Solute-solute Correlations Research Articles

Evaluation of second osmotic virial coefficients from molecular simulation following scaled-particle theory

ABSTRACTWe report a scaled particle theory-based method for evaluation of second osmotic virial coefficients from molecular simulations of dilute species in solution. In this method, we evaluate the work associated with growing a cavity in solution that is perfectly permeable to the solvent but is completely impermeable to the solutes, thereby establishing an osmotic stress between the cavity interior and exterior. Extrapolating our results to determine the solute concentration in contact with a cavity with an infinite radius, we are able to evaluate the solute osmotic pressure and second osmotic virial coefficient. A finite size correction is introduced to account for the impact of effectively concentrating the solutes in the periphery of the simulation box with increasing cavity size. We demonstrate the utility of the proposed method by evaluating second osmotic virial coefficients for methane in water as a function of temperature. The approach proposed here provides a physically transparent route for calculation of second osmotic virial coefficients by direct interrogation of simulation configurations without having to explicitly evaluate the long-range integral over solute-solute correlations required following McMillan-Mayer theory.

Alteration of Water Structure by Peptide Clusters Revealed by Neutron Scattering in the Small-Angle Region (below 1 Å−1)

Solution scattering of neutrons and x-rays can provide direct information on local interactions of importance for biomolecular folding and structure. Here, neutron scattering experiments are combined with molecular-dynamics simulation to interpret the scattering signal of a series of dipeptides with varying degrees of hydrophobicity (GlyAla, GlyPro, and AlaPro) in concentrated aqueous solution (1:20 solute/water ratio) in which the peptides form large segregates (up to 50–60 amino acids). Two main results are found: 1), the shift to lower Q of the so-called water-ring peak (Q≈ 2 Å−1) arises mainly from an overlap of water-peptide and peptide-peptide correlations in the region of 1.3 <Q< 2 Å−1, rather than from a shift of the water signal induced by the presence of the clusters; and 2), in the low-Q region (Q≈ 0.6 Å−1) a positive peak is observed originating from both the solute-solute correlations and changes in the water structure induced by the formation of the clusters. In particular, the water molecules are found to be more connected than in the bulk with hydrogen-bonding directions tangential to the exposed hydrophobic surfaces, and this effect increases with increasing peptide hydrophobicity. This work demonstrates that important information on the (hydrophobic) hydration of biomolecules can be obtained in the very-small-angle region.

Hydration properties and potentials of mean force of nonpolar amino acid residues in water: A pertubation theoretic approach

We use a perturbation approach to calculate the solute-solvent and solute-solute pair correlations for infinitely dilute solutions of nonpolar amino acid side chains in water. In particular, from the solute-solute correlations we derive potentials of mean force for all the distances between different residues of amino acids. Comparison with molecular dynamics simulations performed using GROMOS package shows that the goodness of the approximation varies with the amino acid considered. We discuss in terms of hydrophobicity the different effect that the inclusion of the attractions causes on the solute-water and solute-solute correlations.

Structural Investigation of Solute−Solute Interactions in Aqueous Solutions of Tertiary Butanol

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the solute-solute intermolecular structural correlations in 0.06, 0.11, and 0.16 mole fraction tertiary butanol-water solutions. Empirical potential structure refinement (EPSR) procedures have been used to extract detailed information relating to the intermolecular structure in these systems. A trend from hydrophobic to hydrophilic character of the solute-solute correlations as a function of solute concentration is observed. Of particular note is the domination of nonpolar to nonpolar solute contacts at 0.06 mole fraction concentration compared with a more complex mixture of nonpolar and polar solute-solute intermolecular contact configurations at the higher alcohol concentration.

Differences in hydration structure near hydrophobic and hydrophilic amino acids

We use molecular dynamics to simulate recent neutron scattering experiments on aqueous solutions of N-acetyl-leucine-amide and N-acetyl-glutamine-amide, and break down the total scattering function into contributions from solute-solute, solute-water, water-water, and intramolecular correlations. We show that the shift of the main diffraction peak to smaller angle that is observed for leucine, but not for glutamine, is attributable primarily to alterations in water-water correlations relative to bulk. The perturbation of the water hydrogen-bonded network extends roughly two solvation layers from the hydrophobic side chain surface, and is characterized by a distribution of hydrogen bonded ring sizes that are more planar and are dominated by pentagons in particular than those near the hydrophilic side chain. The different structural organization of water near the hydrophobic solute that gives rise to the inward shift in the main neutron diffraction peak under ambient conditions may also provide insight into the same directional shift for pure liquid water as it is cooled and supercooled.

Do solute-solute interactions affect activation-limited reactions? A Brownian dynamics study

In this study we use Brownian dynamics (BD) to investigate a model activated reaction. We simulate varying activation energy using the probability of reaction on collision. We find that, as the reaction probability decreases (activation energy increases), local structural effects are seen more strongly at ever higher density. In the limit of very low values of the reaction probability, local structural effects on reactivity are seen over the entire density range studied. This provides clear evidence that the degree to which activation energy affects a chemical reaction correlates directly with the extent to which solute-solute correlations may influence that reaction.

