Water mediated attraction between repulsive ions: a cluster-based simulation approach

Could two like ions be attractive to each other in the presence of water? To address this question and to further interrogate the intriguing solvent effects at a molecular level on multiply charged species, a "bottom-up" simulation approach was formulated, from which the inter-ionic potential of mean force and other properties were monitored closely with the gradual addition of the water molecules. This approach was first tested on a commonly studied ion pair (namely, Na+ and Cl-), where excellent agreement with the published bulk-phase data was found. Further application of this approach to the like-ion pair indicated that an attractive interaction between two anions or two cations can be induced by the addition of an appropriate number of water molecules. This result corroborates a recent experimental report of an intriguing folding of a dianionic polymer into a more compact structure with the addition of water molecules in gas phase as well as previous theoretical findings of possible attraction between like-ion pairs in bulk aqueous phases.     

Effect of exchangeable cation on the swelling property of 2:1 dioctahedral smectite--a periodic first principle study

We used both localized and periodic calculations on a series of monovalent (Li+, Na+, K+, Rb+, Cs+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations to monitor their effect on the swelling of clays. The activity order obtained for the exchangeable cations among all the monovalent and divalent series studied: Ca2+ > Sr2+ > Mg2+ > Rb+ > Ba2+ > Na+ > Li+ > Cs+ > K+. We have shown that, in case of dioctahedral smectite, the hydroxyl groups play a major role in their interaction with water and other polar molecules in the presence of an interlayer cation. We studied both type of clays, with a different surface structure and with/without water using a periodic calculation. Interlayer cations and charged 2:1 clay surfaces interact strongly with polar solvents; when it is in an aqueous medium, clay expands and the phenomenon is known as crystalline swelling. The extent of swelling is controlled by a balance between relatively strong swelling forces and electrostatic forces of attraction between the negatively charged phyllosilicate layer and the positively charged interlayer cation. We have calculated the solvation energy at the first hydration shell of an exchangeable cation, but the results do not correspond directly to the experimental d-spacing values. A novel quantitative scale is proposed with the numbers generated by the relative nucleophilicity of the active cation sites in their hydrated state through Fukui functions within the helm of the hard soft acid base principle. The solvation effect thus measured show a perfect match with experiment, which proposes that the reactivity index calculation with a first hydration shell could rationalize the swelling mechanism for exchangeable cations. The conformers after electron donation or acceptance propose the swelling mechanism for monovalent and divalent cations.     

Modeling the interaction between integrin-binding peptide (RGD) and rutile surface: the effect of cation mediation on Asp adsorption

The binding of a negatively charged residue, aspartic acid (Asp) in tripeptide arginine-glycine-aspartic acid, onto a negatively charged hydroxylated rutile (110) surface in aqueous solution, containing divalent (Mg(2+), Ca(2+), or Sr(2+)) or monovalent (Na(+), K(+), or Rb(+)) cations, was studied by molecular dynamics (MD) simulations. The results indicate that ionic radii and charges will significantly affect the hydration, adsorption geometry, and distance of cations from the rutile surface, thereby regulating the Asp/rutile binding mode. The adsorption strength of monovalent cations on the rutile surface in the order Na(+) > K(+) > Rb(+) shows a "reverse" lyotropic trend, while the divalent cations on the same surface exhibit a "regular" lyotropic behavior with decreasing crystallographic radii (the adsorption strength of divalent cations: Sr(2+) > Ca(2+) > Mg(2+)). The Asp side chain in NaCl, KCl, and RbCl solutions remains stably H-bonded to the surface hydroxyls and the inner-sphere adsorbed compensating monovalent cations act as a bridge between the COO(-) group and the rutile, helping to "trap" the negatively charged Asp side chain on the negatively charged surface. In contrast, the mediating divalent cations actively participate in linking the COO(-) group to the rutile surface; thus the Asp side chain can remain stably on the rutile (110) surface, even if it is not involved in any hydrogen bonds with the surface hydroxyls. Inner- and outer-sphere geometries are all possible mediation modes for divalent cations in bridging the peptide to the rutile surface.     

