Nanometer-scale ion aggregates in aqueous electrolyte solutions: guanidinium sulfate and guanidinium thiocyanate

Neutron diffraction experiments and molecular dynamics simulations are used to study the structure of aqueous solutions of two electrolytes: guanidinium sulfate (a mild protein conformation stabilizer) and guanidinium thiocyanate (a powerful denaturant). The MD simulations find the unexpected result that in the Gdm2SO4 solution the ions aggregated into mesoscopic (nanometer-scale) clusters, while no such aggregation is found in the GdmSCN solution. The neutron diffraction studies, the most direct experimental probe of solution structure, provide corroborating evidence that the predicted very strong ion pairing does occur in solutions of 1.5 m Gdm2SO4 but not in 3 m solutions of GdmSCN. A mechanism is proposed as to how this mesoscopic solution structure affects solution denaturant properties and suggests an explanation for the Hofmeister ordering of these solutions in terms of this ion pairing and the ability of sulfate to reverse the denaturant power of guanidinium.     

Like-charge guanidinium pairing from molecular dynamics and ab initio calculations

Pairing of guanidinium moieties in water is explored by molecular dynamics simulations of short arginine-rich peptides and ab initio calculations of a pair of guanidinium ions in water clusters of increasing size. Molecular dynamics simulations show that, in an aqueous environment, the diarginine guanidinium like-charged ion pairing is sterically hindered, whereas in the Arg-Ala-Arg tripeptide, this pairing is significant. This result is supported by the survey of protein structure databases, where it is found that stacked arginine pairs in dipeptide fragments exist solely as being imposed by the protein structure. In contrast, when two arginines are separated by a single amino acid, their guanidinium groups can freely approach each other and they frequently form stacked pairs. Molecular dynamics simulations results are also supported by ab initio calculations, which show stabilization of stacked guanidinium pairs in sufficiently large water clusters.     

Theoretical characterization of laser- and sympathetically-cooled ions in surface-electrode ion traps

Using molecular dynamics simulations we characterize theoretically Coulomb clusters of laser- and sympathetically-cooled ions in a five-wire surface-electrode ion trap. We show that the asymmetry of the trapping potential leads to significantly different cluster structures and ion energy distributions in comparison to conventionally used linear Paul traps and to an asymmetric segregation of the ions in bi-component Coulomb clusters. We explore the impact of our results on the implementation of sympathetic cooling of molecular ions in surface-electrode traps and discuss possible challenges for the realization of such experiments.     

Aqueous divalent metal-nitrate interactions: hydration versus ion pairing

Nitrate aqueous solutions, Mg(NO(3))(2), Ca(NO(3))(2), Sr(NO(3))(2), and Pb(NO(3))(2), are investigated using Raman spectroscopy and free energy profiles from molecular dynamics (MD) simulations. Analysis of the in-plane deformation, symmetric stretch, and asymmetric stretch vibrational modes of the nitrate ions reveal perturbation caused by the metal cations and hydrating water molecules. Results show that Pb(2+) has a strong tendency to form contact ion pairs with nitrate relative to Sr(2+), Ca(2+), and Mg(2+), and contact ion pair formation decreases with decreasing cation size and increasing cation charge density: Pb(2+) > Sr(2+) > Ca(2+) > Mg(2+). In the case of Mg(2+), the Mg(2+)-OH(2) intermolecular modes indicate strong hydration by water molecules and no contact ion pairing with nitrate. Free energy profiles provide evidence for the experimentally observed trend and clarification between solvent-separated, solvent-shared, and contact ion pairs, particularly for Mg(2+) relative to other cations.     

Nanometer-scale ion aggregates in aqueous electrolyte solutions: guanidinium carbonate

Neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations have been used to characterize the structure of aqueous guanidinium carbonate (Gdm2CO3) solutions. The MD simulations found very strong hetero-ion pairing in Gdm2CO3 solution and were used to determine the best structural experiment to demonstrate this ion pairing. The NDIS experiments confirm the most significant feature of the MD simulation, which is the existence of strong hetero-ion pairing between the Gdm+ and CO3(2-) ions. The neutron structural data also support the most interesting feature of the MD simulation, that the hetero-ion pairing is sufficiently strong as to lead to nanometer-scale aggregation of the ions. The presence of such clustering on the nanometer length scale was then confirmed using small-angle neutron scattering experiments. Taken together, the experiment and simulation suggest a molecular-level explanation for the contrasting denaturant properties of guanidinium salts in solution.     

Asymmetric induction in the preparation of helical receptor-anion complexes: ion-pair formation with chiral cations

Asymmetry through ion pairing: Upon addition of chloride and bromide ions, as chiral ammonium salts, to solutions of pyrrole-based π-conjugated linear oligomers, helical structures form with asymmetric induction, which is guided by the formation of diastereomeric ion pairs with chiral counter cations. These ions pairs exhibit circular dichroism (CD) and strong circularly polarized luminescence (CPL) with g(lum) values of greater than 0.1.     

