Cation-ligand Interaction Research Articles

Impact of Ionic Correlations on Selective Salt Transport in Ligand-Functionalized Polymer Membranes

We present a theoretical formalism quantifying the interplay between cation–ligand interaction strength and dynamical ionic correlations on salt diffusivities in the transport of single salt and binary (mixed) salts through ligand-functionalized polymer membranes. The Onsager framework is utilized to capture the effect of correlated ionic motions on salt diffusivities. Such a framework is implemented in the context of a coarse-grained model of systems involving ligand-selective (LS) and/or ligand-agnostic (LA) salts in ligand-functionalized polymer membranes. Our results indicate that ionic correlations speed up the diffusivities of both LS and LA salts relative to the ideal, uncorrelated case. At small values of cation–ligand interaction strength, such a speedup arises from a general trend of positively correlated motion among the mobile ions. On the other hand, ionic correlation-mediated speedup in salt diffusivities at large values of cation–ligand interaction strength can be attributed to the correlated hopping of the cations bound to the ligands in both single and mixed salt systems. At low cation–ligand interaction strengths, ionic correlations reduce the ratio of salt diffusivities of LS to LA salts (relative to the uncorrelated case) in single salt systems. Disparately, ionic correlations enhance the ratio of salt diffusivities of LS to LA salts (relative to the uncorrelated case) at large values of the cation–ligand interaction strengths in both single and mixed salt systems. We observe that single salt systems exhibit higher LS to LA salt diffusivity ratios than their mixed salt analogs up to a certain value of the cation–ligand interaction strength, beyond which the salt diffusivity ratios for the two systems converge.

Impact of Cation–Ligand Interactions on the Permselectivity of Ligand-Functionalized Polymer Membranes in Single and Mixed Salt Systems

We conduct molecular dynamics simulations to study the effects of cation–ligand interactions on salt permeation in ligand-functionalized polymer membranes. Our results indicate that salt partitioning into the membrane increases while the salt diffusivity decreases with enhancement in the cation–ligand interaction strength. Moreover, the increase in partitioning dominates the decline in salt diffusivity, leading to a net enhancement in salt permeation at higher cation–ligand interaction strengths. Equimolar binary mixed salt systems (having a common anion) generally exhibit much more selective partitioning of salts into the membrane as compared to their single salt counterparts. However, single salt systems exhibit much higher ratio of salt diffusivities than their mixed salt analogues. The permselectivity data closely resemble the trends in selective partitioning for both single and mixed salt systems, indicating that salt diffusivity selectivity plays a weaker role than salt partitioning selectivity in dictating the selective permeation of salts across the membrane.

Metal cation-ligand interaction modulated mono-network ionic conductive hydrogel for wearable strain sensor

Metal cation-ligand interaction modulated mono-network ionic conductive hydrogel for wearable strain sensor

Conceptual Design of Yttrium Oxyhydrides: Phase Diagram, Structure, and Properties

The synthesis of stable yttrium oxyhydride-type compounds raised a principal question regarding the key factors that may be responsible for formation routes and structural features of these attractive materials. For solving this challenge, the interplay between chemical composition and crystalline architectures has been theoretically explored via modeling structural transformations caused by the gradual oxidation of the host metal hydride system. The close combination of group-theory methods, mixed-anion chemistry arguments, and relevant density functional theory calculations provided us with the opportunity to suggest and characterize the candidate models for most probable stoichiometric versions of yttrium oxyhydrides. The predicted chemical compositions along with the crystallization results have been summarized in the phase diagram. It is shown that the cation–ligand interaction governs the structural stability which is achieved by matching favorable crystallographic positions of the oxygen and hydrog...