The formation of planar ribbonlike aggregates from stiff polyanions in the presence of anisotropic cations

A dilute salt-free solution of rodlike polyanions in the presence of anisotropic (chain) cations consisting of neutral tails and charged heads is studied. Using Monte Carlo simulation within the framework of the primitive model, different Coulomb coupling regimes were considered. While aggregation in the strong coupling limit is expected, we report new morphology, namely, the formation of ribbonlike nanostructures. At strong electrostatic interaction, the system is found to undergo the self-organization resulting in the formation of planar aggregates that look like a "ladder" of polyanions sandwiched between cationic chains. We investigate the stability of different morphologies and find that these aggregates are thermodynamically stable. Focus has been made on how the chemical structure of anisotropic cations affects the morphology of the aggregates.     

Elemental analysis within the electrical double layer using total reflection X-ray fluorescence technique

A simplified total reflection X-ray fluorescence (TRXF) technique is proposed for the study of the electrical double layer (EDL) near charged monolayers at the air-water interface. In contrast to the parent NTEF (near total external reflection X-ray fluorescence) method, TRXF uses a fixed angle of incidence (below the critical one) and abandons both "spatial resolution" (which is poor anyway) and "absolute calibration" with the bulk reference. These modifications reduce both the duration of experiments and the complexity of the data treatment by 1-2 orders of magnitude and turn TRXF into a truly simple tool for elemental analysis within the EDL. A few TRXF experiments appear sufficient to disprove the model of simultaneous binding of alkali earth metal cations and inorganic anions to negatively charged phospholipid monolayers as proposed in literature. Direct experimental support was provided to the essential feature of the EDL near highly charged interfaces: The main amount of counterions is concentrated in the thin inner part of the EDL irrespective of the electrolyte concentration in the bulk. A study of the counterion competition for the participation in the EDL of highly negatively charged behenylsulfate (BS) monolayers (resulting from packing density limitations) revealed that univalent Cs+ is quite competitive with divalent Ca2+ and Ba2+ (which contradicts the classical Gouy-Chapman model). If the univalent/divalent ion ratio in bulk is 9:1, the corresponding ratio in the EDL is ca. 1.5 for Cs+/Ca2+ and 0.5 for Cs+/Ba2+, whereas the model predicts 0.14 only. Bearing in mind packing density limitations, these values are consistent with a series of sizes for hydrated ions: Cs+ < Ba2+ < Ca2+.     

Selective Interaction Between Proteins and the Outermost Surface of Polyelectrolyte Multilayers: Influence of the Polyanion Type,pH and Salt

Protein adsorption was studied by in-situ ATR-FT-IR spectroscopy of consecutively deposited polyelectrolyte multilayer systems terminated either with poly(ethyleneimine) (PEI) or polyanions, such as poly-(acrylic acid) (PAC), poly(maleic acid-co-propylene) (PMA-P) or poly(vinyl sulfate) (PVS). The influence of the polyanion type, pH and ionic strength was investigated. Negatively charged human serum albumin (HSA) was strongly repelled by multilayers terminated with weak polyanions (PAC, PMA-P), whereas moderate attraction was observed for those terminated with the strong polyanion PVS. Changing the pH from 7.4 to 5 resulted in enhanced HSA adsorption onto PAC-terminated multilayers. An increase in ionic strength diminished the attractive HSA adsorption onto PEI-terminated multilayers. For the PEI/PAC system, the biomedically relevant adsorption of human fibrinogen (FGN) is determined via its isoelectric point in accordance with three other proteins.     