Bond energies and structures of ammonia-sulfuric acid positive cluster ions

New particle formation in the atmosphere is initiated by nucleation of gas-phase species. The small molecular clusters that act as seeds for new particles are stabilized by the incorporation of an ion. Ion-induced nucleation of molecular cluster ions containing sulfuric acid generates new particles in the background troposphere. The addition of a proton-accepting species to sulfuric acid cluster ions can further stabilize them and may promote nucleation under a wider range of conditions. To understand and accurately predict atmospheric nucleation, the stabilities of each molecular cluster within a chemical family must be known. We present the first comprehensive measurements of the ammonia-sulfuric acid positive ion cluster system NH(4)(+)(NH(3))(n)(H(2)SO(4))(s). Enthalpies and entropies of individual growth steps within this system were measured using either an ion flow reactor-mass spectrometer system under equilibrium conditions or by thermal decomposition of clusters in an ion trap mass spectrometer. Low level ab initio structural calculations provided inputs to a master equation model to determine bond energies from thermal decomposition measurements. Optimized ab initio structures for clusters up through n = 3, s = 3 are reported. Upon addition of ammonia and sulfuric acid pairs, internal proton transfer generates multiple NH(4)(+) and HSO(4)(-) ions within the clusters. These multiple-ion structures are up to 50 kcal mol(-1) more stable than corresponding isomers that retain neutral NH(3) and H(2)SO(4) species. The lowest energy n = s clusters are composed entirely of ions. The addition of acid-base pairs to the core NH(4)(+) ion generates nanocrystals that begin to resemble the ammonium bisulfate bulk crystal starting with the smallest n = s cluster, NH(4)(+)(NH(3))(1)(H(2)SO(4))(1). In the absence of water, this cluster ion system nucleates spontaneously for conditions that encompass most of the free troposphere.     

Structural and dynamic properties of concentrated alkali halide solutions: a molecular dynamics simulation study

The physicochemical properties of alkali halide solutions have long been attributed to the collective interactions between ions and water molecules in the solution, yet the structure of water in these systems and its effect on the equilibrium and dynamic properties of these systems are not clearly understood. Here, we present a systematic view of water structure in concentrated alkali halide solutions using molecular dynamics simulations. The results of the simulations show that the size of univalent ions in the solution has a significant effect on the dynamics of ions and other transport properties such as the viscosity that are correlated with the structural properties of water in aqueous ionic solution. Small cations (e.g., Li+) form electrostatically stabilized hydrophilic hydration shells that are different from the hydration shells of large ions (e.g., Cs+) which behave more like neutral hydrophobic particles, encapsulated by hydrogen-bonded hydration cages. The properties of solutions with different types of ion solvation change in different ways as the ion concentration increases. Examples of this are the diffusion coefficients of the ions and the viscosities of solutions. In this paper we use molecular dynamics (MD) simulations to study the changes in the equilibrium and transport properties of LiCl, RbCl, and CsI solutions at concentrations from 0.22 to 3.97 M.     

Structure and dynamics of N,N-diethyl-N-methylammonium triflate ionic liquid, neat and with water, from molecular dynamics simulations

We investigated by means of molecular dynamics simulations the properties (structure, thermodynamics, ion transport, and dynamics) of the protic ionic liquid N,N-diethyl-N-methylammonium triflate (dema:Tfl) and of selected aqueous mixtures of dema:Tfl. This ionic liquid, a good candidate for a water-free proton exchange membrane, is shown to exhibit high ion mobility and conductivity. The radial distribution functions reveal a significant long-range structural correlation. The ammonium cations [dema](+) are found to diffuse slightly faster than the triflate anions [Tfl](-), and both types of ions exhibit enhanced mobility at higher temperatures, leading to higher ionic conductivity. Analysis of the dynamics of ion pairing clearly points to the existence of long-lived contact ion pairs. We also examined the effects of water through characterization of properties of dema:Tfl-water mixtures. Water molecules replace counterions in the coordination shell of both ions, thus weakening their association. As water concentration increases, water molecules start to connect with each other and then form a large network that percolates through the system. Water influences ion dynamics in the mixtures. As the concentration of water increases, both translational and rotational motions of [dema](+) and [Tfl](-) are significantly enhanced. As a result, higher vehicular ionic conductivity is observed with increased hydration level.     

