Interplay between hydration and electrostatic attraction in ligand binding: direct observation of hydration barrier at reverse micellar interface

The recognition of a charged biomolecular surface by an oppositely charged ligand is governed by electrostatic attraction and surface hydration. In the present study, the interplay between electrostatic attraction and hydration at the interface of a negatively charged reverse micelle (RM) at different temperatures has been addressed. Temperature-dependent solvation dynamics of a probe H33258 (H258) at the reverse micellar interface explores the nature of hydration at the interface. Up to 45 degrees C, the environmental dynamics reported by the interface-binding probe H258 becomes progressively faster with increasing temperature and follows the Arrhenius model. Above 45 degrees C, the observed dynamics slows down with increasing temperature, thus deviating from the Arrhenius model. The slower dynamics at higher temperatures is interpreted to be due to increasing contributions from the motions of the surfactant head groups, indicating the proximity of the probe to the interface at higher temperatures. This suggests an increased electrostatic attraction between the ligand and interface at higher temperatures and is attributed to the change in hydration. Densimetric and acoustic studies, indeed, show a drastic increase in the apparent specific adiabatic compressibility of the water molecules present in RMs after 45 degrees C, revealing the existence of a softer hydration shell at higher temperatures. Our study indicates that the hydration layer at a charged interface acts both as physical and energetic barrier to electrostatic interactions of small ligands at the interface.     

Hofmeister effects: interplay of hydration, nonelectrostatic potentials, and ion size

The classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloids, and corresponding theories of electrolytes, are unable to explain ion specific forces between colloidal particles quantitatively. The same is true generally, for surfactant aggregates, lipids, proteins, for zeta and membrane potentials and in adsorption phenomena. Even with fitting parameters the theory is not predictive. The classical theories of interactions begin with continuum solvent electrostatic (double layer) forces. Extensions to include surface hydration are taken care of with concepts like inner and outer Helmholtz planes, and "dressed" ion sizes. The opposing quantum mechanical attractive forces (variously termed van der Waals, Hamaker, Lifshitz, dispersion, nonelectrostatic forces) are treated separately from electrostatic forces. The ansatz that separates electrostatic and quantum forces can be shown to be thermodynamically inconsistent. Hofmeister or specific ion effects usually show up above ≈10(-2) molar salt. Parameters to accommodate these in terms of hydration and ion size had to be invoked, specific to each case. Ionic dispersion forces, between ions and solvent, for ion-ion and ion-surface interactions are not explicit in classical theories that use "effective" potentials. It can be shown that the missing ionic quantum fluctuation forces have a large role to play in specific ion effects, and in hydration. In a consistent predictive theory they have to be included at the same level as the nonlinear electrostatic forces that form the skeletal framework of standard theory. This poses a challenge. The challenges go further than academic theory and have implications for the interpretation and meaning of concepts like pH, buffers and membrane potentials, and for their experimental interpretation. In this article we overview recent quantitative developments in our evolving understanding of the theoretical origins of specific ion, or Hofmeister effects. These are demonstrated through an analysis that incorporates nonelectrostatic ion-surface and ion-ion dispersion interactions. This is based on ab initio ionic polarisabilities, and finite ion sizes quantified through recent ab initio work. We underline the central role of ionic polarisabilities and of ion size in the nonelectrostatic interactions that involve ions, solvent molecules and interfaces. Examples of mechanisms through which they operate are discussed in detail. An ab initio hydration model that accounts for polarisabilities of the tightly held hydration shell of "cosmotropic" ions is introduced. It is shown how Hofmeister effects depend on an interplay between specific surface chemistry, surface charge density, pH, buffer, and counterion with polarisabilities and ion size. We also discuss how the most recent theories on surface hydration combined with hydrated nonelectrostatic potentials may predict experimental zeta potentials and hydration forces.     