Calcium-induced phospholipid ordering depends on surface pressure

The effect of sodium and calcium ions on zwitterionic and anionic phospholipids monolayers is investigated using vibrational sum-frequency generation in conjunction with surface pressure measurements and fluorescence microscopy. Sodium ions only subtly affect the monolayer structure, while the effect of calcium is large and depends strongly on the surface pressure. At low surface pressures (approximately 5 mN/m), the presence on Ca2+ results in the unexpected appearance of ordered domains. For pressures between approximately 5 and approximately 25 mN/m, Ca2+ ions induce disorder in the monolayer. For pressures exceeding 25 mN/m, calcium cations expand the monolayer, while simultaneously ordering the lipid chains. Interestingly, effects are similar for both zwitterionic lipids and negatively charged lipids. In both vibrational sum-frequency generation and surface tension measurements, the molecular signature of the association of Ca2+ with the lipids is evident from Ca2+-induced changes in the signals corresponding to area changes of 4 A2/lipid-precisely the surface area of a Ca2+ ion, with evidence for a change in lipid Ca2+ complexation at high pressures.     

Macrocyclic multicenter complexes of nickel and copper of increasing complexity

Multicenter (bi-, tri-, and tetranuclear) tetraazamacrocyclic complexes were self-assembled from Ni and Cu tetraazamacrocyclic mononuclear units and α,ω-diamines as building blocks. The structures of all compounds studied were proved by spectroscopic methods (ESI MS and NMR spectroscopy). Electrochemical experiments revealed reversible one-electron electrode processes at each of the Ni(2+) and Cu(2+) centers with formation of metal cations in oxidation state +3. Long linkers allow bi- and trinuclear complexes with noninteracting metal centers to be obtained. In the case of the short linkers (e.g. ethylenediamine) higher, trinuclear species are formed as major product. The structures of the bis- and tris-macrocyclic systems were confirmed by single-crystal X-ray diffraction. The tris-macrocyclic systems form cations in the shape of triangles partially filled with counterions and solvent molecules. The cations form positively charged layers, which interact in the crystal lattice with the neighboring negatively charged layers of anions. In solution, the trinuclear complexes exhibit strong host-guest interactions with 9,10-dimethyltriptycene due to complementarity of shape and size of this guest molecule. The association constants were determined by NMR spectroscopy and voltammetry, and very good agreement was obtained. The structural flexibility of the tetranuclear complex with long linkers allows for attractive interactions between the metal-complexing macrocycles that result in folding of the molecule. On the contrary, no folding is possible in the case of short linkers consisting of two CH(2) groups.     

The conformational dynamics of humic polyanions in model organic and organo-mineral aggregates

Molecular mechanics calculations and simulated annealing were applied to model humic polyanions originating from lignin. The dynamic behavior of such oxidized lignins in model soil organic complexes, such as an oxidized lignin-carbohydrate complex (LCC) and humic (oxidized LCC)-clay aggregates, was analyzed. Neither ionization nor hydrogen bonding bring significant changes in the conformational properties of oxidized lignin and LCC. Oxidized lignin and LCC oligomers (humic substances in soil) bind to the mineral surfaces, a process that was exemplified in computational experiments on complexes with muscovite. Upon ionization, a lignin-derived oligomer develops strong attractive organo-mineral interactions through cation bridges. Without metal cations, electrostatic repulsion between negatively charged anions and the oxygen-mineral surface prevails, and the two parts of the organo-mineral complex drift apart. This tendency is typical of an oxidized lignin oligomer but not of a topological oxidized LCC.     

Metallo supramolecular assemblies of bis-squaraines by allosteric Ca2+ ion binding

Alkyl chain tethered bis-squaraines bind to Ca2+ ions through the participation of the negatively charged oxygen of the central cyclobutene moiety to form folded H-type aggregates. The initially formed Ca2+ complex is preorganized to facilitate cooperative allosteric binding of Ca2+, resulting in the formation of extended supramolecular arrays. The electronic absorption, IR, and ESI-MS studies support the formation of metallo supramolecular architectures of the folded H-type dimers of the bis-squaraines.     