Microscopic hydration of the sodium chloride ion pair

Hydration of ion pairs is an essential process in various physicochemical phenomena occurring in solutions. Isolated clusters of an ion pair solvated with finite number of waters have been considered as a model system for the critical evaluation of microscopic interactions involved in the process, and theoretical studies have contributed exclusively to the subject up to now. Here we report the first experimental characterization of structure and internal dynamics of hydrated ion pairs, NaCl-(H2O)n (n = 1-3). The measurements of their rotational spectra have proven that the clusters have cyclic forms, in which Na+ and Cl- ions are strongly interacted with the O and H atoms of the solvent molecules, respectively. The Na-Cl distance shows a pronounced increase with the successive addition of water molecules. The separation for n = 3 approaches the value predicted for the contact ion-pair state in aqueous solution by recent molecular dynamics simulations.     

Parameters of monovalent ions in the AMBER-99 forcefield: assessment of inaccuracies and proposed improvements

The monovalent ion parameters used by the AMBER-99 forcefield are shown to exhibit physically inaccurate behavior in molecular dynamics simulations of strong 1:1 electrolytes. These errors arise from an ad hoc adaptation of Aqvist's cation parameters. The result is the rapid formation of large, unphysical clusters at concentrations that are well below solubility limits. The observed unphysical behavior poses a serious challenge for simulating ions around highly charged polymers such as nucleic acids. In this communication, we explain the source of this unphysical behavior. To facilitate the continued use of the popular AMBER parameters, we prescribe a simple fix whereby Aqvist's cations and anions are used in conjunction with the AMBER forcefield for nucleic acids. A preliminary test of this strategy suggests that the proposed fix is reasonable and is likely to be generalizable for simulating diffuse and specific ion binding to nucleic acids.     

Equilibrium clusters in concentrated lysozyme protein solutions

We have studied the structure of salt-free lysozyme at 293 K and pH 7.8 using molecular simulations and experimental SAXS effective potentials between proteins at three volume fractions, ?=0.012, 0.033, and 0.12. We found that the structure of lysozyme near physiological conditions strongly depends on the volume fraction of proteins. The studied lysozyme solutions are dominated by monomers only for ?≤0.012; for the strong dilution 70% of proteins are in a form of monomers. For ?=0.033 only 20% of proteins do not belong to a cluster. The clusters are mainly elongated. For ?=0.12 almost no individual particles exits, and branched, irregular clusters of large extent appear. Our simulation study provides new insight into the formation of equilibrium clusters in charged protein solutions near physiological conditions.     

Simulation of the embryonic stage of ZnS formation from aqueous solution

We investigate the processes of cluster formation and growth of ZnS from aqueous solution using molecular dynamics simulation techniques. The influence of both temperature and concentration is studied. We show that, at lower temperatures, the crucial process is the transformation of an outer-sphere Zn/S complex to an inner-sphere ion pair. Further growth of the latter is fast to generate negatively charged planar clusters. These clusters interact to form more stable, closed structures, which are found to be the global minima configurations in vacuo. At higher temperatures, no outer-sphere ion pairs are formed, and the larger cluster configurations form much more quickly.     

Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling Constants—Comparison with Salt Bridges and Molecular Systems

We present deuteron quadrupole coupling constants (DQCC) for hydroxyl‐functionalized ionic liquids (ILs) in the crystalline or glassy states characterizing two types of hydrogen bonding: The regular Coulomb‐enhanced hydrogen bonds between cation and anion (c–a), and the unusual hydrogen bonds between cation and cation (c–c), which are present despite repulsive Coulomb forces. We measure these sensitive probes of hydrogen bonding by means of solid‐state NMR spectroscopy. The DQCCs of (c–a) ion pairs and (c–c) H‐bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids. At low temperatures, the (c–c) species successfully compete with the (c–a) ion pairs and dominate the cluster populations. Equilibrium constants obtained from molecular‐dynamics (MD) simulations show van't Hoff behavior with small transition enthalpies between the differently H‐bonded species. We show that cationic‐cluster formation prevents these ILs from crystallizing. With cooling, the (c–c) hydrogen bonds persist, resulting in supercooling and glass formation.     

Molecular dynamics simulation of solution structure and dynamics of aqueous sodium chloride solutions from dilute to supersaturated concentration

Constant-temperature and constant-pressure (NpT) molecular dynamics simulations were performed to study the effects of salt concentration ranging from dilute to supersaturated concentrations on solution structure and dynamical properties of aqueous sodium chloride solutions at 298 K. The rigid SPC/E model was used for water molecules, and sodium and chloride ions were modeled as charged Lennard–Jones particles. Na⁺–Cl⁻ radial distribution functions showed the presence of contact ion pairs and solvent separated ion pairs. The coordination numbers of Na⁺–Cl⁻ ion pairs increased with salt concentration up to saturated concentration, although the number of contact ion pairs was almost constant in supersaturated regions. The tracer diffusion coefficients of both ions decreased with salt concentration up to saturated concentration, while that of sodium ion was almost constant in supersaturated regions. The tracer diffusion coefficients of both ions were therefore quite close to each other. The constant number of the contact ion pairs and the almost equality of the tracer diffusion coefficients of both ions would lead to the formation of clusters in supersaturated solutions.     

