as studied by flow field-flow fractionation

Size distribution of humic substances is a central issue when one considers stability against aggregation and transportation of HS with bound metal ions. In the present study, the effects of low molecular weight weak organic acids, which are commonly used as pH-buffering agents, on the hydrodynamic size of purified Aldrich humic acid (PAHA) at pH 7 were investigated by means of flow field-flow fractionation (Fl-FFF). The modal size of the PAHA fractogram with 3-morpholino propane sulfonic acid (MOPS) was similar to that obtained without organic acids, whereas that with tris(hydroxymethyl)aminomethane (Tris) increased and that with 2-(N-morpholino)ethanesulfonic acid (MES) decreased relative to that without organic acids. Differences in the charges of the weak organic acids were likely the cause of this difference in the fractograms. After selecting MOPS as an appropriate pH-buffering agent, the size-dependency of metal cation binding (UO22+ and Eu3+) by PAHA was studied at low loading conditions (10−6–10−8M). The binding of the metal ions was not homogeneous with respect to the size of the PAHA particles; larger particles had greater affinity for both UO22+ and Eu3+. The modal size for the hydrodynamic diameter of PAHA was 2nm, whereas the modal size for the bound metal ions per unit mass of PAHA was 5nm. This contrast indicates that PAHA particles with 5nm sizes strongly bind UO22+ and Eu3+ in terms of their affinity and the number of relevant sites. Size variation of PAHA induced by the addition of multivalent cations was insignificant in the present study.

Time-dependent fluorescence Stokes shift and molecular-scale dynamics in alginate solutions and hydrogels

Alginates are water-soluble polysaccharides that bind metal cations like Ca2+, producing hydrogels. Here, we have determined time-dependent fluorescence Stokes shift of a guest fluorophore to elucidate molecular length-scale local dynamics within alginate-based solutions and hydrogels. We find a major bulk water-like fast response emanating from large water pools interspersed between the polysaccharide chains, together with a minor but significant slow response. The possible origin of the latter is discussed in terms of either water molecules constituting the polysaccharide hydration shells or ion distribution and diffusion around the fluorophore dipole, or microscopic structural inhomogeneity inside the alginate-based media.

UV photofragmentation and IR spectroscopy of cold, G-type β-O-4 and β-β dilignol-alkali metal complexes: structure and linkage-dependent photofragmentation.

Ultraviolet photofragmentation spectroscopy and infrared spectroscopy were performed on two prototypical guaiacyl (G)-type dilignols containing β-O-4 and β-β linkages, complexed with either lithium or sodium cations. The complexes were generated by nanoelectrospray ionization, introduced into a multistage mass spectrometer, and subsequently cooled in a 22-pole cold ion trap to T ≈ 10 K. A combination of UV photofragment spectroscopy and IR-UV double resonance spectroscopy was used to characterize the preferred mode of binding of the alkali metal cations and the structural changes so induced. Based on a combination of spectral evidence provided by the UV and IR spectra, the Li(+) and Na(+) cations are deduced to preferably bind to both dilignols via their linkages, which constitute unique, oxygen-rich binding pockets for the cations. The UV spectra reflect this binding motif in their extensive Franck-Condon activity involving low-frequency puckering motions of the linkages in response to electronic excitation. In the pinoresinol•Li(+)/Na(+) complexes involving the β-β linkage, the spectra also showed an inherent spectral broadening. The photofragment mass spectra are unique for each dilignol•Li(+)/Na(+) complex, many of which are also complementary to those produced by collision-induced dissociation (CID), indicating the presence of unique excited state processes that direct the fragmentation. These results suggest the potential for site-selective fragmentation and for uncovering fragmentation pathways only accessed by resonant UV excitation of cold lignin ions.

FRET study of G-quadruplex forming fluorescent oligonucleotide probes at the lipid monolayer interface

Spectral properties and G-quadruplex folding ability of fluorescent oligonucleotide probes at the cationic dioctadecyldimethylammonium bromide (DODAB) monolayer interface are reported. Two oligonucleotides, a 19-mer bearing thrombin binding aptamer sequence and a 21-mer with human telomeric sequence, were end-labeled with fluorescent groups (FAM and TAMRA) to give FRET probes F19T and F21T, respectively. The probes exhibited abilities to fold into a quadruplex structure and to bind metal cations (Na+ and K+). Fluorescence spectra of G-quadruplex FRET probes at the monolayer interface are reported for the first time. Investigations included film balance measurements (π-A isotherms) and fluorescence spectra recording using a fiber optic accessory interfaced with a spectrofluorimeter. The effect of the presence of DODAB monolayer, metal cations and the surface pressure of monolayer on spectral behavior of FRET probes were examined. Adsorption of probe at the cationic monolayer interface resulted in the FRET signal enhancement even in the absence of metal cations. Variation in the monolayer surface pressure exerted rather modest effect on the spectral properties of probes. The fluorescence energy transfer efficiency of monolayer adsorbed probes increased significantly in the presence of sodium or potassium ion in subphase, which indicated that the probes retained their cation binding properties when adsorbed at the monolayer interface.