X-ray absorption spectroscopy of vanadium dioxide thin films across the phase-transition boundary

X-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy of the V L edge and O K edge were performed on VO{sub 2} thin films rf sputtered at various conditions. The spectra give evidence of the changes in the electronic structure depending on the film quality. XAS of the O K edge shows a decrease of the spacing between 3d{sub {pi}} and 3d{sub {sigma}} bands by 0.8 eV with concurrent broadening of both bands for the sample sputtered at lower substrate temperature and consequently having more polycrystalline and disordered character. 3d{sub {sigma}} band position appears to be more sensitive to the sample quality, indicating that the cation-ligand interaction is mostly affected likely due to the distortion of the local O coordination surrounding a V ion. The observed variation of the spectra in films of different morphologies may reflect the changes of the density of states responsible for the considerable variation of the metal-insulator transition (MIT) properties reported for VO{sub 2} thin films synthesized at different conditions. The study of the temperature dependence of the XAS spectra including repeated measurements across the MIT revealed both reversible and irreversible V L-edge and O K-edge changes. The thermal cycling of the VO{sub 2} filmsmore » through the MIT shows irreversible shifts of the conduction bands toward lower photon energies apparently caused by the sample deterioration due to the lattice transformations at the MIT. The signature of a phase transition in a VO{sub 2} film at MIT temperature (T{sub MIT}) is clearly seen in the XAS O K-edge spectra which show reversible switches of the 3d{sub {pi}} and 3d{sub {sigma}} bandwidths by approximately 20% depending on the sample being above or below T{sub MIT}.« less

Formation constants for complexes of transition-metal cations with O- and N-donor ligands in aqueous solutions

A method has been developed for the theoretical calculation of stability constants for transitionmetal complexes proceeding from molecular properties. An analytical form of the cation-ligand interaction potential including the partial bond covalence has been developed. The potential is expressed through molecular parameters and a covalence parameter; the latter may be determined from experimentally measured or theoretically calculated gas-phase data. The stability constants have been calculated for 1: 1 complexes of divalent metal cations with hydroxide anion, acetic, glycolic, and lactic acid anions, and ammonia in aqueous solutions. The covalence parameters have been calculated for the bonds of metal cations with O-and N-donor ligands.

Development, Validation, and Applications of Anisotropic Polarizable Molecular Mechanics to Study Ligand and Drug-Receptor Interactions

A correct representation of intermolecular interaction energies is necessary for reliable drug-receptor docking studies. While ab initio quantum chemistry with extended basis sets is the most accurate tool for that purpose, its use is precluded for very large molecular complexes. This constitutes the incentive for the development of accurate molecular mechanics potentials, in which the first-order electrostatic, and the second-order polarization energy contributions, are of essential importance. In this paper, we review the most important steps in the development of anisotropic, polarizable molecular mechanics (APMM) procedures. Among these, we illustrate validation tests of the ab initio-grounded, polarizable molecular mechanics potential, SIBFA (Sum of Interactions Between Fragments Ab initio computed). These are done by comparisons with parallel quantum-chemical (QC) results on representative multiply hydrogen-bonded complexes and polycoordinated complexes of one, or of two, divalent metal cations. For both kinds of complexes, the need to reproduce the non-additivity of the QC interaction energies is emphasized. One difficulty arises upon handling flexible molecules, due to the need to account simultaneously and consistently for the onset of inter- and intra-molecular polarization and charge-transfer effects. A new approach in the context of SIBFA was recently developed towards this aim, and tested in two cases of conformation-dependent cation-ligand interaction energies. The first relates to the complexes formed between the mecapto-carboxamide anion, an essential building-block of several Zn-metalloenzyme inhibitors, and Zn(II). The second relates to the complexes of the tetra-anionic pyrophosphate anion, a key building-block of ATP and GTP, with one or two divalent Zn(II) cations used as a probe. In the domain of applications, two recent studies are then presented. The first is the docking of the captopril drug to the active site of the binuclear Zn(II)- beta-lactamase enzyme. The second is the complex of a non-hydrolyzable analog of ATP with the active site of a binuclear Mg(II)-dependent kinase. An extension to an open-shell cation, Cu(II), is finally presented. The encouraging results presented in this review show that APMM procedures could be used in large-scale studies of ligand and drug-receptor interactions.