Ion solvation in water clusters

We describe here molecular dynamics computer simulations performed to study the solvation of ions (Cl^? and Na⁺) in water clusters. Our simulations show that the calculated structure and dynamics of the clusters is very sensitive to the potential model which is used to describe the interactions. From the comparison with thermodynamic data and data from the photoelectron spectra we conclude that in Cl^?(H₂O)_n (n≤20) clusters the ion is located on the surface of the cluster.     

Rotational dynamics of thiocyanate ions in highly concentrated aqueous solutions

The thiocyanate (SCN(-)) anion is known as one of the best denaturants, which is also capable of breaking the hydrogen-bond network of water and destabilizing native structures of proteins. Despite prolonged efforts to understand the underlying mechanism of such Hofmeister effects, detailed dynamics of the ions in a highly concentrated solution have not been fully elucidated yet. Here, we used a dispersive IR pump-probe spectroscopic method to study the dependence of vibrational lifetimes and rotational relaxation times of thiocyanate ions on KSCN concentration in D(2)O. The nitrile stretch mode is used as a vibrational probe for dispersed IR pump-probe and FTIR measurements. To avoid possible self-attenuation of the IR pump-probe signal by highly concentrated SCN(-) ions, we added a small amount of (13)C-isotope-labeled thiocyanate ions (S(13)CN(-)) and focused on the excited-state absorption contribution to the IR pump-probe signal of the (13)C-isotope-labeled nitrile stretch mode. Quite unexpectedly, the vibrational lifetime of S(13)CN(-) ions is independent of the total KSCN concentration in the range from 0.46 m (molality) to 11.8 m while the rotational relaxation time of S(13)CN(-) ions is linearly dependent on the total KSCN concentration. By combining the present experimental findings with the fact that the dissolved ions of KSCN salt have a strong tendency to form a large ion cluster in a highly concentrated aqueous solution, we believe that the ion clusters consisting of potassium and thiocyanate ion pairs in D(2)O behave like ionic liquids and the ions inside ion clusters are weakly bound by electrostatic Coulombic interactions. The ability of SCN(-) ions to form ion clusters in aqueous protein solutions seems to be a key to understand the Hofmeister ion effect. We anticipate that the present experimental results provide a clue for further elucidating the underlying mechanism of the Hofmeister ion effects on protein stability in the future.     

Computer Simulation of Water Clusterization on Chlorine Ions: 2. Microstructure

The molecular structure of Cl^–(H₂O) _n clusters, n = 1–60, in equilibrium with vapor, and the cluster with n = 500 was studied by the Monte Carlo method. The first hydrated layer of a cluster is formed in unsaturated water vapors. The second hydrated layer begins to be formed in saturated vapor. The position of hydrated layers is not changed with an increase in cluster size and coincides with the position of the hydrated layers of ions in aqueous solutions of weak electrolytes. Orientational order in a cluster also has the layered structure. The orientation of molecules between the layers is random. The stability of the first layer is ensured only due to direct interactions with ions, whereas the stability of subsequent layers is due to cooperative interactions between molecules and between molecules and ions. As temperature decreases, the effect of ion displacement to the cluster surface becomes stronger.     

Computer simulation of ion cluster speciation in concentrated aqueous solutions at ambient conditions

Dynamic simulations are used to investigate ion cluster formation in unsaturated aqueous NaCl at 25 degrees C. Statistical, structural, and dynamic properties are reported. An effort is made to identify general behaviors that are expected to hold beyond the limitations of the force field. Above approximately 1 M, clusters with more than ten ions begin to form after approximately 10-20 ns of simulation time, but no evidence of irreversible ion aggregation is observed. Cluster survival times are estimated, showing that the kinetics become increasingly complex as salt is added, leading to multiple decay rates. Cluster dipole moment distributions show characteristic peaks that reflect the preferred conformations of clusters in solution. These are modulated by electrostatic and liquid-structure forces and are described in detail for clusters of up to five ions. For a given size and charge, the cluster morphology is independent of salt concentration. Below approximately 2 M, clusters affect the structure of water in their first hydration shells, so dipole moments parallel to the cluster macrodipoles are induced. These effects show a weak dependence with concentration below approximately 2 M, but vanish in the 2-3 M range. A possible connection with the structural transition recently suggested by NMR data in concentrated electrolytes is discussed. The effects of electrostatics on cluster speciation and morphology are discussed based on results from a set of simulations carried out with the ionic charges removed.