The Structural and Dynamic Properties of some Transition Metal Aqua Cations: Results from Neutron Scattering

The following paper comprises a survey of the role neutron scattering methods have played to help understand the origins of the diverse properties of electrolyte solutions which contain transition metal cations. It is seen how neutron diffraction and isotopic substitution is able to resolve the local structure around contrasting ions, such as Cr3+ , Ni2+, Fe3+ , Fe2+, Cu2+, without recourse to sophisticated modelling procedures. Quasielastic neutron scattering (QNS) provides insight into the dynamics of the protons in solution. The results enable one to distinguish between cations whose water molecules are coordinated on time scales larger than 5 x 10-9 s, shorter than 10-10s, or intermediate between those two limits. QNS also provides information on the existence of a second relatively short-lived hydration shell distinct from the bulk water.

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.

Structure of aqueous glucose solutions as determined by neutron diffraction with isotopic substitution experiments and molecular dynamics calculations

Neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations have been used to examine the structuring of solvent around d-glucose in aqueous solution. As expected, no significant tendency for glucose molecules to aggregate was found in either the experiments or the simulation. To the extent that solute pairing does occur as the result of the high concentration, it was found to take place through hydroxyl-hydroxyl hydrogen bonds, in competition with water molecules for the same hydrogen-bonding sites. A detailed analysis of the hydrogen-bonding patterns occurring in the simulations found that the sugar hydroxyl groups are more efficient hydrogen bond donors than acceptors. From the comparison of the MD and NDIS data, it was found that while the modeling generally does a satisfactory job in reproducing the experimental data the force fields may produce sugar rings that are too rigid and thus may require future revisions.

X-ray and neutron scattering studies of the hydration structure of alkali ions in concentrated aqueous solutions

The presence of ions in water provides a rich and varied environment in which many natural processes occur with important consequences in biology, geology and chemistry. This article will focus on the structural properties of ions in water and it will be shown how the 'difference' methods of neutron diffraction with isotopic substitution (NDIS) and anomalous X-ray diffraction (AXD) can be used to obtain direct information regarding the radial pair distribution functions of many cations and anions in solution. This information can subsequently be used to calculate coordination numbers and to determine ion-water conformation in great detail. As well as enabling comparisons to be made amongst ions in particular groups in the periodic table, such information can also be contrasted with results provided by molecular dynamics (MD) simulation techniques. To illustrate the power of these 'difference' methods, reference will be made to the alkali group of ions, all of which have been successfully investigated by the above methods, with the exception of the radioactive element francium. Additional comments will be made on how NDIS measurements are currently being combined with MD simulations to determine the structure around complex ions and molecules, many of which are common in biological systems.

Neutron diffraction studies of electrolytes in null water: a direct determination of the first hydration zone of ions

A method of neutron diffraction is described which enables the first hydration zone of small cations to be investigated at atomic resolution. It is shown that the cation structures of aqueous electrolyte solutions dissolved in a 'null' mixture of water (H(2)O) and heavy water (D(2)O), can be calculated directly from the neutron scattering patterns. The hitherto unresolved structure around Na(+) is used to illustrate the power of this method, the accuracy of which is discussed formally with reference to standard nickel chloride solutions. Possible applications to a variety of other systems and at different thermodynamic states are proposed.

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.

An anomalous x-ray diffraction study of the hydration structures of Cs+ and I− in concentrated solutions

Anomalous x-ray diffraction experiments were carried out on concentrated aqueous solutions of sodium iodide (6 molal) and cesium iodide (3 molal). Data were gathered at two energies below the absorption edges of the Cs+ and I- ions in order to avoid contributions from fluorescence. The statistics and quality of the raw data were improved by the use of a focusing analyzer crystal. Differences were taken between the data sets and used to calculate the hydration structures of Cs+ and I-. The structures found are more complex than anticipated for such large ions with relatively low charge densities and show evidence of ion-pair formation in both solutions. A two-Gaussian fit to the Cs+ data gives information about the Cs+-O and Cs+-I- correlations. The central position of the Gaussian representing the Cs+-O was fixed at 3.00 A, that is, the maximum of this contribution. The other parameters were allowed to vary freely, giving a Cs+-I- distance of 3.84+/-0.05 A and coordination numbers of 7.9 and 2.7, respectively, for the Cs+-O and Cs+-I- correlations. The results on the structure of I- in the 6 molal NaI aqueous solution were also fitted to a model based on Gaussians; this gives correlations for I- -O and I- -Na+ at 3.17+/-0.06 and 3.76+/-0.06 A with respective coordination numbers of 8.8 and 1.6. The structure of I- in the 3 molal CsI solution shows overlapping contributions due to I- -H, I- -O, and I- -Cs+. The best Gaussian fit gives two peaks centered at 3.00+/-0.08 and 3.82+/-0.04 A and shows that the latter two correlations are unresolved. The hydration structures are compared with those of other alkali and halide ions. The results are also found to be in good agreement with those obtained from standard x-ray diffraction and computer simulation.

EXAFS studies of structural changes in fragile glasses of zinc nitrate and nickel nitrate hydrates

The ionic structures of aqueous solutions of two sets of transition metal nitrates have been studied in the liquid and glass states by EXAFS spectroscopy. Experiments were carried out on Zn(NO3)2.xH2O, with x=2, 6, 12 and NiNO.9HO over the temperature range 30<T(K)<250. The glass transition regime was monitored by means of an in-situ DSC probe. The EXAFS data were analysed by recently developed Monte Carlo procedures, enabling a discussion of the glass structure in terms of pairwise and higher order correlations. Results for the zinc nitrate hydrates show complex behaviour depending on the concentration. This behaviour is explained in terms of first hydration shell stability and NO3- penetration. This result contrasts with that for the equivalent correlation in nickel nitrate, and is taken as evidence for a more extensive free energy landscape of zinc nitrate hydrates. The results are also consistent with the known hydration properties of Zn2+ and Ni2+, and help explain why Zn2+ is biologically active in solution.

The hydration structure of guanidinium and thiocyanate ions: implications for protein stability in aqueous solution

Neutron diffraction experiments were carried out on aqueous solutions containing either guanidinium or thiocyanate ions. The first-order difference method of neutron diffraction and isotopic substitution was applied, and the hydration structures of two of nature's strongest denaturant ions were determined. Each ion is shown to interact weakly with water: Guanidinium has no recognizable hydration shell and is one of the most weakly hydrated cations yet characterized. Hydration of thiocyanate is characterized by a low coordination number involving around one hydrogen-bonded water molecule and approximately two water molecules weakly interacting through "hydration bonds." The weak hydration of these denaturant ions strongly supports suggestions that a major contribution to the denaturant effect is the preferential interaction of the denaturant with the protein surface. By contrast, solute species such as many sugars and related polyols that stabilize proteins are strongly hydrated and are thus preferentially retained in the bulk solvent and excluded from the protein surface.

The Structure of Aqueous Electrolyte Solutions: Comparis on of Computer Simulation and Experiment

This review compares results of neutron and X-ray diffraction experiments with computer simulation and theoretical calculation for aqueous electrolyte solutions at the atomic level in terms of the partial radial distribution functions of several ionic solutions, and includes results for the ion-water and ion-ion structure of systems containing alkali ions, alkaline earth ions, transition metal cations and a few anions