Ligand preorganization in metal ion complexation: molecular mechanics/dynamics, kinetics, and laser-excited luminescence studies of trivalent lanthanide complex formation with macrocyclic ligands TETA and DOTA.

The molecular mechanics and dynamics calculations, kinetics, and laser-excited luminescence studies were carried out for trivalent lanthanide (Ln(3+)) complexes of macrocyclic polyaminopolycarboxylate ligands TETA and DOTA (where TETA is 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid and DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to further understand the observed thermodynamic, kinetic, and structural properties and to examine how ligand preorganization affects metal ion complexation. Excitation spectroscopy (emission monitored at 614.0 nm) of the (7)F(0) --> (5)D(0) transition of Eu(3+) was used to study the aqueous properties of the Eu(3+)-TETA system. A stopped-flow spectrophotometric method was used to study the formation kinetics of the aqueous Ce(3+)-TETA/DOTA systems in the pH range 6.1-6.7. Molecular mechanics calculation results are consistent with the proposed mechanism of Ln(DOTA)(-) formation, i.e., formation of a carboxylate O-bonded precursor, followed by metal ion moving into the preformed macrocyclic cavity. For Ln(TETA)(-) formation, at least two carboxylate O-bonded intermediates have been predicted and Ln(3+) ion assisted reorganization of the TETA ligand is present. The calculated bond distances and overall structures of Ln(DOTA)(-) and Ln(TETA)(-) were in agreement with the single-crystal and solution NMR structural data. The origin of the difference in thermodynamic stability of Ln(DOTA)(-) and Ln(TETA)(-) complexes and the corresponding formation intermediates is mainly due to the differences in water-occupancy energy (i.e., whether there is an apical coordinated water molecule), the ligand strain energy, and the cation-ligand interaction energy. Kinetic studies revealed that the formation rates of the Ce(TETA)(-) complex are smaller at lower pH and temperature but become greater at higher pH and temperature, as compared to those of the Ce(DOTA)(-) complex. This is attributed to the lanthanide ion and both mono- and di-hydroxide ion assisted TETA conformational reorganization and higher kinetic activation parameters. The presence of a di-hydroxide ion assisted intermediate rearrangement pathway could make the Ce(TETA)(-) complex formation rate faster at higher pH, and the higher activation barrier makes Ce(TETA)(-) complex formation rate slower at lower pH, as compared to those of the Ce(DOTA)(-) complex.

Adsorption Mechanism and Structure of the Montmorillonite Complexes with (CH3)2XO (X=C, and S), (CH3O)3PO, and CH3–CN Molecules

The formation of complexes with different ligands in the interlayer space of montmorillonite saturated in Na+, Mg2+, Ca2+, Co2+, Cu2+, Ni2+, Fe3+, and Cr3+ was studied. Acetone, acetonitrile, dimethyl sulfoxide, and trimethylphosphate were used as ligands. The nature of the complexes was studied by means of X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, microcalorimetry, and ab initio quantum mechanical methods. In all cases, the organic ligands penetrate into the interlayer space at room temperature, forming complexes stable in vacuum with the interlayer cations. The ligand–cation ratio depends on the valence of the saturating cation. The cation–ligand interaction in these complexes has an ion–dipole electrostatic nature. The complexes are formed by the direct interaction of the oxygen or nitrogen atom of ligand and the interlayer cation. Using the quantum mechanical approach, allow us to determine the disposition of the ligand in these complexes. In all cases, only one layer of ligands is present in the stable complexes.

A Thermodynamic Study of Complexation of Ciprofloxacin and Lomefloxacin with Aluminium Ion

Gibbs energy, enthalpy and entropy for the reactions of ciprofloxacin and lomefloxacin with aluminium ion were determined. No significant differences were observed for Gibbs energy values for these fluorquinolones while significant differences were found for enthalpy and entropy. The highly positive values observed for the entropy give evidence for a strong cation-ligand interaction and point out the role of hydration in the complex formation. As the value of the entropy for the reaction with ciprofloxacin is higher than that observed for lomefloxacin one concludes that the former ligand interacts more strongly with the metal ion than the latter.