IDEA: interface dynamics and energetics algorithm

IDEA, interface dynamics and energetics algorithm, was implemented, in FORTRAN, under different operating systems to mimic dynamics and energetics of elementary events involved in interfacial processes. The code included a parallel elaboration scheme in which both the stochastic and the deterministic components, involved in the developed physical model, worked simultaneously. IDEA also embodied an optionally running VISUAL subroutine, showing the dynamic energy changes caused by the surface events, e.g., occurring at the gas-solid interface. Monte Carlo and ordinary differential equation system subroutines were employed in a synergistic way to drive the occurrence of the elementary events and to manage the implied energy flows, respectively. Biphase processes, namely isothermal and isobaric adsorption of carbon monoxide on nickel, palladium, and platinum surfaces, were first studied to test the capability of the code in modeling real frames. On the whole, the simulated results showed that IDEA could reproduce the inner characteristics of the studied systems and predict properties not yet experimentally investigated.     

Cation exchange in lipophilic G-quadruplexes: not all ion binding sites are equal

Lipophilic guanosine derivatives that form G-quadruplexes are promising building blocks for ionophores and ion channels. Herein, cation exchange between solvated cations (K+ and NH4+) and bound cations in the G-quadruplex [G1]16.4Na+.4DNP- was studied by electrospray ionization mass spectrometry and solution 1H, 15N NMR spectroscopy. The ESI-MS and 1H NMR data provided evidence for the formation of mixed-cationic Na+, K+ G-quadruplexes. The use of 15NH4+ cations in NMR titrations, along with 15N-filtered 1H NMR and selective NOE experiments, identified two mixed-cationic intermediates in the cation exchange pathway from [G1]16.4Na+.4DNP- to [G1]16.4NH4+.4DNP-. The central Na+, bound between the two symmetry-related G8-Na+ octamers, exchanges with either K+ or NH4+ before the two outer Na+ ions situated within the C4 symmetric G8 octamers. A structural rationale, based on differences in the cations' octahedral coordination geometries, is proposed to explain the differences in site exchange for these lipophilic G-quadruplexes. Large cations such as Cs+ can be exchanged into the central cation binding site that holds the two symmetry-related C4 symmetric G8 octamer units together. The potential relevance of these findings to both supramolecular chemistry and DNA G-quadruplex structure are discussed.     

Metal ion coordination with an asymmetric fan-shaped dendrimer at the air-water interface

Metal coordination to monolayers of 4-{10-[4-(3,5-bis-benzyloxy)-phenyl]-anthracen-9-yl}-benzoic acid ([G1-An]-CO(2)H, G1) and 4-(10-{4-[3,5-bis-(3,5-bis-benzyloxy)-benzyloxy]-phenyl}-anthracen-9-yl)-benzoic acid ([G2-An]-CO(2)H, G2) at the air-water interface and to Langmuir-Blodgett (LB) films was investigated using surface pressure-area isotherms, ultraviolet-visible (UV-vis) spectroscopy, atomic force microscopy (AFM), and X-ray reflectivity (XRR). Surface pressure-area isotherms show that G1 and G2 have different limiting areas according to the type of subphase. The limiting area of G1 and G2 increased more with Al(3+) than with Eu(3+) in the subphase. This result indicates that the hydrophilic core group is anchored to ions in the water via bidentate chelates with the carboxylate oxygen atoms of G1 and G2. Circular domains and aggregates were observed for the LB film. The different behavior of Eu(3+) and Al(3+) complexes is originated from the intrinsic nature of the ion, i.e., coordination number.     

Transfer of actinide ion at the interface between aqueous and nitrobenzene solutions studied by controlled-potential electrolysis at the interface

A novel electrochemical method based on controlled-potential electrolysis has been developed for the elucidation of the ion transfer at the interface between two immiscible electrolyte solutions (ITIES). A relationship between the applied interfacial potential (E_app) and the amount of the ion transferred (A_tr) was investigated after an electrolytic equilibrium was attained by controlled-potential electrolysis. The A_tr was determined chemically or radiometrically instead of by current measurement. It was found that (i) controlled-potential electrolysis was applicable to the study of the transfer of such hydrophilic ions as transition metal ions which gave no appreciable current within the potential window in voltammetry or polarography at ITIES, (ii) controlled-potential electrolysis in combination with a sensitive analytical method enabled a study of the transfer reaction of an ion of very dilute concentration, and (iii) even when the transfer reaction of an ion was irreversible or quasi-reversible, a standard ion transfer potential could be determined by controlled-potential electrolysis without using a kinetic parameter. The controlled-potential electrolysis method developed was applied to the transfer reactions of actinide ions such as UO₂ ²⁺ and Am³⁺ from aqueous solution to nitrobenzene solution in the absence or presence of an ionophore facilitating the transfer. The Gibbs energy for the transfer of actinide ion and a stability constant of the complex between an actinide ion and the ionophore in nitrobenzene solution were determined from log D versus E_app plots (D the ratio of the concentration of the ion in nitrobenzene solution to that in aqueous solution). The feasibility of controlled-potential electrolysis as a method for electrolytic separation of actinide ions is discussed.     