Molecular simulation and electron paramagnetic resonance (EPR) studies of rapid bimolecular reactions in supercritical fluids

We present comparisons among Brownian dynamics simulations, molecular dynamics simulations, and electron paramagnetic resonance spectroscopic studies of the Heisenberg spin-exchange reaction between nitroxide free radicals at near-infinite dilution in near-critical and supercritical ethane. We discuss the effects of correlations in the solute-solute and solvent-solute radial distribution functions on the rate constants for collision and reaction. We find that the enhancements in the local density of solvents around solutes strongly affect the rate constant for solvent-solute encounters. This result holds implications for those reactions where collisional-energy transfer from solvent to solute is the rate-limiting step. While the rate of collisions between solutes is strongly affected by solute-solute correlations for all densities, the reaction rate constant is affected by such local density augmentations only for certain combinations of density and collision lenght scale. Rate constants estimated computationally and experimentally show the same qualitative trend as a function of density. Collision lifetimes estimated from the simulations show a strong density dependence. These lifetimes reflect the competing effects of the intermolecular force and the potential of mean force and are distinctly bimodal at the higher densities.

Self-assembly of biopolymeric structures below the threshold of random cross-link percolation

Self-assembly of extended structures via cross-linking of individual biomolecules often occurs in solutions at concentrations well below the estimated threshold for random cross-link percolation. This requires solute-solute correlations. Here we study bovine serum albumin. Its unfolding causes the appearance of an instability region of the sol, not observed for native bovine serum albumin. As a consequence, spinodal demixing of the sol is observed. The thermodynamic phase transition corresponding to this demixing is the determinative symmetry-breaking step allowing the subsequent occurrence of (correlated) cross-linking and its progress up to the topological phase transition of gelation. The occurrence of this sequence is of marked interest to theories of spontaneous symmetry-breaking leading to morphogenesis, as well as to percolation theories. The present results extend the validity of conclusions drawn from our previous studies of other systems, by showing in one single case, system features that we have hitherto observed separately in different systems. Time-resolved experimental observations of the present type also bring kinetic and diffusional processes and solute-solvent interactions into the picture of cross-link percolation.

IMPACT OF NEUTRON SCATTERING ON THE STUDY OF WATER AND AQUEOUS SOLUTIONS

Neutron scattering is making a major impact on our physical and chemical understanding of water structure in aqueous solution. In combination with the technique of isotope substitution it can probe, via the partial pair correlation functions of the system, both the structural aspects of the water itself in solution and the correlation of water molecules with individual dissolved ions and molecules. Moreover it can also investigate the way the solute species distribute themselves in solution. The use of the so-called first-order difference method to look at the hydration of inorganic ions in solution has already been widely reported in several previous reviews, so this account concentrates largely on recent advances in the application of the second order hydrogen-deuterium substitution method to the study of water under non-ambient conditions, and to the measurement of water structure in a variety of solutions and heterogeneous media. It is also shown how the same technique can be used to investigate the nature of solute-solute correlations in solution, and the way water organizes itself around organic molecules and ions in solution.

Solute-solute and solute-solvent correlations in dilute near-critical ternary mixtures: mixed-solute and entrainer effects

The asymptotic expressions for the solute-solute, cosolute-cosolute (cosolvent-cosolvent), solute-cosolute (solute-cosolvent), solute-solvent, and cosolute-solvent (cosolvent-solvent) total correlation function integrals of infinitely dilute ternary mixtures approaching the solvent's critical point are derived. All these integrals scale as the solvent's isothermal compressibility and diverge at the solvent's critical point. The sign of those diverging quantities depends on the behavior of the short-ranged infinite dilution solute-solvent and cosolute-solvent direct correlation function integrals. Some important implications of the peculiar microscopic behavior of these mixtures are discussed, such as (a) the irreconcilable incompatibility between the microstructure of dilute near-critical mixtures and the physical basis underlying the van der Waals one-fluid conformal solution mixing rules, (b) the prediction of mixed-solute and/or entrainer effects for ternary systems whose individual solute-solvent or solute-cosolvent binaries behave as either weakly attractive or repulsive mixtures, (c) the independence of the mixed-solute effect on the solute-solute, solute-cosolute, and cosolute-cosolute interactions, and (d) the independence of the entrainer effect on the solute-solute, solute-cosolvent, and cosolvent-cosolvent interactions. 41 refs.

Solute-solute correlations in aqueous solutions of tetramethylammonium chloride

Neutron diffraction with hydrogen/deuterium isotope substitution on the methyl group hydrogen atoms of the solute is used to investigate the distribution of the tetramethylammonium (TMA) ion, (CH3)4N+, in aqueous solution of 4·0 molal tetramethylammonium chloride at room temperature. The centre-centre correlation function for TMA ions is calculated from the HH correlation function for the solute by deconvolution of a spherical form factor representing the distribution of methyl hydrogen atoms. The centre-centre function shows a broad distribution of TMA … TMA distances, but indicates that close contacts occur frequently. This suggests that the charge interaction of TMA is not important in determining the distribution and that the TMA ion behaves more like an apolar solute.

Structure and thermodynamics of dilute liquid multicomponent systems

We present an approximation based on standard integral-equations for the determination of the structure and thermodynamics of dilute liquid N-component systems. The low concentration limit is taken on the level of the Ornstein-Zernike equations; the system of N(N + 1)/2 coupled integral-equations is decoupled by this procedure into two lower dimensional problems: one describing the pure solvent, the other describing the solvent-solute correlations, the solute-solute correlations being neglected. As a consequence this method is far less time consuming than the exact problem and enables us to handle even dilute ternary systems. Extensive calculations based on two model potentials of binary and ternary systems show that this approximation gives very good agreement up to 5 per cent minority concentration.