Complexation of Ln 3+ lanthanide cations with phosphoryl-containing OPR 3 ligands: a quantum-mechanics study

We report a series of ab initio quantum-mechanical calculations on M 3+ (M = La, Eu and Yb) and MCl 3 model complexes of OPR 3 ligands (R = H, Me, Et and Ph) to assess the role of R substituents and Cl − counterions on the intrinsic cation-ligand interaction energy. The calculations reveal a marked selectivity in the ligand series, as well as in the cation series. In the absence of counterions, for a given M 3+ cation, the binding sequence follows the order H < Me < Et < Ph, owing to polarization and charge-transfer effects. For a given OPR 3 ligand, the cation binding follows the sequence La 3+ < Eu 3+ < Yb 3+, as expected based on the decrease in ionic radius in this lanthanide series. Geometry optimization shows that, as the M 3+⋯OPR 3 interaction increases, the OP bond lengthens and the O⋯M 3+ distance shortens. Similar trends are observed in the R 3PO⋯MCl 3 complexes but are less pronounced, because of the ligand-anion repulsive interactions and electron transfer from Cl − to the M 3+ cation. The importance of these results in the context of designing efficient ionophores for lanthanide and actinide cations is discussed.

Glutamate substitution in repeat IV alters divalent and monovalent cation permeation in the heart Ca2+ channel

In voltage-gated ion channels, residues responsible for ion selectivity were identified in the pore-lining SS1-SS2 segments. Negatively charged glutamate residues (E393, E736, E1145, and E1446) found in each of the four repeats of the alpha 1C subunit were identified as the major determinant of selectivity in Ca2+ channels. Neutralization of glutamate residues by glutamine in repeat I (E393Q), repeat III (E1145Q), and repeat IV (E1446Q) decreased the channel affinity for calcium ions 10-fold from the wild-type channel. In contrast, neutralization of glutamate residues in repeat II failed to significantly alter Ca2+ affinity. Likewise, mutation of neighboring residues in E1149K and D1450N did not affect the channel affinity, further supporting the unique role of glutamate residues E1145 in repeat III and E1446 in repeat IV in determining Ca2+ selectivity. Conservative mutations E1145D and E1446D preserved high-affinity Ca2+ binding, which suggests that the interaction between Ca2+ and the pore ligand sites is predominantly electrostatic and involves charge neutralization. Mutational analysis of E1446 showed additionally that polar residues could achieve higher Ca2+ affinity than small hydrophobic residues could. The role of high-affinity calcium binding sites in channel permeation was investigated at the single-channel level. Neutralization of glutamate residue in repeats I, II, and III did not affect single-channel properties measured with 115 mM BaCl2. However, mutation of the high-affinity binding site E1446 was found to significantly affect the single-channel conductance for Ba2+ and Li+, providing strong evidence that E1446 is located in the narrow region of the channel outer mouth. Side-chain substitutions at 1446 in repeat IV were used to probe the nature of divalent cation-ligand interaction and monovalent cation-ligand interaction in the calcium channel pore. Monovalent permeation was found to be inversely proportional to the volume of the side chain at position 1446, with small neutral residues such as alanine and glycine producing higher Li+ currents than the wild-type channel. This suggests that steric hindrance is a major determinant for monovalent cation conductance. Divalent permeation was more complex. Ba2+ single-channel conductance decreased when small neutral residues such as glycine were replaced by bulkier ones such as glutamine. However, negatively charged amino acids produced single-channel conductance higher than predicted from the size of their side chain. Hence, negatively charged residues at position 1446 in repeat IV are required for divalent cation permeation.