Hydrodynamic voltammetry at the liquid|liquid interface: facilitated ion transfer and the rotating diffusion cell

A new approach to the voltammetric investigation of facilitated ion transfer processes is reported. The technique uses a rotating diffusion cell approach to induce laminar flow in the organic phase of a liquid|liquid electrochemical cell. The interface between two immiscible electrolyte solutions (ITIES) was stabilised against rotation with either γ-alumina or a track-etched polyester membrane. The resultant voltammetry is shown to be consistent with the Koutecký–Levich equation enabling kinetic parameters associated with facilitated transfer of sodium by dibenzo-18-crown-6 across the water|1,2-dichloroethane interface to be evaluated. In particular, the use of the more hydrophilic alumina membrane permits the uncertainties regarding the use of the membrane-stabilised ITIES, namely the interfacial position, to be eliminated.     

Evidence for the hydration effect at the semiconductor phospholipid-bilayer interface by TiO2 photocatalysis

The interactions of TiO2 with phospholipid bilayers found in cell membrane walls were observed to perturb the bilayer structure under UVA light irradiation. The structure changes in the phospholipid bilayers upon contact with TiO2 under light and in the dark were followed by X-ray diffraction. Hydration effects at the semiconductor-phospholipid interface played an important role in the degradation of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) bilayers taken as cell wall lipid bilayer models. Evidence is provided that the fluidity of the phospholipid bilayers plays a significant role when interacting in the dark with the TiO2 or in processes mediated by TiO2 under light irradiation.     

Dynamic behavior of water within a polymer electrolyte fuel cell membrane at low hydration levels

Protonic conduction across the membrane of a polymer electrolyte fuel cell is intimately related to the dynamic behavior of water present within the membrane. To further the understanding of water dynamics in these materials, quasielastic neutron scattering (QENS) has been used to investigate the picosecond dynamic behavior of water within a perfluorosulfonated ionomer (PFSI) membrane under increasing hydration levels from dry to saturation. Evaluation of the elastic incoherent structure factor (EISF) reveals an increase in the characteristic length-scale of confinement as the number of water molecules in the membrane increases, tending to an asymptotic value at saturation. The fraction of elastic incoherent scattering observed at high Q over all hydration levels is well fit by a simple model that assumes a single, nondiffusing hydronium ion per membrane sulfonic acid site. The quasielastic component of the fitted data indicates confined dynamic behavior for scattering vectors less than 0.7 A(-1). As such, the dynamic behavior was interpreted using continuous diffusion confined within a sphere at Q < 0.7 A(-1) and random unconstrained jump diffusion at Q > 0.7 A(-1). As the number of water molecules in the membrane increases, the characteristic residence times obtained from both models is reduced. The increased dynamical frequency is further reflected in the diffusion coefficients predicted by both models. Between low hydration (2 H2O/SO3H) and saturation (16 H2O/SO3H), the continuous spherical diffusion coefficient changes from 0.46 +/- 0.12 to 1.04 +/- 0.12 (10(-5) cm2/s) and jump diffusion indicates an increase from 1.21 +/- 0.03 to 2.14 +/- 0.08 (10(-5) cm2/s). Overall, the dynamic behavior of water has been quantified over different length scale regimes, the results of which may be rationalized on the basis of the formation of water clusters in the hydrophilic domain that expand toward an asymptotic upper limit with increased hydration.     