CnNCl和SDS在水溶液中的相互作用

CnNCl和SDS在水溶液中的相互作用;滴定式微量热法; 烷氧基羟丙基三甲基氯化铵; 十二烷基硫酸钠;胶束;疏水作用     

Influence of surfactant concentration on the surface morphology of hollow silica microspheres and its explanation

The surface morphology of hollow silica microspheres has influence on their applications. After a thorough investigation of the deposition of silica nanoparticles on polystyrene (PS) beads and the surface morphology and texture of the resultant hollow silica shells with scanning electron microscopy, transmission electron microscopy, and N2-sorption measurements, the influence of surfactant [cetyltrimethylammonium bromide (CTAB)] concentration on the surface morphology of hollow silica microspheres templated by PS beads is explained. Previously, CTAB was believed to turn the surface charge of PS beads from negative into positive so that negatively charged silica could be deposited on the PS template. Here, we show CTA+ cations preferentially assemble with silica species to form silica-CTA+ composite nanoparticles. Since the zeta potential of silica-CTA+ composite nanoparticles is smaller than that of pure silica nanoparticles, these composite nanoparticles encounter less repulsion when they are deposited on the surface of PS beads and close to each other. As more CTAB is added, the silica-CTA+ nanoparticles are less negatively charged, and more compact and smooth hollow silica microspheres are obtained.     

Stimulation and binding properties of polyelectrolyte multilayers verified by ATR‐FTIR spectroscopy

We report on the deposition and properties of multilayers composed of reactive polymers on planar surfaces. As reactive polymers the poly cations poly(ethylene‐imine), poly(L‐lysine) (PLL), poly(allylamime) (PAA) and the polyanions poly(acrylate) (PAC), poly(vinylsulfate), poly(maleate‐co‐olefines) were used. ATR‐FTIR Spectroscopy was adopted to study deposition, binding and stimulation properties of polymer multilayers. The binding of charged species of different molecular size such as rhodanide anions and sodium oleate from solution was examined, whereby binding was found to be dependent on the charge of the outermost layer. For these two analytes a selectivity parameter Q, defined as the ratio between the adsorbed amount obtained at the negatively charged and that at the positively charged surface, respectively, was determined. Furthermore, swelling experiments on multilayer assemblies of PLL and PAC exposed to mixtures of ethanol/water (10–70% EtOH content) were carried out. Our experiments gave evidence, that the PLL layers showed a more significant increase in density than the PAC layers. The conformation of PLL incorporated into multilayers could be changed by pH variation.     

Influence of the polyelectrolyte molecular weight on exponentially growing multilayer films in the linear regime

Alternated deposition of polyanions and polycations on a charged solid substrate leads to the buildup of polyelectrolyte multilayer (PEM) films. Two types of PEM films were reported in the literature: films whose thickness increases linearly and films whose thickness increases exponentially with the number of deposition steps. However, it was recently found that, for exponentially growing films, the exponential increase of the film thickness takes place only during the initially deposited pairs of layers and is then followed by a linear increase. In this study, we investigate the growth process of hyaluronic acid/poly(L-lysine) (HA/PLL) and poly(L-glutamic acid)/poly(allylamine) (PGA/PAH) films, two films whose growth is initially exponential, when the growth process enters the linear regime. We focus, in particular, on the influence of the molecular weight (Mw) of the polyelectrolytes. For both systems, we find that the film thickness increment per polyanion/polycation deposition step in the linear growth regime is fairly independent of the molecular weights of the polyelectrolytes. We also find that when the (HA/PLL)n films are constructed with low molecular weight PLL, these chains can diffuse into the entire film during each buildup cycle, even for very thick films, whereas the PLL diffusion of high molecular weight chains is restricted to the upper part of the film. Our results lead to refinement of the buildup mechanism model, introduced previously for the exponentially growing films, which is based on the existence of three zones over the entire film thickness. The mechanism no longer needs all the "in" and "out" diffusing polyanions or polycations to be involved in the buildup process to explain the linear growth regime but merely relies on the interaction between the polyelectrolytes with an upper zone of the film. This zone is constituted of polyanion/polycation complexes which are "loosely bound" and rich in the polyelectrolyte deposited during the former deposition step.     