Complexation thermodynamics of crown ethers. Part 3. 12-Crown-4 to 36-crown-12: from rigid to flexible ligand

Complex stability constants and thermodynamic parameters have been determined for the complexation of alkali metal ions with large-sized crown ethers, i.e. 24-crown-8 to 36-crown-12, in methanol at 25 °C. Using the present and reported data, the complexation behaviour of 12-crown-4 through 36-crown-12 is discussed comparatively and globally from the thermodynamic point of view, complex stability sequence for each cation as a function of ring size shows a characteristic two-peak profile with a global maximum and a smaller local maximum at a larger ring size, both of which may be interpreted in terms of the size-fit concept. Thermodynamically, the complexation of alkali metal ions with the crown ethers is absolutely enthalpy-driven in methanol, but the cation selectivity determined by the size–fit relationship is shown to be mostly entropy-governed. The large, but ring size insensitive, enthalpy and entropy changes for large crown ethers are rationalized by the mutually resembling cation–ligand interaction, degree of desolvation, and substantial structural freezing upon encapsulating complexation forming a highly flexible 3D cavity. However, the size–fit concept still appears to play a subsidiary role even in the complexation by larger crown ethers, although its control becomes fairly loose.

Enthalpy–entropy compensation in complexation of cations with crown ethers and related ligands

Thermodynamic parameters indicate that the enthalpy–entropy compensation effect holds in general for complexation of cations with glymes/podands, crown ethers, cryptands, and macrocyclic antibiotics in various solvents. Thus the ΔH–TΔS plots give fairly good linear relationships for each type of ligand, with different slopes (α) and intercepts (TΔS0) characteristic of the type. The nature of the cation–ligand interaction is discussed in terms of α and TΔS0 values.

Far-infrared and Raman study of lithium and sodium cryptates in nonaqueous solvents

Far-infrared spectra of sodium and lithium cryptates were observed in several nonaqueous solvents. The spectra are characterized by a broad band whose frequency is independent of the solvent or of the anion and which is assigned to the vibration of the cation in the cryptand cavity. The band frequencies were 234±2, 218±1, 243±3, and 348±1 cm−1 for Na+-C222, Na+-C221, Li+-C221, and Li+-C211 cryptates, respectively. These bands were found to be Raman-inactive, indicating that the cation-ligand interaction is very largely electrostatic in nature.

Regulation of parathyroid hormone secretion in vitro: quantitative aspects of calcium and magnesium ion control.

In studies of parathyroid hormone secretion in organ culture, the quantitative aspects of calcium and magnesium ion control were compared. The response of the glands in vitro paralleled their function in vivo, indicating that the system was a valid model for studying hormone regulation. There was a first-order relationship between hormone release and the total concentration of divalent cation (calcium plus magnesium); calcium and magnesium ions were equivalent in blocking the release of hormone. These observations emphasize the physiologic regulation of parathyroid hormone secretion by both calcium and magnesium ions and suggest the possibility of a cation-ligand interaction in or at the surface of the parathyroid cell. (Endocrinology88: 1477, 1971)

Oxidation of methanol and formaldehyde on Fe 2O 3 in comparison with MoO 3 and a mixed Mo 6+Fe 3+O catalyst

The oxidation of methanol and formaldehyde was followed on Fe 2O 3 at 220 °C with and without oxygen in the gaseous phase. The main difference between these two processes is the formation of hydrogen during the reaction without oxygen and the evolution of water in the opposite case. The effect of small amounts of preadsorbed pyridine on the oxidation of methanol was also studied. The inhibition effect of preadsorbed pyridine on methanol oxidation on the mixed Mo 6+Fe 3+O catalyst together with the infrared spectra of adsorbed pyridine may be explained by a cation-ligand interaction of both reactants with the transitional metal ions of the catalyst. The behavior of the mixed Mo 6+Fe 3+O catalyst in the oxidation of methanol resembles in many aspects that of MoO 3 in an unreduced state.