Liquid interface functionalized by an ion extractant: the case of Winsor III microemulsions

The present work shows for the first time that tributylphosphate (TBP), the major ion extractant used in the reprocessing of spent nuclear fuel, acts efficiently as a cosurfactant in the formation of three-phase microemulsions. The system is composed of water, dodecane, TBP, and an extremely hydrophilic sugar surfactant, n-octyl-β-glucoside. The investigation of the three-phase region (Winsor III), the so-called "fish-cut" diagrams, revealed that TBP exhibits cosurfactant behavior comparable to that of classical cosurfactants n-pentanol and n-hexanol. Upon increasing the cosurfactant/surfactant molar ratio, TBP appears to be more efficient than single-chain alcohols in raising the spontaneous curvature of the adsorbed surfactant film toward oil. This is a direct consequence of the different lateral packing of TBP and n-pentanol or n-hexanol in the mixed surfactant film, with TBP having three alkyl chains and so a higher hydrophobic volume than those n-alcohols. This property is underlined by the interfacial film composition, which is determined by the chemical analysis of the excess phases. It gives a surfactant to cosurfactant molar ratio of 1:1 for TBP and 1:3 for n-hexanol. Moreover, the local microstructure of the microemulsion becomes dependent on the addition of salt when n-alcohol is replaced by TBP. A specific salt effect is also observed and rationalized in terms of the complexing property of TBP and Hofmeister's effects. Treatment of the small-angle neutron scattering (SANS) data gives access to (i) the length scales characterizing the microemulsions (i.e., the persistence length, ξ, and aqueous or organic domain sizes, D*) and (ii) the specific surface, Σ. It results that a subtle change is highlighted in the TBP microemulsion structure, in terms of connectivity, according to the type of salt added.     

Sequence-dependent binding of dipeptides by an artificial receptor in water

An artificial dipeptide receptor (1) was designed and observed to bind the deprotonated dipeptide Ac-D-Ala-D-Ala-OH in buffered water with K = 33,100 M(-1), whereas other dipeptides such as Ac-Gly-Gly-OH or Ac-D-Val-D-Val-OH were bound less efficiently, by factors of more than 10 (K < 3000 M(-1)). The efficient binding and the pronounced sequence selectivity are the result of a combination of strong electrostatic contacts and size-discriminating hydrophobic interactions. To provide such a combination, a guanidiniocarbonylpyrrole cation was attached to a novel cyclotribenzylene-substituted alanine derivative 5, to provide a hydrophobic bowl-shaped cavity just large enough to bind a methyl group but not any larger alkyl chains, thus causing the receptor to prefer alanine to valine. We describe the synthesis of 1 and the evaluation of its complexation properties in UV and fluorescence titration studies.     

Calcium binding by fulvic acids studied by an ion selective electrode an ultrafiltration method

The interaction between Ca(2+) and two well-characterized fulvic acids (Armadale and Laurentide FA) has been studied at 0.100 and 0.010M sodium nitrate using a fixed concentration of fulvic acid (100 ppm) and varying amounts of calcium (0.005-0.020 mmoles). Free calcium concentration was determined by in situ measurements employing a calcium electrode. For Armadale FA, free calcium was additionally determined via an ultrafiltration technique followed by atomic absorption measurements. For both fulvic acids, Ca(2+) binding was observed to be decreased by an increase in the ionic strength of the system. At the lower ionic strength the tendency for binding is dependent on the fulvic acid-to-metal ratio while at the higher ionic strength, the binding is insensitive to changes in the fulvic acid-to-metal ratio (an observation corroborating the contention that calcium binding to humic substances is primarily electrostatic). Comparison of the computed overall complex formation functions shows that values obtained from the ultrafiltration method were higher than those obtained using the calcium electrode. The binding of calcium was similar for the two fulvic acids.     

Cation assisted binding and cleavage of dinitrogen by uranium complexes

The role of alkali promoters in N₂ cleavage by metal complexes remains poorly understood despite its relevance to the industrial production of ammonia from N₂. Here we report a series of alkali bound-oxo-bridged diuranium(iii) complexes that provide a unique example of decreasing N₂ binding affinity with increasing cation size (from K to Cs). N₂ binding was found to be irreversible in the presence of K. A N₂ complex could be isolated in the solid state in the presence of the Rb cation and crystallographically characterized, but N₂ binding was found to be reversible under vacuum. In the presence of the Cs cation N₂ binding could not be detected at 1 atm. Electrochemical and Computational studies suggest that the decrease in N₂ binding affinity is due to steric rather than electronic effects. We also find that weak N₂ binding in ambient conditions does not prevent alkali assisted N₂ cleavage to nitride from occurring. More importantly, we present the first example of cesium assisted N₂ cleavage leading to the isolation of a N₂ derived multimetallic U/Cs bis-nitride. The nitrides readily react with protons and CO to yield ammonia, cyanate and cyanide.