Multiple charging of poly(propylene glycol) by binary mixtures of cations in electrospray

Single, double and triple charging of poly(propylene glycol) (PPG) (Mn = 1900 g/mol) in the presence of binary mixtures of cations (Li+, Na+, K+, Cs+, and NH4+) under electrospray ionization (ESI) conditions were investigated. For these studies, sodium ion was selected as the reference cation, and the resulting ion-intensities were evaluated as a function of the [Na+]/[C+] ratio (where C+ is the other cation, i.e., Li+, K+, Cs+ and NH4+). A linear relationship was found between INa+/IC+)and [Na+]/[C+] (INa+ and IC+ stand for the intensity of the singly charged PPG molecules cationized with Na+ and C+ ions, respectively). The slope of the INa+/IC+--[Na+]/[C+] plot (alpha) indicates the binding selectivity of Na+ ions to PPG chains with respect to cation C+. In the case of the doubly charged PPG chains, the INaNa2+/INaC2+ and INaC2+/ICC2+ versus [Na+]/[C+] ratio also yield straight lines with slopes of approximately alpha/2 and 2alpha, respectively (INaNa2+, INaC2+ and ICC2+ are the intensity of the doubly charged PPG chains cationized with two Na+ ions, Na+ and C+ ions, and two C+ ions, respectively). Similarly, linear dependences with the [Na+]/[C+] ratio for the corresponding intensity ratios of the triply charged PPG were found. Based on the value of alpha, the selectivity of the cations was found to increase in the order of Li+ < Cs+ approximately Na+ < K+ approximately NH4+. The observed relative ion intensities are interpreted on the basis of the solution state equilibrium between PPG and the cations. In addition, the investigations showed that the abundances of the doubly and triply charged PPG-containing mixed cations can be optimized in a simple way using the value of alpha.     

The study of terbium regenerated bacterirhodopsin

Bacteriorhodopsin (bR) is the only retinal-contain- ing protein in the purple membrane of Halobacterium halobium[1]. Upon illumination, the protein undergoes a photocycle and pumps protons across the cell mem-brane[2,3]. It has been found that well-washed…     

Determination of total acidity and of divalent cations by ion chromatography with n-hexadecylphosphocholine as the stationary phase

An ion chromatographic (IC) method is reported for simultaneous determination of total acidity (H+), Ba2+, Ca2+, and Mg2+ in aqueous samples. A standard ODS silica column modified by coating with n-hexadecylphosphocholine was used as the separation column. Water alone was used as the eluent, with conductivity detection of the sample ions. An excess of sodium iodide was added to each sample so that both H+ and divalent cations were always eluted with iodide as the counterion. The elution order was Ba2+, Mg2+, Ca2+, and H+ with H+ being eluted much later than the divalent cations. Acetic acid and several other weak acids could also be separated because all the protons were transposed from acetic acid (HAc) to HI by the sodium iodide. Detection limits for 100 microl injection, S/N=3 were in the low micromolar range for the divalent cations and approximately 0.3 mM for H+/I-. This method was used successfully for simultaneous determination of total acidity, magnesium and calcium in HCl-type of hot-spring water.     

不同形态的氧氟沙星在凹凸棒土上的吸附特征

通过控制反应体系的pH值,探究了阳离子、兼性和阴离子形态的氧氟沙星(OFL,3种形态分别记为OFL~+,OFL~±和OFL~-)在凹凸棒土(ATP)上的吸附特征.实验结果表明,OFL~+主要通过与ATP表面的Ca~(2+),Mg~(2+)进行阳离子交换吸附于ATP上,当其吸附量较高时,会存在少量的氢键;OFL~±和OFL~-可与ATP表面的铁氧化物、铝氧化物进行表面络合,也可与溶液中从ATP中溶解出的Ca~(2+)和Mg~(2+)形成络合物,再通过静电作用吸附于ATP上.在中性至微碱性(pH=7.10~7.70)条件下,由于Ca的电负性小于Mg,[Ca~(2+)-OFL]+不能稳定地存在于溶液中,使得OFL±与Ca~(2+)进行阳离子交换而与Mg~(2+)形成络合物,再通过静电作用吸附于ATP上.当OFL主要以OFL~-形态存在于溶液中时(p H=9.00~10.00),Ca~(2+)和Mg~(2+)均可与OFL~-形成络合物,再通过静电作用吸附于ATP上.     

Design of a calcium-binding protein with desired structure in a cell adhesion molecule

Ca2+, "a signal of life and death", controls numerous cellular processes through interactions with proteins. An effective approach to understanding the role of Ca2+ is the design of a Ca2+-binding protein with predicted structural and functional properties. To design de novo Ca2+-binding sites in proteins is challenging due to the high coordination numbers and the incorporation of charged ligand residues, in addition to Ca2+-induced conformational change. Here, we demonstrate the successful design of a Ca2+-binding site in the non-Ca2+-binding cell adhesion protein CD2. This designed protein, Ca.CD2, exhibits selectivity for Ca2+ versus other di- and monovalent cations. In addition, La3+ (Kd 5.0 microM) and Tb3+ (Kd 6.6 microM) bind to the designed protein somewhat more tightly than does Ca2+ (Kd 1.4 mM). More interestingly, Ca.CD2 retains the native ability to associate with the natural target molecule. The solution structure reveals that Ca.CD2 binds Ca2+ at the intended site with the designed arrangement, which validates our general strategy for designing de novo Ca2+-binding proteins. The structural information also provides a close view of structural determinants that are necessary for a functional protein to accommodate the metal-binding site. This first success in designing Ca2+-binding proteins with desired structural and functional properties opens a new avenue in unveiling key determinants to Ca2+ binding, the mechanism of Ca2+ signaling, and Ca2+-dependent cell adhesion, while avoiding the complexities of the global conformational changes and cooperativity in natural Ca2+-binding proteins. It also represents a major achievement toward designing functional proteins controlled by Ca2+ binding.     

Enhanced aggregation of alginate-coated iron oxide (hematite) nanoparticles in the presence of calcium, strontium, and barium cations

Early-stage aggregation kinetics studies of alginate-coated hematite nanoparticles in solutions containing alkaline-earth metal cations revealed enhanced aggregation rates in the presence of Ca2+, Sr2+, and Ba2+, but not with Mg2+. Transmission electron microscopy (TEM) imaging of the aggregates provided evidence that alginate gel formation was essential for enhanced aggregation to occur. Dynamic light scattering (DLS) aggregation results clearly indicated that a much lower concentration of Ba2+ compared to Ca2+ and Sr2+ was required to achieve a similar degree of enhanced aggregation in each system. To elucidate the relationship between the alginate's affinities for divalent cations and the enhanced aggregation of the alginate-coated hematite nanoparticles, atomic force microscopy (AFM) was employed to probe the interaction forces between alginate-coated hematite surfaces under the solution chemistries used for the aggregation study. Maximum adhesion forces, maximum pull-off distances, and the work of adhesion were used as indicators to gauge the alginate's affinity for the divalent cations and the resulting attractive interactions between alginate-coated hematite nanoparticles. The results showed that alginate had higher affinity for Ba2+ than either Sr2+ or Ca2+. This same trend was consistent with the cation concentrations required for comparable enhanced aggregation kinetics, suggesting that the rate of alginate gel formation controls the enhanced aggregation kinetics. An aggregation mechanism incorporating the gelation of alginate is proposed to explain the accelerated aggregate growth in the presence of Ca2+, Sr2+, and Ba2+.