N₂ binding affinity decreases markedly in a series of isostructural U(iii)–alkali ions complexes with increasing cation size. N₂ binding is undetectable in the Cs analogue, but the first example of cesium-assisted N₂ cleavage to bis-nitride was observed at ambient condition.     

Transfer behavior of cobaltous ion across the water/ nitrobenzene interface facilitated by 2,2'-bipyridine

The transfer behavior of cobaltous ion across the water/nitrobenzene interface facilitated by 2,2'-bipyridine has been investigated by cyclic voltammetry. The transfer species are successive complexes formed between Co(II) and bipyridine. Coupled chemical reactions occur not only in the aqueous phase but also in the organic phase during the electrolytic transfer. Irreversible transfer phenomena were found from voltammograms obtained in the Co(II)-bipyridine system and discussed.     

Cation exchange at the mineral-water interface: H3O+/K+ competition at the surface of nano-muscovite

This article describes a (39)K nuclear magnetic resonance (NMR) spectroscopic study of K (+) displacement at the muscovite/water interface as a function of aqueous phase pH. (39)K NMR spectra and T 2 relaxation data for nanocrystalline muscovite wet with a solid/solution weight ratio of 1 at pH 1, 3, and 5.5 show substantial liquid-like K (+) only at pH 1. At pH 3 and 5.5, all K (+) appears to be associated with muscovite as inner- or outer-sphere complexes, indicating that H 3O (+) does not displace basal surface K (+) beyond the (39)K detection limit under these conditions. In our pH 1 mixture, only approximately 1/3 of the initial basal surface K (+) population is located more than 3-4 A from the surface. (29)Si and (27)Al MAS NMR spectra and SEM images show no evidence of dissolution during the (39)K experiments, consistent with the liquid-like (39)K fraction originating from displaced basal surface K (+). Assuming no muscovite dissolution or interlayer exchange, the K (+)/H 3O (+) ratio relevant to the solution/surface exchange equilibrium is controlled by the total amount of K (+) on the surface and H 3O (+) in solution (K (+) surf/H 3O (+) aq). These parameters, in turn, depend on the basal surface area, solution pH, and the solid/solution ratio. The results here are consistent with significant displacement of surface K (+) only under conditions where the initial K (+) surf/H 3O (+) aq ratio is less than approximately 1. Computational molecular models of the muscovite/water interface should account for both K (+) and H 3O (+) in the near-surface region.     

Pore size distributions of ion exchangers and relation to protein binding capacity

The pore structure of chromatographic media directly influences macromolecular transport and adsorption, and consequently separation resolution and loading capacity in chromatographic separations. The pore size distribution (PSD) is therefore a central structural characteristic of chromatographic materials and a critical determinant of chromatographic behavior. In this work the PSDs of a set of commercial anion exchangers were determined by inverse size-exclusion chromatography (ISEC). The PSDs were further utilized to develop relations to functional properties of adsorbents, such as intraparticle diffusivity, and static and dynamic binding capacities. We find that the detailed PSD is useful in semi-quantitative understanding of chromatographic behavior. However, more accurate prediction of column behavior requires more thorough knowledge of the pore structure, specifically the connectivity of the pore network, as well as improved understanding of the function of grafted resins.     

Direct measurement of ion accumulation at the electrode electrolyte interface under an oscillatory electric field

The ionic charge accumulation at the metal-electrolyte interface is directly measured by using differential interferometry as a function of magnitude and frequency (2-50 kHz) of external electric field. The technique developed probes the ion dynamics confined to the electrical double layer. The amplitude of modulation of the ions is linearly proportional to the amplitude of applied potential. The linearity is observed up to high electrode potentials and salt concentrations. The frequency response of the ion dynamics at the interface is interpreted in terms of the classical RC model.